does jetting affect idle quality (switched 750DP to 950HP)? - Chevelle Tech
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post #1 of 12 (permalink) Old Nov 7th, 03, 9:39 PM Thread Starter
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I am curious to know if jetting has any effect on idle quality at all. Common sense tells me that it wouldn't affect the idle circuit but everytime I jet up to support the motor in cool air my 750 and now this 950HP is giving me fits at the stoplight.

Tonight I switched out my 750 DP for a Davinci 950 HP and all I can say is WOW. The throttle response is razor sharp and even feels stronger in the mid range but I am having intermittent problems maintaining a consistent idle in gear.

The carb is used and came jetted 85 all the way around. Primary Power valve is 6.5, secondary is plugged, and all four throttle blades are drilled (not sure size just yet).

All four idle adjust screws are set at 1.5 turns out and the car will respond to adjustment (it dies if I start turning them down to 1/2 turn). The car idles nice and crisp at 950 - 1000 with 10-11 in vac. Great throttle response. Sometimes when it goes in gear it maintains nice vacuum and 900 rpm. Othertimes, especially coming off the throttle at a stop light the idle hunts up and down (500 - 900rpm) with the vacuum bouncing up down between 10 - 5). If I put it in neutral and rev the engine once or twice it will usually clean up and allow it to idle cleanly in gear. All I can think of is the excess jetting is loading the engine up with excess fuel at times???

I realize the factory 950hp comes with 79's all the way around but I wanted to try the carb as is since Davinci set it up that way. I will say that I had similar issues begin to manifest itselft with my 750DP (jetted 74/86) as well, which makes me think poss vacuum leak?? I have sealed the intake, blower, and carb twice looking for a leak but no luck.

Any advice/experiences would be much appreciated.

Chris Dufresne<br />Waldorf Maryland<br />71 Chevelle, 383, B&M 144 Supercharger(7.5lb boost), 8.5 to 1 CR, AFR 210, UD solid 272/282, TH350 w/2800 stall, 12 bolt limited slip w/3.73, 275/60/15 BFG Drag Radial. 1.76 60ft, 1/8 mile: 7.48 @ 93.41; 1/4 mile: 11.72 @ 115.47
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post #2 of 12 (permalink) Old Nov 7th, 03, 10:30 PM
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If the vac is droping below 6 1/2 in. at idle in gear you will have to swap out the 65 power valve as the PV opening at idle will richen up the mixture big time.
A 45 PV may be all you need.

1971 Chevelle SS
454 4 speed
Original Nevada Silver D88 black stripe car
(GM documented by George Zappora from vintage services)

1969 Chevelle SS
396 / 4 spd
L-78 / M-22
(GM documented)

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post #3 of 12 (permalink) Old Nov 8th, 03, 6:33 AM
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I do believe the main jets will effect air/fuel mix at idle. Especially with holes in the throttle blades. That pulls air through the venturies which would pull fuel out of the jets.

I put on a 950HP from Allstate carbs. It came with holes in the throttle blades. Idle was extreamly rich. Jetting was 78/88 or something like that. I had a 750 on the car before and the primary jets were 72s. I put 72s in the 950 primaries and 80s, I think, in the rear. Fixed the idle right up.

I don't think you'd want the jetting square if you have a power valve on one side and not the other, either.


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post #4 of 12 (permalink) Old Nov 8th, 03, 2:20 PM
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Hey Let me post some reading from a guy that helped me,I know it is for BG carbs but it works well on holley's with 4 corner ideling,also I tried alot of things and no one ever told me some of this so keep an open mind and try it it works if you have any problems you can email him or myself.

by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. New 2-19-03

This tech paper will discuss basic set-up of the Barry Grant Speed Demon carbs to establish an initial starting point for a good-running carb.

The BG carbs are similar to the Holley carbs in their layout and design. The BG carbs have some refinements that make them easily tunable for most street applications, providing this tuning is done correctly and in the right sequence. BG carbs come factory-set so that they will run well right out of the box, but I have found that the setup can be refined and tweaked a little to optimize the carb a little better. It is also important that the tuner/owner understand these tuning processes when re-adjusting idle speeds and idle mixtures so as not to get the carb settings really messed up.

This paper will only discuss basic initial setup. For tuning and tweaking, check out my paper on How to Tune a Holley - the tuning processes for the BG are very similar once the initial setup described here has been completed.

Tech Tip #1
BG carbs use mostly the same gaskets as a Holley. However, the throttle plate gasket (the gasket between the bottom throttle plate and the upper body of the carb) on a BG has 4 idle fuel transfer holes that are not in the same location as the Holley. If you use a Holley gasket, you will not get any fuel flow through the idle metering circuit on the BG, and the idle mixture screws will not work. You can use the Holley gasket, but you need to slot the idle fuel transfer holes in the gasket to match the holes in the BG carb.

Tools and Equipment Required
As a minimum, you will need the following tools:

1. Vacuum Gauge
2. Small cup to drain fuel into
3. Screwdrivers
4. Box end wrenches
5. Spark plug removal tools
6. Rags

Here is my recommended sequence and procedure for doing a basic BG set-up:

1. Bench-Set the Idle Speed & Idle Mixture.
The BG Speed Demon carbs have 4-corner idle metering. This means that they meter idle fuel and idle air through all 4 of the throttles - primary and secondary. For this system to work properly, it is absolutely critical that all 4 of the throttle blades ALWAYS be set at the same setting (NEVER set idle speed by only adjusting the primary idle speed screw), and all 4 of the idle mixture screws should be set to the same metering setting. This will assure that the carb is balanced and working right from the beginning. Once you get the engine up and running well, a slight difference in mixture screws between the primary and secondary side may be required, but start by balancing everything out as follows:

Before installing the carb to the engine (if you have installed it, yank it off), turn the carb upside-down on your workbench. If you look at the throttle bores just below the edge of the throttle plates (“butterflies”), you will see a vertical slot. Open the throttles a little to see the whole slot. This slot is called the “transfer slot,” and it provides a fuel discharge transition circuit between the idle circuit (which discharges fuel out of the round idle discharge holes below the throttle plates) and the main metering circuit (which discharges fuel out of the main discharge nozzles once airflow through the venturies is high enough to pull the fuel through the nozzles). The transition slot receives its fuel from the idle metering supply circuit.

With the throttles fully closed against their idle stop screws (not on the fast idle cam), noting that the secondaries and primaries both have separate idle stop screws, there should be exactly .020” of the transition slot exposed below the throttle plates. Use a .020” feeler gauge to measure this: Place the feeler gauge on the throttle plate up against the transition slot and adjust the idle speed screw so that the slot JUST BARELY disappears behind the feeler gauge. At .020” slot exposure, the slot will appear to be a perfectly square hole. Adjust the primary and the secondary idle speed screws so that both of the throttle shafts are at this same position.

Once the primary and secondary throttles have been set to this initial idle speed setting (which should make your car idle very close to the correct idle rpm range), it is your job as a tuner to assure that any further idle speed changes occur by adjusting both of the screws equally from this point on. Never adjust the idle speed by only adjusting the primary screw: if you adjust the primary idle speed by ¼ turn, you MUST adjust the secondary idle speed screw ¼ turn as well. Keep the two throttles adjusted the same.

Now, turn all 4 of the idle mixture screws all the way IN until they LIGHTLY seat, and then back them all out ¾ turn.

2. On-Engine Settings.
You can now bolt the carb onto the engine and hook up your fuel and choke (if you’re running a choke). If you have a choke, make sure you hook up the wire to a switched 12-volt source. Note that the “hot” wire going to the “+” side of the coil is not 12 volts due to the resistor wire in the ignition circuit, so don’t use the coil wire for your choke. BG chokes tend to be set very rich from the factory, so you might want to loosen the 3 choke cover screws and rotate the black choke housing cover so that the choke plate begins to open - in its factory setting, it is tightly closed. Note that the BG carbs do not have in-carb fuel filters like a Q-Jet, so you MUST run an in-line filter between your fuel pump and the carb (don’t install filters on the suction side of the fuel pump).

Start the engine and allow it to warm up. If you have a choke, you can adjust the fast idle screw to your preference at this time. If you do not have a choke, you can turn BOTH idle speed screws in the same amount (usually about ½ turn-or-so) to keep the engine running during this warm-up period. NOTE how much you turn them both in. While the engine is warming up, you can check and verify your float levels: There are 3 lines on the bowl sight glasses: for a street car, set the primary and the secondary float levels to the lower line. The float levels are set by loosening and adjusting the float adjusters on the tops of the float bowls. If your float levels need to be lowered, do this slowly, as the fuel in the bowl will only drop as fast as the engine is using the fuel at idle speed.

As the engine warms up, make sure the choke is opening, and get the engine off fast idle. Once the engine is up to normal operating temperature, start playing with your idle speed screws: Adjust both of the screws equally to obtain the slowest practical idle. This should be very close to your bench setting. If the 2 screws need to be turned IN more than a full turn from the bench setting, you need to consider installing idle bleed restrictors as outlined in the BG documentation that came with your carb, since cranking the idle speed screws IN too far will make you engine idle on the transition circuit instead of on the idle circuit. Once a low idle speed has been obtained, you are running on the idle circuit. You can now adjust idle mixtures.

If desired, you can now hook up a vacuum gauge to use as a tuning aid. Using a small screwdriver, turn one of the primary mixture screws IN ¼ turn and observe the reaction of the engine. Turn the screw back out to its original setting, and then turn it OUT ¼ turn. Observe the engine response. This test will tell you if you need to go IN or OUT from the original setting. Once this has been determined, go back to the original setting, and turn ALL of the screws 1/8 turn at a time in the direction needed until best idle is obtained. While doing this, the idle speed may need to be lowered/adjusted to keep the car at a slow idle. Be sure to adjust both idle speed screws the same. Once the optimum idle mixture has been obtained in this fashion, go back and set up your idle speed to the final rpm desired using both screws equally.

With the engine now good and hot, re-check your float levels (making sure the car is on a level surface). Float levels will change slightly as the engine/carb/fuel heats up. If the levels seem a little high, you can slowly discharge some fuel out of the bowls by actuating the accelerator pump lever(s). If the level remains the same, you need to lower the float slightly.

This completes the initial setup of your BG carb. You should now have a very good idle, and off-idle throttle response should be crisp and instant.

3. Accelerator Pump Arm.
To assure proper operation of the accelerator pump, make sure that the pump arm is properly set up against the pump arm screw (spring loaded screw on the lever). With the engine OFF, verify that there is no gap at all between the end of the screw and the pump lever - it should have a little bit of “pre-load.” Verify that the SLIGHTEST movement of the throttle produces an instant discharge of fuel out of the discharge nozzles. Now, open the throttle fully and verify that there is still a little bit of travel left in the pump arm (make sure it’s not bottomed out and jammed solid).

4. Throttle Cable Check.
Before you go for a drive, make sure you have full throttle travel, and make sure you have a positive throttle stop. One of the most common performance problems I see are carbs that do not go to Wide Open Throttle (WOT) due to improperly adjusted linkage. Also make sure that your throttle linkage does not restrict the carb from a full return to idle speed. Make sure you have a throttle return spring attached (don’t rely on the carb throttle shaft spring to do the work for you). Now, with a helper in the car, observe the throttle shaft lever as the helper slowly presses the gas pedal to the floor. Verify that the throttle opens fully. Verify also that the gas pedal hits the floor or a fabricated pedal stop just as the throttle hits its wide open point. If you rely on the carb to stop your gas pedal, you will bend and destroy your carb - your leg has more power in it than the sheet metal lever on your carb. You may need to swap throttle cable attach points on the carb throttle lever, and you may need to play with your throttle linkage geometry to make this all work right, but it’s imperative to check and correct as required.

5. Secondary Opening Rate (Vacuum Secondaries).
If you’re running a vacuum secondary BG, you can gain some performance by playing a little with the secondary diaphragm spring. BG sets up the secondary opening rate very conservatively to avoid a secondary tip-in bog. But this results in secondaries that open very slowly, and often they fail to ever open fully. You can buy a secondary spring assortment kit from BG, Summit, Holley, Jeg’s, or your local NAPA store and play with this a little. I have had best success using the lightest spring in the kit or the second-lightest spring. Use the lightest spring you can that prevents a bog when you go into the secondaries.

Questions, Comments & Technical Assistance
If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an e-mail. Also, if you need any technical assistance or advice regarding this process, or other maintenance issues, feel free to contact me:

[email protected]
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post #5 of 12 (permalink) Old Nov 8th, 03, 2:24 PM
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more good info to pass along

I’ve been seeing a lot of discussion and
questions regarding distributor vacuum advance control units; what do they
do, which ones are best, what was used on what, etc., etc. To clarify
some of this, I thought I’d summarize a few facts and definitions, and
provide a complete part number and specification listing for all vacuum
advance control units used by Chevrolet on the points-style distributors.
I’m also providing a listing of the specs for all other GM (non-Chevrolet)
control units, but without the specific application listed for each (it
would take me a bit too much time to research each part number by
application across each of the GM Motor Divisions – it took me long enough
to compile just the Chevy stuff…!). This latest revision to this paper
also includes the HEI listings (the HEI distributors use a longer control
unit, so the non-HEI and HEI vacuum advance control units CANNOT be

As always, I’m going to include the disclaimer that
many of these are my own comments and opinions based on my personal tuning
experience. Others may have differing opinions & tuning techniques from
those presented here. I have made every attempt to present factual,
technically accurate data wherever possible. If you find factual errors
in this information, please let me know so I can correct

The vacuum advance control unit on the
distributor is intended to advance the ignition timing above and beyond
the limits of the mechanical advance (mechanical advance consists of the
initial timing plus the centrifugal advance that the distributor adds as
rpm comes up) under light to medium throttle settings. When the load on
the engine is light or moderate, the timing can be advanced to improve
fuel economy and throttle response. Once the engine load increases, this
“over-advance” condition must be eliminated to produce peak power and to
eliminate the possibility of detonation (“engine knock”). A control unit
that responds to engine vacuum performs this job remarkably

Most GM V8 engines (not including “fast-burn” style heads),
and specifically Chevys, will produce peak torque and power at wide open
throttle with a total timing advance of 36 degrees (some will take 38).
Also, a GM V8 engine, under light load and steady-state cruise, will
accept a maximum timing advance of about 52 degrees. Some will take up to
54 degrees advance under these conditions. Once you advance the timing
beyond this, the engine/car will start to “chug” or “jerk” at cruise due
to the over-advanced timing condition. Anything less than 52 degrees
produces less than optimum fuel economy at cruise speed.

additional timing produced by the vacuum advance control unit must be
tailored and matched to the engine and the distributor’s mechanical
advance curve. The following considerations must be made when selecting a
vacuum advance spec:

How much engine vacuum is produced at cruise?
If max vacuum at cruise, on a car with a radical cam, is only 15 inches
Hg, a vacuum advance control unit that needs 18 inches to peg out would be
a poor selection.

How much centrifugal advance (“total timing”) is
in effect at cruise rpm? If the distributor has very stiff centrifugal
advance springs in it that allow maximum timing to only come in near
red-line rpm, the vacuum advance control unit can be allowed to pull in
more advance without the risk of exceeding the 52-degree maximum limit.
If the engine has an advance curve that allows a full 36-degree mechanical
advance at cruise rpm, the vacuum advance unit can only be allowed to pull
in 16 more degrees of advance.

Are you using “ported” or “manifold”
vacuum to the distributor? “Ported” vacuum allows little or no vacuum to
the distributor at idle. “Manifold” vacuum allows actual manifold vacuum
to the distributor at all times.

Does your engine require
additional timing advance at idle in order to idle properly? Radical cams
will often require over 16 degrees of timing advance at idle in order to
produce acceptable idle characteristics. If all of this initial advance
is created by advancing the mechanical timing, the total mechanical
advance may exceed the 36-degree limit by a significant margin. An
appropriately selected vacuum advance unit, plugged into manifold vacuum,
can provide the needed extra timing at idle to allow a fair idle, while
maintaining maximum mechanical timing at 36. A tuning note on this: If
you choose to run straight manifold vacuum to your vacuum advance in order
to gain the additional timing advance at idle, you must select a vacuum
advance control unit that pulls in all of the advance at a vacuum level 2”
below (numerically less than) the manifold vacuum present at idle. If the
vacuum advance control unit is not fully pulled in at idle, it will be
somewhere in its mid-range, and it will fluctuate and vary the timing
while the engine is idling. This will cause erratic timing with
associated unstable idle rpm. A second tuning note on this: Advancing
the timing at idle can assist in lowering engine temperatures. If you
have an overheating problem at idle, and you have verified proper
operation of your cooling system components, you can try running manifold
vacuum to an appropriately selected vacuum advance unit as noted above.
This will lower engine temps, but it will also increase hydrocarbon
emissions on emission-controlled vehicles.

Thus, we see that there
are many variables in the selection of an appropriate control unit. Yet,
we should keep in mind that the control unit is somewhat of a “finesse” or
“final tuning” aid to obtain a final, refined state of tune; we use it to
just “tweak” the car a little bit to provide that last little bit of
optimization for drivability and economy. The vacuum advance unit is not
used for primary tuning, nor does it have an effect on power or
performance at wide open throttle.

With these general (and a little
bit vague, I know…) concepts in mind, let’s review a few concepts and
terms. Then it’s on to the master listing of specs and

Part Number
There are many different sources for these
control units. Borg Warner, Echlin, Wells, and others all sell them in
their own boxes and with their own part numbers. Actually, there are very
few manufacturers of the actual units: Dana Engine Controls in
Connecticut manufactures the units for all three of the brands just
mentioned, so it doesn’t make much difference who you buy from: They’re
made by the same manufacturer. The part numbers I have listed here are
the NAPA/Echlin part numbers, simply because they are available in any
part of the country.

Every vacuum advance control unit built
by Dana, and sold under virtually any brand name (including GM), has a
stamped ID number right on top of the mounting plate extension. This ID,
cross referenced below, will give you all specifications for the unit. So
now, when you’re shopping in a junkyard, you’ll be able to quickly
identify the “good” vs. the “bad” control units.

Starts @
Vacuum is measured in “inches of Mercury.” Mercury has the
chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to
as “Hg. The “Start” spec for the control unit is a range of the minimum
vacuum required to get the control unit to just barely start moving. When
selecting this specification, consideration should be made to the amount
of vacuum that a given engine produces, and what the load is on the engine
at this specification. For example, an engine with a very radical cam may
be under very light load at 7 inches Hg, and can tolerate a little vacuum
advance at this load level. Your mom’s Caprice, on the other hand, has
such a mild cam that you don’t want the vacuum to start coming in until 9
– 10 inches Hg. For most street driven vehicle performance applications,
starting the vacuum advance at about 8” Hg produces good

Max Advance
Since the vacuum advance control unit is a
part of the distributor, the number of degrees of vacuum advance is
specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking
about these control units, it is important that you know whether the
person you’re talking to is referring to the distributor degrees, or if
he’s talking crankshaft degrees. All of the listings shown in the
following chart, and in any shop manual & technical spec sheet, will refer
to distributor degrees of vacuum advance. You must DOUBLE this number to
obtain crankshaft degrees (which is what you “see” with your timing
light). Thus, a vacuum advance control unit with 8 degrees of maximum
advance produces 16 degrees of ignition advance in relationship to the
crankshaft. When selecting a unit for max advance spec, the total
centrifugal timing at cruise must be considered. Thus, a car set up to
produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum
advance control unit producing 16 degrees of crankshaft advance. This
would be an 8-degree vacuum advance control unit.

Max Advance @
This is the range of manifold vacuum at which the maximum vacuum
advance is pegged out. In selecting this specification, you must consider
the vacuum produced at cruise speed and light throttle application. If
your engine never produces 20” Hg, you better not select a control unit
requiring 21” Hg to work.

The following listing (Non-HEI) is as
follows: The first two part number listings are the two numbers that are
most commonly used in a Chevrolet performance application. The “B1” can
is the most versatile and user-friendly unit for a good performance street
engine. As you can see, it was selected by GM for use in most high
performance engines due to its ideal specs. The “B28” can was used on
fuel injected engines and a few select engines that produced very poor
vacuum at idle. The advance comes in very quick on this unit – too quick
for many performance engines. Do not use this very quick unit unless you
have a cam/engine combination that really needs an advance like this. It
can be used as a tuning aid for problem engines that do not respond well
to other timing combinations, and can be successfully used in applications
where direct manifold vacuum is applied to the can (see paragraph and
discussion on this above)

After this, the listing is by Echlin part
number. The Chevrolet applications are listed first by application,
followed by a complete listing of all of the units used on any GM product
(all GM units are interchangeable, so you can use a Cadillac or GMC Truck
unit on your Vette, if that’s what you want to do).


P/N ID# Application
Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18

1964 Corvette exc. FI
1964 Impala, Chevy II

1965 396 High Perf.
1965-67 283, 409

1966-68 327 exc. Powerglide
1967-68 All 396

1969 Corvette 427 High Perf.
1969 396 Exc. High
1969 Corvette 350 TI

1969-70 302 Camaro
1970 400 4-bbl

1970 396 High Perf.
1970 Corvette 350 High Perf.

1973-74 454 Exc. HEI

B28 1965 409 High Perf. 3-5 8 @ 5.75-8

1965 327 High Perf.
1966 327 High Perf.

1964-67 Corvette High Perf.


VC1605 B9 1965 impala 396 Exc. High Perf.
7-9 10.3 @ 16-18
1965 327 All Exc.
1969 327 Camaro, Chevelle, Impala

1969-70 Corvette 350 Exc. High Perf.
350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle,
Impala High Perf.
1971-72 350 2-bbl AT

1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide
9-11 8 @ 16-18
1968 327 Impala AT

1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl
5.5-8 12 @ 14-18
1969-70 350 2-bbl

B20 1965 396 Impala High Perf 5-7 8 @
1966-67 Corvette Exc. High Perf.

1966-67 Impala 427 Exc. High Perf.
1966-68 327
Powerglide Exc. High Perf.
1969 307 All

1969-70 396, 427 Camaro, Chevelle High Perf.

1970 400 2-bbl
1970 307 MT

1973 Camaro 350 High Perf.

B21 1971 350 2-bbl 7-9 10 @
1971-72 400, 402

1971-72 307 AT

VC1802 B22
1971-72 350 4-bbl 7-9 8 @

Other Part Numbers & Specs:

VC700 B3
8-10 11.5 @ 19-21
VC1415 M1
6-8 10 @ 13-15
VC1420 M2
5-7 11 @ 16-17
VC1650 B12
8-10 10 @ 15-17
VC1725 B18
8-10 12 @ 13-16
VC1740 A5
6-8 12 @ 15-17.5
VC1755 A7
8-10 12.5 @ 18-20.5
VC1804 B24
6.5-8.5 10 @ 12-14
VC1805 M13
6-8 12 @ 14.5-15.5
VC1807 B25
5-7 8 @ 13-15
VC1808 B26
5-7 8 @ 11-13
VC1809 B27
5-7 9 @ 10-12
VC1812 B30
5-7 12 @ 11.75-14

The following listing (HEI)
is as follows: The first four part number listings are the 4 numbers that
are most commonly used in a Chevrolet performance application. The “AR12”
can is the most versatile and user-friendly unit for a good performance
street engine. The AR 15 and AR23 are almost identical, with only slight
variations in their “start-stop” specs. The “AR31” can is the HEI
equivalent to the “B28” Hi-Perf can used on the early engines: The advance
comes in very quick on this unit – too quick for many performance engines.
Do not use this very quick unit unless you have a cam/engine combination
that really needs an advance like this. It can be used as a tuning aid
for problem engines that do not respond well to other timing combinations,
and can be successfully used in applications where direct manifold vacuum
is applied to the can (see paragraph and discussion on this

After this, the listing is by Echlin part number. All GM
HEI vacuum advance units are interchangeable, so you can use a Cadillac or
GMC Truck unit on your Vette, if that’s what you want to do.


P/N ID# Application
Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

AR12 1975 350 Buick 7-9 7 @

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc,
Calif. 3-5 7.5 @ 9-11
1974 400 All
1977 305 El Camino

1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf,
Incl. Calif
1975 350 Z-28

1977 305 Buick Skylark

VC1853 AR23 1976 350 All
Calif. 5-7 7.5 @ 11-12.5

1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc.
1975 350 4-bbl

1974 350 All w/1112528 Distr.
1978 350/400 Heavy
Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31
2-4 8 @


N/A 1978-79 Vette Special Hi Perf N/A N/A

1979 305 El Camino Calif.
1978-79 350 Blazer &
1979 Buick 305/350

AR1 1976 454 Caprice, Impala 3-5 9
@ 6-8
1975 454 Caprice, Chevelle, Monte,

VC1826 AR2
5-7 12 @ 10-13

VC1827 AR3
5-7 9 @ 9-11

VC1828 AR4 1975-76 350 Buick
& Olds 6-9 10 @ 12-14
1976 350

VC1831 AR7
6-8 12 @ 14-16

VC1832 AR8 1975-76
455 Buick Electra 4-6 12 @ 12-14

AS1 1975-76 500 Cadillac Exc. Calif. 4-6 14
@ 15-16

VC1834 AR9
4-6 13 @ 13-16

VC1835 AS2 1975-76
350 Olds 5.5-7.5 12 @ 15-17

AR10 1977 305 All Hi Alt, Exc. Calif. 3-5 9
@ 11-13
1977 350 All exc. Calif.

1977 350 Vette Exc. Calif, Exc. Hi Perf
1976 305
All Exc. Calif
1976 350 All Exc. Vette, Exc.
1976 350 Vette Exc. Calif., Exc. Hi
1975 262, 350 All w/2-bbl carb

1975 350 All 4-bbl w/ 1112880 & 1112888 Distr.

1977 305 Chev Truck Light Duty
1975-76 350 El
Camino 2-bbl

VC1837 AR11 1976 305 Blazer, Exc.
Calif 6-8 12.5 @ 10.5-13.5

1976 350/400/455 Pontiac 4-bbl

VC1839 AR13
4-6 12 @ 11-13

VC1840 AR14 1975-76
350/400/455 Pontiac Firebird 6-8 10 @ 9-12

AS3 1975-76 500 Cadillac Calif. 5-7 10 @

VC1842 AS4 1976 350 Olds Cutlass
5-7 12 @ 13-15

VC1844 AR16
3-5 12 @ 13.5-15.5

VC1845 AS5 1978-79
425 Cadillac w/F.I. 4-6 14 @ 14-16

1977 425 Cadillac

VC1846 AR17 1977 301 Buick Skylark
3-6 13 @ 10-13
1977 301

VC1847 AS6 1978 403 Motor Home
4-6 12 @ 12-14
1977-79 350/403 Buick
LeSabre Hi Alt, Riviera, Olds
1977-79 350/403
Pontiac Hi Alt

VC1848 AR18
4-6 12 @ 9-12

VC1849 AR19
4-6 12 @ 7-10

VC1850 AR20 1977 350/400
Pontiac 4-6 10 @ 8-11

AR21 1977-79 350 Buick LeSabre, Century 5-7 12 @
1978-79 350 Pontiac

AR22 77-78 305/350/400 Chev Truck, Heavy Duty 7-9 5
@ 12-14
1975-76 350/400 Chev Truck Heavy

VC1854 AR24
3-5 13 @ 10-13

VC1855 AS7 1977-79
260 Olds Cutlass 3-5 15 @

VC1856 AR25
3-6 15 @ 10-14

VC1857 AR26
3-6 12 @ 13-16
VC1858 AR27 1978-79 305 All
3-6 9 @ 11-13
1978 350 Camaro

1978 305 Chev Truck, M/T, Light Duty
1978 350
Chev Truck Hi Alt
1978 305/350 Buick & Olds

1978-79 305 Pontiac

VC1859 AR28 1979 350 Vette Exc
Hi Perf 3-6 10 @ 9-12
305 w/1103282 Distr., Incl. El Camino A/T
1979 350
Camaro, Impala, Nova, Malibu, Monte
1979 350
1979 350 Buick Century

1978 305/350 Buick & Olds
1978-79 305 Pontiac Hi

VC1860 AR29
3-6 12 @ 10-13

VC1861 AR30 1978-79
301Buick 3-5 13 @ 11-13

1979 301 Olds
1978-79 301 Pontiac

AR32 2-4 10 @

VC1864 AR33 1978 305 Chev Truck, A/T, Light
Duty 4.5-6.5 13 @ 11-13

VC1865 AR34
1973-74 350 Vette Special Hi Perf 3-5 15 @

VC1866 AS8 1978-79 425 Cadillac w/carb
3-5 14 @ 13-15

VC1867 AS9
2-4 10 @ 8-10

VC1868 AR35 1979 305 Chev
Truck & El Camino 2-4 10 @ 6-9

1979 305 Buick & Olds
1979 305 Pontiac

VC1869 AS10
2-4 12 @ 8-11

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post #6 of 12 (permalink) Old Nov 8th, 03, 2:33 PM
Tech Team
Join Date: Dec 1999
Location: Southhaven, MS, USA
Posts: 410

And finally this
I know it is also of info but my cam combo
I only get 6"to7" of vacuum at Idle I have been to DUI about my distributor and they didn't know this info. Now like I said I talked to alot of others and tried alot of there ways and it never seemed to work with them finally saying I dont know.The guy who wrote these I emailed and he responded every day and every thing he told me the car would do with each change
i did was on the money!!!! [img]graemlins/beers.gif[/img]

This tech paper will discuss basic set-up and
tuning of Holley vacuum secondary carbs for optimum street performance and

Holley carbs have been used by hot
rodders for many decades now. Holley offers a very tuneable, modular carb
design, yet it’s surprising how few people understand the basic
tuning principles and procedures for these carbs. This tech paper will
discuss some basic tuning procedures and techniques to help you get your
setup optimized a little better.

Holley carbs have several
operating systems, all of which can be tuned independently. Often, an
improper setup of one system will give false indications of a problem in
another system. Attempting to cure Holley problems by tuning the wrong
system results in a really messed up carb, and a lot of frustration for
tuners and car owners.

Holleys have the following basic operating

Idle System
Accelerator System
Main Metering
Power System
Secondary System

These systems at times
overlap in their operation. Not only does each system need to be properly
tuned, but its timing and “overlap” with other systems is
critical to proper performance.

Tech Tip #1
Before you go
trying to fix all the errors of the previous carb tuner, set your carb up
to the stock Holley spec for your carb List Number (stamped into the front
of the air horn). Holley does a pretty decent job of supplying a
ready-to-run carb, so the jetting, pump cam, shooter sizing, and spring
rates are usually pretty close. This will give you a good baseline
starting point for your tuning process. You can either call your local
Holley distributor for the stock specs on your List Number carb, or go to
this website to check out the baseline

Tech Tip
When buying tuning parts for your Holley you have two
Obviously, Holley offers original parts at the Holley price.
I highly recommend original Holley rebuild kits, gaskets, and power
valves. BG, or Barry Grant, also makes parts for Holleys, and offers them
at a fraction of the Holley price. BG is a great source for items such as
the accelerator pump cam kits.

Tech Tip #3
Virtually all
musclecars (with a only a few notable exceptions) came from the factory
with a dual plane intake manifold, also known as a
“180-degree” manifold. A dual plane intake is characterized by
having two separate sides (right and left) which feed two different
“levels” inside the manifold. Half of the cylinders are
“fed” from the one level, and the other half are
“fed” from the second level. A dual plane manifold promotes
higher intake velocity at lower rpm, improving low-end torque and low-end
throttle response. A dual plane manifold also promotes very good vacuum
at idle and at low speed.

At very high rpm, a dual plane manifold
will loose some efficiency. For this reason, there are single plane
manifolds available. Known also as “360-degree” manifolds,
the single plane manifolds typically have large intake runners and a
large, open plenum under the carb. Very good for high-rpm use, these
manifolds can be a real challenge to tune for low-end, street rpm use. A
single plane manifold will typically significantly reduce vacuum, and this
causes several problems.

Intake vacuum is used by the carb as a
“signal” for many different events to occur. As vacuum is
reduced, caused by large plenum intakes and large overlap cams, the vacuum
“range” becomes much more narrow. The carb tuner must make
the various events occur within this narrow tuning range, while hitting a
much more narrow “tolerance” for overlapping events to occur.
Typically, a large-plenum intake will require richer main jetting and a
richer pump shot to avoid a lean stumble hesitation upon initial
acceleration. The idle transition may even require extensive recalibration
of the idle restrictors – not an operation for the

My recommendation: don’t use a single
plane intake on a street driven car unless you have a lot of cubic inches
and you’re willing to put in a lot of time tuning the idle
transition. If you’re experiencing an off-idle stumble on a car
with a single plane manifold, your problem may be in the manifold and not
with the carb. Consider spending the $$ for a dual plane intake and saving
yourself some frustration.

Tech Tip #4
Which way to tune the
accelerator pump – richer or leaner?
Hesitation, stumble, or
backfire is usually indicative of a lean condition.
throttle” is usually indicative of a rich condition.
A rich
condition may also be accompanied by a small puff of black smoke upon
initial acceleration.

Tech Tip #5
When removing the float bowl,
pull one of the lower float bolt screws first. Place a small container,
such as the plastic cap off of a spray paint can, under the float bowl,
and catch the fuel as it drains out. When you put it all back together,
you can use a small kitchen funnel (just don’t tell your wife about
it) to pour the drained fuel back into the bowl by dumping the fuel down
the bowl vent tube (the tube sticking up right at the choke).

Tip #6
When using a Holley on a car with a bumpy cam, it is often
necessary to screw the idle speed screw in far enough that the ported
vacuum slot in the throttle bore is uncovered, producing significant
ported vacuum to the distributor vacuum advance at idle. When the engine
is rev’ed up, the vacuum source advances the timing, as it should.
When the throttle is closed, the rpm will very gradually bleed down as
vacuum advance gradually fades: the engine will not settle down
immediately to the correct idle speed. To eliminate this problem, you can
adjust the secondary opening screw to allow the secondary throttle plates
to be cracked open a little more than the spec requirement. This will
bleed a little air through the secondaries at idle, increasing idle speed
and allowing the primaries to be closed down to correct the ported vacuum

Tools and Equipment Required
As a minimum, you will
need the following tools:

1. Vacuum Gauge
2. Small cup to
drain fuel into
3. Screwdrivers
4. Box end wrenches
5. Spark
plug removal tools
6. Rags

Here is my
recommended sequence and procedure for doing a basic Holley

1. Set the float level.
You’ll be amazed how many
people try tuning a Holley without ever checking the float level. An
incorrect float level can give you all kinds of symptoms and problems, so
get this one set right off the bat. Most Holleys have a sight plug on the
side of the float bowls, and have externally adjustable floats. With the
engine HOT and running (float level will change with engine temperature
– make sure you’re completely warmed up before setting this),
put a rag under the sight plug area and pull the plug. Adjust the float
level until fuel just barely starts to dribble out of the plug, then lower
it until the dribble barely stops. The slotted screw in the center of the
adjustment is the locking feature – loosen it and turn the hex nut
CW to lower the float level, CCW to raise it.

2. Make sure your
Power Valve is in the ballpark.
Most “stock” Holleys come
with a power valve in the 6.5” range. This means that the power
valve will open and start dumping fuel into the discharge when manifold
vacuum hits 6.5” Hg (Mercury) of vacuum. A lot of cars with big
overlap cams, single plane manifolds, or operating at high altitude will
not pull 6.5” vacuum with the transmission in “drive.”
This means that the power valve is flooding the engine with fuel at idle,
and you’ll never get the tuning right unless you fix it.

up a vacuum gauge to the intake manifold or to one of the vacuum ports at
the base of the carb. With the transmission in “drive” (or in
neutral on a manual car), note the vacuum reading. Your power valve
should be rated at about 2” below the actual vacuum reading. On a
car running at 6” of vacuum in drive, you will need a 4.5 or a 3.5
power valve.

3. Now get the main metering circuit in the
Now that your power valve is not flooding you out, you can
get the main metering jets tuned in. If your main metering circuit is
either too lean or too rich, you will not be able to properly set up the
accelerator pump circuit. Trying to fix a bog on acceleration with pump
cams and discharge nozzles when the main metering is off is like chasing
your tail. The main metering system also affect the idle circuit, so you
need to get the main jets in line before going on to the other tuning

Pop a new set of spark plugs in the engine (don’t
use the new Platinum or other exotic metal plugs- they won’t read
right). Now take the car out for a drive around a few blocks. Get it up
to steady-state cruise rpm, and make a few moderate accelerations. Pull
the car back into the garage, allowing it to idle as little as possible.
Shut it down and pull a few of the plugs. The plugs should have a very
light, off-white or tan color around the nose of the insulator. If
you’re pure white (completely clean), you’re probably running
too lean. If you’re picking up black soot, you’re too rich.
Raise or lower primary jet sizes by 2 sizes at a time and repeat this test
until your plugs are showing the correct color.

4. Tweak your idle
Screw both of the screws in until they gently bottom out, and
turn them both out 1-1/4 turns. This is a good starting point. With a
tach on the engine to monitor rpm, start the engine and turn the screws ½
turn at a time, turning both screws the same amount, until the best rpm
reading is observed. Then turn them in 1/8 to ¼ turn to lean it out just
a tad. Once set, I like to shut the engine off and turn both of the
screws all the way in, counting the turns, until they gently bottom out
again. Just to make sure they’re both set the same. Then back them
out to the setting you had.

5. You’re finally ready to
fine-tune the accelerator pump circuit.
Many people start by tuning
this, and can never get the stumble tuned out of the car. By making sure
that the other systems are approximately right, you can now set this
system up so that it’s not trying to compensate for other

There are two tuning parameters to the accelerator
pump circuit: Pump Cam and Discharge Nozzle.

The pump cam, a
little plastic, color-coded part located on the throttle shaft with a
single screw, will determine the total volume of the pump shot.

The discharge nozzle, or “shooter,” is available in
different sizes, and determines the rate of discharge. In other words, it
is possible to install a large-volume pump cam, and have a small-diameter
“shooter” which “bleeds” the total volume in over
a longer duration. Or you can install a big “shooter” which
allows the entire pump shot to be dumped in almost immediately.

There is no real scientific way of coming up with the exact
correct combination for these parts. As a rule, of the car stumbles,
backfires, or hesitates on initial throttle opening, try a larger shooter
(the size is stamped on the body, and they’re available in
.001” increments). If the car has a “lazy throttle,”
indicative of a rich condition, try a smaller pump cam at first.

The last step in the initial tuning process is to set the secondary
opening rate.
The secondary opening rate is determined by the spring
in the secondary diaphragm. A soft spring will allow the secondaries to
open sooner. Install softer springs until the car develops a slight
stumble from the opening rate, then go back one spring level. This will
allow the secondaries to open as quick as possible while avoiding a

A common mistake made by “performance” tuners
is to remove the secondary diaphragm check ball. The check ball is
installed in the secondary diaphragm housing, and helps to control the
secondary opening rate. Removal of the ball will result in immediate
opening of the secondaries with very little mass air flow through the
primaries, with a resultant massive stumble upon acceleration.

cannot normally get the vacuum secondaries to open by flicking the
throttle and rev’ing the rpm with the car in neutral. This is not a
valid test of the functionality of the secondaries.

7. Now, if you
want to fine-tune the primary jetting, you can do one of three
For maximum horsepower, you will need to run the car through a
measured distance and note its speed at the end of the distance. Using one
of the available performance computers, or running the car at a track, is
best for this. The correct size jets are the jets which give the car the
highest mph at the end of the measured distance (not the shortest elapsed
time). Again, change jet sizes 2 sizes at a time to see if the speed
increases or decreases.

For maximum economy, drop the jet size
until the car develops a slight stumble upon acceleration, then go up two

For a really good, scientific way of setting up the jetting,
check out this

Questions, Comments
& Technical Assistance
If you have questions or comments regarding this
article, or if you notice any errors that need to be corrected (which is
quite possible since I’m writing this from memory…), please
feel free to drop me an e-mail. Also, if you need any technical
assistance or advice regarding this process, or other maintenance issues,
feel free to contact me:

[email protected]

aces member #03260
69 convertible
68 convertible
72 Olds 442 convertible

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post #7 of 12 (permalink) Old Nov 8th, 03, 10:51 PM Thread Starter
Tech Team
Join Date: Apr 2002
Location: waldorf, maryland
Posts: 46

Thanks for all the background information. I have been digging around all weekend looking for tips on how to properly set up a four corner idle set up.

After tearing the carb down and setting it back up I still could not get a consistent idle in gear but it had killer throttle response and pulls harder than it ever did with the 750DP, plus the additional flow now allows the engine to build 7 vice 6 lbs of boost.

I checked timing and saw I only had 12 initial but 34 I bumped to 18 initial which allowed me to turn idle down a bit. It now idles nicely in gear and has awesome driving manners [img]graemlins/hurray.gif[/img] ....however I now have 42 total which is way too much for this supercharged setup...arrgh. I ended up dialing in 14 degrees retard when boost is present (via the MSD) to bring timing back down to 28 total when full boost is present. However, I am extremely nervous MSD will not do its job and I will end up with full timing and massive detonation. So far it seems to idle nice, pull hard with no noticable detonation that I can tell. I think I may have fixed my situation by just bumping up the timing.

Now...any advice on how to limit the centrigal advance built into the HEI? Is it a JB weld/grind deal the advancing slots or can I limit it another way?

I am off to the track tomorrow...its going to be sunny, 39 deg and only 40% humidity , hoping for some new bests with the cold crisp air, extra boost and free breathing carb.


Chris Dufresne<br />Waldorf Maryland<br />71 Chevelle, 383, B&M 144 Supercharger(7.5lb boost), 8.5 to 1 CR, AFR 210, UD solid 272/282, TH350 w/2800 stall, 12 bolt limited slip w/3.73, 275/60/15 BFG Drag Radial. 1.76 60ft, 1/8 mile: 7.48 @ 93.41; 1/4 mile: 11.72 @ 115.47
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post #8 of 12 (permalink) Old Nov 9th, 03, 12:13 AM
Tech Team
Join Date: Dec 1999
Location: Southhaven, MS, USA
Posts: 410

Did you read the vacuum advance post?
First are you running ported or full manafold for your advance?
Second what is the p/n on your advance?

Now I run full manafold it alows me to set the inital to 12 for starting and I use the vacuum advance to bring it in.The one I use comes in at 3.5to5" now this will advance my timming for big cams such as mine and alows it to smooth out.
This will bring you back down to about 36 total. I have ran a locked out dist. for 16 initial but this did not work very well.

aces member #03260
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68 convertible
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post #9 of 12 (permalink) Old Nov 9th, 03, 12:22 AM
Tech Team
Join Date: Dec 1999
Location: Southhaven, MS, USA
Posts: 410

Hey look at this article


This tech paper will discuss setting the timing
on a Chevy V8. This procedure also applies to other GM V8s.

procedure outlined here differs from the Service Manual, and is based on
my years of experience doing this work in the quickest, least painful,
most economical way while keeping the level of quality high. It is
recognized that other people will have different methods of doing things,
and may disagree with specific methods and procedures that I

How to Set the Timing
When you think about it, setting the
timing at idle speed makes no sense at all: You don’t operate your
car at idle, and timing changes as the rpm changes. Fact is, the timing
spec at idle speed is provided as a simple way for most people to set the
timing, and is not a good procedure for optimum performance.
block Chevys (and most other GM performance V8 engines) perform best when
the total timing (full centrifugal advance plus the initial timing setting
with vacuum advance disconnected) is all in by 2,500 – 2,800 rpm and
is set to 36 – 38 degrees. If you have an adjustable timing light,
this is very easy to check. If you don’t, you need to scribe a
36-degree mark on your harmonic balancer. Here’s how:
the circumference of your harmonic balancer using a sewing tape measure
(or other flexible tape measure). Get it as accurate as you can. Take
this measurement and divide by 10. The number you get is the distance to
36 degrees. Measure this distance CLOCKWISE from your existing harmonic
balancer timing mark and place a clear mark on the balancer.
your distributor cap and rotor. Remove the 2 centrifugal advance springs.
Install the rotor and the cap (without the springs). Disconnect the vacuum

NOTE: This procedure cannot be used on the HEI ignition
systems. Removal of the springs will cause an artificially over-advanced
condition that will never be achieved with the springs in place. You can
use the basic technique described in this paper with the HEI units
(setting timing up to 36 degrees), but to check total timing, you must
install a set of soft springs. You cannot remove the springs altogether.
With the soft springs in place, rev the engine until the centrifugal
advance is pegged out. Adjust for 36 degrees total. Then re-install your
original springs.

Start the engine. It may kick back a little due
to the advance coming in immediately without the springs. If you’re
using an adjustable timing light, set the light to 36 degrees advanced.
Now rev the engine just a little while observing the timing marks with the
light. It shouldn’t take much rpm to peg out the advance without
the springs installed. With an adjustable light set at 36 degrees, align
the stock timing marks with “0” when the timing is
“pegged out.” With the non-adjustable light, align your new
36-degree mark with “0.” Rev the engine a little to make sure
the timing will not advance any further. Shut it down.
Pop the cap and
rotor and re-install the springs. Put everything back together, but leave
the vacuum disconnected. Start it up. For future reference, make a note
of the timing setting at idle. This is your new curb idle timing spec.
Now give the engine a few quick rev’s past 3,000 rpm and verify that
the full timing (36 degrees) is coming in. If it’s not, you need to
change to a softer set of springs until you get full 36-degree advance
before 3000 rpm. (NOTE: A stock set of springs will usually not allow
full centrifugal advance to come in before redline rpm. If you have stock
springs installed, don’t rev the engine beyond its limits to try to
force full advance in.)
Shut it down and hook up the vacuum. Now do a
road test.
The 36-degree 2500 rpm advance curve is optimum for
performance, but may require premium fuel. Lug the car around, and punch
the throttle at low rpm while listening for detonation (“engine
knock”). If you’re getting any audible knock, you MUST retard
the timing. Retard the timing in 2-degree increments until engine knock
stops. Engine knock will seriously damage engine components if not
corrected. If you get no knock, you may see slightly improved performance
at 38 degrees total timing. This is particularly true if you’re
running at high altitude.
If you have no engine knock under
acceleration, but the car “chugs” or “jerks” at
cruising speed (light throttle application), you are getting too much
vacuum advance on top of the mechanical advance. You may need to change
out the vacuum advance diaphragm with an adjustable unit available from
aftermarket sources. Adjust these units so that you get the most vacuum
advance possible without any “chugging” or
“jerking” at cruise speed.
Your timing is now set for best
possible performance. Make note of the new setting, and use this for your
future tune-up work.

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post #10 of 12 (permalink) Old Nov 9th, 03, 1:10 AM Thread Starter
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Location: waldorf, maryland
Posts: 46

Thanks Mike, I prob just need to experiment a bit more with the vacuum advance. I have always had it on a ported vacuum source as GM recommends, which hasn't done much for my idle situation. I will hook it back up to full manifold vacuum and see how much advance it gives me. This should give me the additional advance my engine appears to want at idle/part throttle and then retard itself once boost is built up, letting me keep total timing down where it should be.

I haven't checked any of the part numbers yet, been too focused on the carb before I realized my care really wanted more timing.

Chris Dufresne<br />Waldorf Maryland<br />71 Chevelle, 383, B&M 144 Supercharger(7.5lb boost), 8.5 to 1 CR, AFR 210, UD solid 272/282, TH350 w/2800 stall, 12 bolt limited slip w/3.73, 275/60/15 BFG Drag Radial. 1.76 60ft, 1/8 mile: 7.48 @ 93.41; 1/4 mile: 11.72 @ 115.47
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post #11 of 12 (permalink) Old Nov 9th, 03, 3:23 AM
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Location: Up to my neck
Posts: 961

I believe the ease of adjustment is the reason that the MSD distributors are so popular.
You just change a bushing to get more initial with the same total.
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post #12 of 12 (permalink) Old Nov 9th, 03, 9:18 AM
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Location: Pa
Posts: 1,324

it sounds like your idle screws are too far out especialy if you have 10" of vacuum at idle.close the rears to 3/4 turn out.
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