Idleing prob with UD 288/296F10 and 950HP - Chevelle Tech
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post #1 of 19 (permalink) Old Nov 22nd, 03, 5:35 PM Thread Starter
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For the life of me I cannot get my car to idle the way I want it to. I can get it to idle either in gear or in park but never together. With 12* of timing I can back the idle screw all the way off and it is still around 1500rpm with the mixture screws were they are best, and in gear it sits about 750-800rpm about where I want it. If I back the timing off to get it to idle around 1000rpm in park, as soon as I drop in in gear it goes to about 500rpm and will die if I leave it there. I'm about to go nuts. I put 35 power valves in it to try to help with a rich idle (still there) and an off idle stumble (which is gone), but the carb is at the factory settings otherwise.

So how can I get it to idle in park about the same rpm in gear? I know there will be a drop when I put it in gear, but this is crazy the way it is working now, I would like it to idle about 1000 in park and 700-800 in gear... thanks in advance

*1970 Chevelle Malibu
*460ci UD288/296 950HP 10.6:1
3.73 12 Bolt /TH350
*11.96 ET 111.66 MPH
*God Bless America
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post #2 of 19 (permalink) Old Nov 22nd, 03, 7:57 PM
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You should probably state the torque converter size and stall speed. That will factor into the problem.
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post #3 of 19 (permalink) Old Nov 22nd, 03, 8:06 PM Thread Starter
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The converter is a B&M 2400.. but it is pretty tight, I know that is a factor but I had this engine in before with some changes (same cam though) and a 3310 and it would drop about 200rpm.

*1970 Chevelle Malibu
*460ci UD288/296 950HP 10.6:1
3.73 12 Bolt /TH350
*11.96 ET 111.66 MPH
*God Bless America
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post #4 of 19 (permalink) Old Nov 22nd, 03, 8:07 PM
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sounds like the rear throttle blades may be open a little to much. back the screw for the rears out about 1/4 turn and see if you can get the adjustment back on the front idle screw where it needs to be. HTH Dave

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BEST 60'=1.35
BEST 1/8=6.27
BEST 1/8mph=108.14
BEST 1/4=9.98
BEST 1/4mph=131.86

fall of 07-spring of 08----10.85-10.48=.37 gain
fall of 08-spring of 09----10.48-10.17=.31 gain
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post #5 of 19 (permalink) Old Nov 22nd, 03, 9:02 PM
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Chris, I agree with Sheetmetal that the extra rpm could be caused by rear throttle being open too much. If the carb is still set up square jetted all the way around and you are using all four idle circuits then it is recommended to try and keep the both the front/rear adjusted the same amount. I ended up taking the carb off setting front/back throttle equally then adjust each a half turn at a time until the idle was down where I needed it. Good luck.

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 #6 of 19 (permalink) Old Nov 22nd, 03, 9:41 PM
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Eiteh that or possibly a simple vacuum leak.
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post #7 of 19 (permalink) Old Nov 22nd, 03, 10:54 PM
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you mentioned some changes, did you mess with the timing advance? or lighter advance springs? it may be you don't have enough advance when you drop it into gear, check your timing when in park, then check it in gear .you may see a big difference.
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post #8 of 19 (permalink) Old Nov 22nd, 03, 11:15 PM
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You shouldn't change the timing to get a lower idle speed.

With that cam I think you should be running about 18-20 degrees initial.

Reset your initial,
close the secondary's a touch,re-adjust the idle mixture screws.

If by resetting your initial timing at 18-20 degrees puts your total timing at 38 degrees or more you need to take some of the mechanical advance out of the distributor.On a non HEI this is done by installing a larger advance stop or welding up the advance stop slot.

Changing the mechanical advance on a HEI requires a different advance cam.This is very hard to come by.In fact I do believe that GM never made an advance cam that will work with that much initial and let you have a good total.
This is why I always recommend that people use a MSD distributor.The advance is easy to change with them.The distributor it's self will not give you more power.They give you more tuneablilty.Wich when dialed in makes a smoother,more driveable vehicle.It will also be more responcive to the pedal.
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post #9 of 19 (permalink) Old Nov 23rd, 03, 7:27 AM
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Thrasher is right on with the need for more initial advance becasue your 12 deg btdc is just too retarded causing you havic at idle/low engine speeds. Most per cams need approx 16-18 deg btdc intial advance so crank it up to a minimum of 16 deg btdc and go from there.

I for the life of me don't know why the cam mfg don't include a little note with their perf cams stating the need to run a lot more intial advance than the stock cam would run. This issue gives people problems all the time which could be so easily avoided if this info was with each perf cam. With all the testing thats been done over the years at these cam mfg's they have plenty of data to supply a good approx starting point for intial timing on mild to wild setups for any SBS/BBC so why don't they do it,who knows.


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post #10 of 19 (permalink) Old Nov 23rd, 03, 11:31 AM
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If you are running a very large cam have you tried running full manafold vacumm to advance.I had same problem with mine but I am running HEI at 12 and advance will pull it to 16 when I start it I'm also run a 750hp.Here is an artical from Lars about 4corrner Idling that might help. If you need more let me know I can help with timming.It did the trick for me but I had to work the timming first I only pull 6.5 in at idle.

How to Set Up a BG Speed Demon

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:

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69SSRat is offline  
post #11 of 19 (permalink) Old Nov 23rd, 03, 4:51 PM
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You are going to need to "lock out" the timing on this engine. Forget the advance, you're wasting your time. Set the timing and lock it in place.
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post #12 of 19 (permalink) Old Nov 23rd, 03, 5:38 PM
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now you are getting carried away, i run the same cam and the same carb, with a curved hei all in by 3000 rpms, 12 deg advance and 36 total. no idling problems whatsoever. with a victor jr, and 1" spacer and only 10 " vacum. it's all in the advance curve and at what deg of increments it starts to come in at.i agree that the msd is the easiest to set up, but he has to get a light on it and document what is happening , watch at what rpm it is advancing (how much) . 12 deg at 1500rpm he says, then when it drops in gear the idle drops to 500 rpm then dies out. he is probobly at around 4/6 deg at 500 rpms. he has to watch the advance rate and probably change it.sure he can lock in the advance, but this setup is not that wild, he probably cruises the car around town using pump gas and if he is locked in at 36 deg ,on a hot day he may detonate,and have hot cranking problems.
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post #13 of 19 (permalink) Old Nov 23rd, 03, 6:21 PM
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I have to agree with MR68 I run a HEI myself and had your problems but at first i ran a locked out distributor and for the street I wouldn't go that way. I only run 6to6.5 at idle.I looked up the advance that was 2" below that set timing and this fixed my problem.
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post #14 of 19 (permalink) Old Nov 23rd, 03, 6:32 PM
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Take the advance weight return springs out for a quick test. Reset your timing to whatever total advance (less vacuum adv) you want. Drive it a little and see how it responds. Maybe if you had a little looser converter you could run the dist locked out but even with youyr 2400 you should be able to. I had a similar setup but with a 3200 stall 10" and ran it locked out, far more stable idle with a 292/302 solid. Since the engine sees very little load below the TC rated stall, detonation should not be any problem. Wasn't in my case and I ran 38 degrees locked.

Try it locked out with no vacuum advance.
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post #15 of 19 (permalink) Old Nov 23rd, 03, 6:45 PM
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Originally posted by streetwize:
You are going to need to "lock out" the timing on this engine.
That's a bit extreme!

On the track go for it.
On the street you shouldn't need to be locked out with that cam.


You simply need a little more initial,dial in your idle mixture screws,reset your idle speed.

The info provided above by 69SSRat is good,follow it.

Like I said just make sure you don't end up with more than 36degrees total timing.

What distributor do you have?
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