oversized carb and single-plane intake - Chevelle Tech
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post #1 of 11 (permalink) Old Nov 17th, 03, 9:26 PM Thread Starter
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if my combo turns out to be slightly "over-carbed"is there any way to tune the carb(such as smaller jets or nozzles) to make the engine "think" the carb is smaller rather than actually getting a smaller carb?
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post #2 of 11 (permalink) Old Nov 17th, 03, 9:30 PM
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Just how much too big is everything? The only thing you can do is tune the carb correctly and live with the low speed torque loss.
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post #3 of 11 (permalink) Old Nov 17th, 03, 9:32 PM
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As Mike said...you can tune the air/fuel ratio, but where you'll suffer is from loss if signal though the booster which will make for lazy throttle response.
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post #4 of 11 (permalink) Old Nov 17th, 03, 9:57 PM
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Use a higher gain boaster

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post #5 of 11 (permalink) Old Nov 17th, 03, 10:17 PM Thread Starter
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i checked the math formula for choosing the right- size carb for engine size and rpm and it came to 833cfm.it looks like i am 17cfm over-sized but since it will launch at around 4000 rpm with .456 gears maybe it can overcome that obstacle.
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post #6 of 11 (permalink) Old Nov 17th, 03, 10:35 PM
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my engine was dynoed with a 950hp and a 775 barry grant. there was only a 6hp difference. right now i have a 750hp on it. i may go back to the 950hp at some point after i get the bugs out of the 750. 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 #7 of 11 (permalink) Old Nov 17th, 03, 10:46 PM Thread Starter
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thanks for the helpful input guys!by the way,sheetmetal,i checked out the photos and specs of your mill.... [img]graemlins/thumbsup.gif[/img] s-w-e-e-t!
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post #8 of 11 (permalink) Old Nov 18th, 03, 8:42 PM
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17cfm. Don't even worry a little bit about it.
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post #9 of 11 (permalink) Old Nov 18th, 03, 10:22 PM Thread Starter
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thanks mike! [img]graemlins/thumbsup.gif[/img] i won't!
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post #10 of 11 (permalink) Old Nov 18th, 03, 11:20 PM
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In the book "How to Hot Rod Big Block Chevys" they suggest stagger jetting when using a single plane on a big block. What size is your engine? By your name I'm guessing big block
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post #11 of 11 (permalink) Old Nov 19th, 03, 4:27 AM
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This is an Artical that Lars sent me it helped me on my holley Hp carb and told me some things I didn't know about 4 corner Idle.He also has another that talks about the single plane intakes and how to tune them I will try to post it also as a link.His explanation is alot better than mine.

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:

[email protected]

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68 convertible
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