Stumbling and backfire - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 9 (permalink) Old Dec 23rd, 02, 06:02 AM Thread Starter
 
Join Date: Dec 2002
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Help please: 454, Holley 750 on Performer, points distributor. Everything ran perfect, installed new fuel pump and intake manifold (didn't remove valve covers). After assembly, it starts and idles fine, runs fine (maybe a slight stumble) under light or moderate acceleration, but stumbles and backfires when stomping on it or hitting it quickly in neutral.

So far: replaced fuel filter, verified TDC and firing order, played with timing (still a problem anywhere from 8 - 18*), replaced plugs/cap/rotor, swapped on another carb (brand new from perfectly running engine). Plug wires are routed properly with looms, they are newer and in good shape. Manifold vacuum is a steady 16 at idle, sprayed around intake and carb to check for leaks. Problem still exists and frustrations are high!

Could it be points and condensor, even though they were fine before the swap? Still a possibility of vacuum leak, but what else to look for to verify?

Any and all suggestions are welcome and greatly appreciated. Thanks.
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post #2 of 9 (permalink) Old Dec 23rd, 02, 06:20 AM
 
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Location: memphis tn us
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I had the exact problem on my 396. turned out the exhaust crossover at the front of the manifold was leaking into the carb. couldn't find the leak from the outside.
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post #3 of 9 (permalink) Old Dec 23rd, 02, 06:52 AM Thread Starter
 
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So the only way to find it was to pull the intake and look at the gaskets? I'm assuming you saw some crossfire on the gasket between the exhaust crossover and intake port? What would cause this as the intake was bolted to torque specs in the correct order.
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post #4 of 9 (permalink) Old Dec 23rd, 02, 09:47 AM
 
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My leak was not between the intake and the head. It was between the carb and the intake. I should have put in the block off plates when I installed the intake. Right in front of the primary holes in the intake are to holes that heat the carb. the heat shield I put on was cut out and did not hold the gasket down under the carb and the exhaust was getting in the primary holes.
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post #5 of 9 (permalink) Old Dec 24th, 02, 02:16 AM
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Eric
 
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Your problem is with the accelerator pump calibration. This is common with Holley carbs and is easily fixed. Swapping carbs may not do anything to fix the problem because both carbs may have the same accelerator pump calibration settings.

If it backfires through the carb when you hit the gas, you are getting what is called a "lean stumble." If it does not backfire and you get black smoke out the tailpipes you have a rich stumble. I think yours is a lean stumble.

There are two components used to change your accelerator pump calibration: pump cam and squirter. The pump cam controls the AMOUNT of fuel pumped by the accelerator pump; the steeper the accelerator pump cam is, the more fuel it will squirt. The squirter controls the length of TIME the pump shot lasts; the larger the squirter the faster it will squirt the fuel.

A Holley 750 cfm O-3310 carb (vac secondary) comes standard with an Orange pump cam (it's located behind the throttle lever on the drivers side) and a #31 squirter (located just under the choke blade in the front of the carb). You can get a kit with an assortment of pump cams for about $10 at your local speed shop or from Sumit, Jegs, etc. The squirters can be bought individually at the same places and come in increments of 3 (#28, #31, #34, etc).

Before you go and change any parts make sure that the lever that moves the pump doesn't have any slack in it. there is a screw and nut with a spring in it on the front left corner of the carb that can be adjusted so it is just touching the pump lever. You should also check and make sure that when at WOT the pump lever can travel at least another .050" so it is not bottoming out the pump.

If the linkage adjustment is correct and it still stumbles you need to determine whether to change the pump cam or the squirter. If the car immediately stumbles and backfires then go to a larger cam first. If the car goes for a split second then stumbles and pops then go to a SMALLER squirter to extend the pump shot.

Sometimes with large cams and single plane intakes you need to go with both a larger cam and a smaller squirter (more gas spread over a longer time). With a big engine like yours I think a larger cam should fix the problem by itself but if you go up a couple sizes on the accelerator pump cam and the problem gets only a little better you might also have to use a larger squirter.

Hope this helps - sorry so long.
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post #6 of 9 (permalink) Old Dec 24th, 02, 06:23 AM Thread Starter
 
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Eric - thanks for your reply. We will have to look into it. The other carb we swapped was an Edelbrock 750, and it actually ran worse with that on - so it may very well have to do with carb configuration. Kinda strange though, the only difference was going from an older Offenhauser 180 to a Performer RPM. Would that change alone cause such a carburation adjustment necessary?
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post #7 of 9 (permalink) Old Dec 24th, 02, 07:12 AM
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Eric
 
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I'm not too familiar with the Offy manifolds, but a change in plenum size can definately affect the engine's pump shot needs. Larger plenum usually means larger pump shot, larger runners would have the same affect. If the pump shot was "barely OK" with the Offy manifold, the RPM could have pushed it over the edge. When you get the bugs out, I'm sure you'll really like the RPM.

That pop back through the carb is a sure sign of a lean stumble though . . . it HAS to be an accel pump issue.
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post #8 of 9 (permalink) Old Dec 24th, 02, 07:37 AM Thread Starter
 
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All right...off to the parts store! Thanks again Eric for your expertise. Merry Christmas.
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post #9 of 9 (permalink) Old Dec 24th, 02, 04:36 PM
 
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Location: Keysville, VA USA
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Just for sh!ts and giggles.....check those points......sounds like they may be too close.

Phil
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