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Old 08-13-2008, 11:04 AM
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Really Cold Blooded

My car has a Chevy 350 crate engine with an Edelbrock 4bbl. carb. In the mornings it starts and idles just fine, but it has a habit of cutting out or stumbling on light acceleration until completely warm. Afterwards, it runs just fine. I recently had some suspension work done on it and the guy who did it adjusted the electric choke slightly and made some minor adjustments to the mixture screws. Now the problem is worse. It still starts fine and idles OK, but the stumble under light acceleration is more pronounced and now it also backfires through the carb. and exhaust until completely warm. I'd like to take it to a shop that specializes in carbureted cars, but I'm hesitant to leave it overnight as this problem only occurrs after a cold start. It's been a long time since I've worked on carbureted cars, but I'll get back into it if I have a sense of direction before starting. Based on my limited knowledge, I'm convinced this is a carburetor problem, but I wouldn't rule anything out. Since this is essentially a cold start problem, could the ignition (i.e. the timng) also be part of the problem? Once the car has been started and run for a while it runs great other than a very slight shudder under light throttle inputs.

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Old 08-13-2008, 11:33 AM
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Quote:
Originally Posted by 10mikeymike
My car has a Chevy 350 crate engine with an Edelbrock 4bbl. carb. In the mornings it starts and idles just fine, but it has a habit of cutting out or stumbling on light acceleration until completely warm. Afterwards, it runs just fine. I recently had some suspension work done on it and the guy who did it adjusted the electric choke slightly and made some minor adjustments to the mixture screws. Now the problem is worse. It still starts fine and idles OK, but the stumble under light acceleration is more pronounced and now it also backfires through the carb. and exhaust until completely warm. I'd like to take it to a shop that specializes in carbureted cars, but I'm hesitant to leave it overnight as this problem only occurrs after a cold start. It's been a long time since I've worked on carbureted cars, but I'll get back into it if I have a sense of direction before starting. Based on my limited knowledge, I'm convinced this is a carburetor problem, but I wouldn't rule anything out. Since this is essentially a cold start problem, could the ignition (i.e. the timng) also be part of the problem? Once the car has been started and run for a while it runs great other than a very slight shudder under light throttle inputs.
Yep that's what it were, a carb problem.

It would like a richer mixture when cold, as too much fuel is in a liquid state probably forming a puddle on the bottom of plenum of the manifold.

Electric chokes have the issue of not being responsive to engine temp. They have their own electric heat source which controls the choke with absolutely no connection to what the engine really needs based upon its temperature.

This motor either has no heat cross over or one that's shut off with the intake gasket, or is one of those air gap manifolds.

In either case the engine is telling you that the cold mixture is too lean.

That can be fixed with a tighter closure of the choke.

Applying heat to the intake under the plenum or as preheated air entering the carb as the factory does with the snorkel air cleaners.

Readjusting the carb mixture screws for a richer idle.

Setting the cold idle speed up higher. This will warm the engine faster and will keep velocity inside the runners up so fuel gets mechanically stirred with the air, holding the mixture closer to something the motor likes without having to pour more fuel into the engine all the time.

A big help in warming an engine is the modern in-the-radiator engine oil heat exchanger. This would bring the engine temp up faster, which is more compatible with an electric choke's cycle time.

Bogie
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