fuel dumping

[chevynova777]

i have a nova with a chevy 350 small block and an edelbrock 750 4 barrel carb. electric choke. i bought the car with this engine already installed the way it is, however when i start the car it runs smooth until it warms up and it starts to have an idling problem sort of like a timing issue or somethig of that nature. this engine doesnt even have 5000 miles on it and was built with new parts in and out. its going through gas like a top fuel drag car. i took out the plugs and they were as black as anything i have ever seen. i changed them started the car back up and had the same problem fine for about 15 minutes and BAM plugs are burnt. so my question is, is it dumping too much fuel is the carb to big maybe the wires are in the wrong firing order and if they are how do i determine wat the order is i dont know where to start any help would be greatly appreciated. oh and by the way i smell lots of gas when i turn this beast on dont know if that will help at all. thanks ahead for your wisdom and knowledge

[snshddog]

First off yes, 750 is too much carb for a stock 350. It can be used but hard to tune in. I wouldnt go bigger than a 600. The other problem I keep seeing is when people set up a carb many of them set the idle circuit way too rich so it almost gives a loap like its got a big cam in it because it "sounds cool" If it drives down the road ok and you want to save money then rejet it and tune it in right.

[chevynova777]

well the engine has suped up heads and performance rockers and lifters and so forth so should i still put a 600 in there to level it off and.

[snshddog]

I would say yes, we built a good 400 and the street dyno showed the 600 to work better than the 750 the customer started with. I have removed plenty of big carbs in favor of smaller and the customer is never dissapointed. On a SBC i only use between 400 and 600 cfm unless its a race car. Race is all different rules but thats never on the road.

[MrPbody]

Flow "rating" of the carb is more important to maximum RPM desired, and the CID of the engine. For the problem stated here, only the "primary" side of the carb is involved, so the CFM rating isn't really all that relevant. Also, contrary to popular belief, the air/fuel mix on a carb has nothing to do with the CFM rating and a "lean/rich" condition. A 750, if properly "jetted", won't change the A/F ratio over a 600 CFM carb, also properly jetted. What DOES change, is the velocity of the mixture as it enters the intake manifold, and ultimately, the cylinder. Smaller carb, higher velocity (for "square bore" carbs, Q-Jets really "hose" this philosophy...). Higher velocity directly relates to cylinder "packing". For more low-speed power, more velocity... More revs, more CFM.

I also agree, though, a "750" is a bit on the "large" size. We use a 650 (Holley, NEVER E-carbs) on most 350 "Street" engines, not intended to rev beyond 5,500 or so. The 400 though, at 6,000 RPM, is "using" about 720 CFM. Using the old "rule of thumb" that you should have a carb 10% "larger" than actual CFM (for average volumetric efficiency), means a 400 would more "like" a 750 or 800 at that rev range. For the 400 small blocks, we use a 750. For 400 Pontiacs, 850 is needed. Pontiac intake ports are a bit more "efficient" than small block (stock versus stock), so they "like" a little more air/fuel at a given rev.

What type of choke mechanism does this carb have? Electric or "manual"? This symptom sounds more to me like the choke isn't "coming off". The "clue" is that it runs well "cold".

Jim

[danielsatur]

The 4-barrel carb works like an American standard Toilet!

It sounds like an electric choke adjustment, the throttle plate is off during warm up.