Engine Shuts Off Question

[JacobSchni]

I've had this problem but only expierenced it when i floor it. If i floor my truck, when i let off it kills. It doesnt seem like its bogging, it just stops running. Itll still start back up though(somtimes easy, somtimes not so easy). I rarely beat my truck by flooring it, but i need to know why this would be happening. What could be causing this?

[cadillac_al]

Must not be a Quadrajet. My Edelbrock will do that but it runs fine otherwise. If I stomp on it, it takes off on the fly. If I stomp on it and change my mind a split second later, it will stall everytime. I have never had a Quadrajet do that to me.

[JacobSchni]

no its a quadrajet, and i dont let off right away even if i floor it for around 10 to 15 seconds then let off it stalls

[rich weyand]

I'll take a shot at one possibility.

Gas and air have different momentum. When you open the throttle, the air moves first, and the gas has to catch up. That would result in a lean condition, which would cause the engine to stall, so carburetors include accelerator pumps to shoot some extra gas in the barrels to make up for the sudden in-rush of air until the gas circuit catches up.

Similarly when you shut down the throttle, the air stops immediately but the gas continues to flow for a bit. This results in a rich condition, which isn't usually a problem, because you have to be way rich for the engine to stall. That being said, if you're set up way too rich to begin with, that extra gas into the engine can cause the engine to stall.

How was the carb tuned? Is it the original carb? If so, it should have been set up right from the factory. Could it be leaking gas through the casting plugs, giving you a rich condition to start with? In that case, when you shut down the throttle, and vacuum in the manifold increases, more gas will be sucked through the casting plugs, as well as the gas that continues for a bit when the air stops, and you could go rich enough to stall it.

Does it idle rich, do you know? If it idles rich because of casting plug leaks, the oil will be black at oil change time, not just dark brown. Have the casting plugs been fixed to stop leaking? Have you put an A/FR meter on it to see what the mixture is doing? Do you ever get after-fires (booms in the exhaust) when you get off the throttle?

One thing that isn't too bad to do to try to clean it up is to pull the carb and fix the casting plugs to eliminate that as the potential problem. If they are leaking, your gas mileage will also go up when you fix them.
http://www.highperformancepontiac.co...r/viewall.html

[motornut]

check this
http://67-72chevytrucks.com/vboard/s...d.php?t=579621

[JacobSchni]

Its the original carb, it was just rebuild this month by my grandpas friend. He then helped tune it for me. It run great besides after I floor it and let off... it just kills. There is no booms in the exhaust it just instantly kills.

[rich weyand]

Can you ask him if he repaired the casting plugs?

[JacobSchni]

Yea I'll try to get to home tonight

[cadillac_al]

OK Qjet after all. How does it idle? Does it start and idle perfectly? If you drive moderately without kicking the secondaries does it drive normally? To stall after a wide open run sounds like an over rich condition to me. If it was the well plugs or the float then it should idle terribly. If it idles good and moderate driving is good, then the secondaries must be overly rich for some reason. If the secondary needles weren't changed then maybe the builder bent the secondary needle hanger in an attempt at "more power". Or the secondary air flap is just opening too much too soon raising those needles more than they need to be. It shouldn't be too hard to sort out; especially since the builder is a relative.

[JacobSchni]

That makes sense. Could it also be that his is the orig carb and I think it was on a stock 305 and now its on a 350 crate engine