Hi All, I had the Quadrajet rebuild. Car starts fine, hot or cold. Idles great. Runs great, but when I stomp on it,( or even a heavy foot) it accelerates hard and long. But, when I let off the pedal, it sputters and stumbles until I restart the car. Visually, the secondaries don't appear to be stuck or sticking. 455 with headers and a cam. Any ideas?
Who rebuilt it, and what kind of a kit was used? As a first guess, I'd say that maybe you need a dashpot, though no Q-Jet that I've ever run had one, or exhibited this problem. Could be that the float level is set too high, but that's only a conjecture, at this point.
The carb was rebuilt by Quadrajet Power about 500 miles ago. It has been sent back once for the same issue. The problem is better but still persists.
Hey Brian. Couple of questions. What is your fuel pressure at WOT? Is raw fuel entering carb, or overflowing through vent when this happens?
Hi Mark. Thanks for jumping in. I've no way to tell what the fuel pressure is. I'll get the car to act up, leave it running and check for overflowing. I can say this, I don't smell raw fuel.
Another thing to check on is your distributor advance weights. If the surfaces aren't good an clean, and springs in good condition, they can have a problem retracting and you may have a high advance situation until shut down. Just trying to help think of all the possibilities. We have thousands of our carbs out in the world, and I'm not aware of this type of issue. I can always be wrong, of course, but I'm also not sure internally what could change after shut down. Other than the possibility of fuel flow not stopping after WOT. My thought on fuel pressure is your pump possibly pushing too much pressure at higher RPM and needle/seat not holding it. You wouldn't notice on hard acceleration, because the engine is taking all the fuel, but coming back to idle or low rpm, fuel pressure could be high enough to push past needle/seat. You could just go WOT all the time??
WOT forever? What a ride that would be! The carb was rebuilt because it was practically impossible to start when cold, then once started, couldn't be driven until it was pretty well warmed up. Once It was up to temp, It had no issues. I'm no pro, but I think excessive fuel pressure at high RPM would eventually clear up and as the excess fuel burned off? I've never just let it run more than a block or two after the issue. But I will and see if it ever clears up. I'll check for overflow and the condition of parts under the cap too. It may take me awhile, the next three days are pretty well booked., and then, well, Monday.
I checked the vac advanced yesterday. It holds vacuum. I was going to purchase a cap and rotor yesterday, but the parts shop I stopped in had the rotor but no cap. I bought nothing there. I'm going to RockAuto now. I'll also mention that the car has a Stinger ignition that I have no experience with. Electronic ignition. Points eliminated. I think i'll throw a timing light on it and check the base timing, get it to act up and then before I restart the car, check the timing again while its stumbling.
Are you sure the air valve is sticking open? With the engine off open the valve with your finger repeatedly. Push it fairly hard against its stop. Did it stick open?
I checked under the cap. The rotor looked good but the center contact in the MSD cap was a bit worn. I've replaced both. The weights and springs look new. No rust or dirt. Gave em a shot of WD-40 anyway.. I also manually, with the engine off, pushed open the secondaries fairly hard lots of times, it does not stick. I then went for a spin. The problem is still there. I'm headed down to the garage soon to check timing at idle. Then, I'll take the car out for a "stress test" get it to act up, and before I shut it off, (which clears the problem) check the timing again.
The Stinger S4 retards the timing continually 1.5-2* per 1000 RPM. What you want is fairly advanced timing in nice and early. Something like 38* in by 3000 RPM, so that by 5500 RPM, it will retard back to 34*. You need to determine exactly what your timing is and when it is all in. Such a simple thing to do, yet so many have a problem doing so, or even understanding ignition in general. Also, I know if you reverse the polarity on an MSD magnetic pick up, the engine will run at very advanced timing. I believe the Stinger is the same. In addition, I have heard of the Stinger boxes going bad. That may or may not be the problem. If you can't figure it out, maybe swap in another ignition. You can trigger an MSD box with the Stinger distributor.
A few days late I know,,,The car was idling fine. I checked the timing mark, and took it out for a spin. Ran great until I put my foot in it. Accelerates hard and long, but when I left off the gas, it starts stumbling as if it wants to die. So I pulled over and checked the timing- it was at the same degree of timing as when as when I started the run, and still stumbling. (I'll thank Staged70 for marking the timing mark on the balancer.) I was going to swap carbs, but I just sold the car the other carb was on..
Very strange. If it is the carburetor, it is either going very lean, or very rich. You need to find a way to substitute another carburetor to rule in or rule out the carburetor.
I'm searching for a known good carb for testing...in the meantime, I'm also searching for a good thread about timing. Base timing? All in? How tight, or loose, should the springs on the weights be? They seem like they may be a little loose. But I dunno.
