My son is restoring a '62 Olds Starfire with a 394 c.i. v-8, which has a Rochester Quadrajet carburetor. The engine is virtually brand new, with new plugs, wires, rotor, points, distributor cap, and all internals. I bought a kit for the carburetor and installed it in the Quadrajet. The engine now has a low rpm "cough" type miss, sometimes more pronounced, like a huge flat spot where it seems like the engine is starving for fuel. This occurs under light normal acceleration. We have the factory service manuals for the car, but this type of problem isn't addressed anywhere in them. We did have an aftermarket electronic ignition module in it, which we took out. We re-installed new points and condenser, with no effect at all. We also installed a new coil, with no effect. It seems like an ignition type miss, but I am beginning to think it must be something I did wrong in the carb kit installation.

Any ideas where to begin?

Thanks!

 

Did you SOAK the carb before installing the new rebuild parts????........Sounds like you have a restriction in the accelerator pump circuit

 

A "miss" and a "flat spot" are quite different. A miss would affect usually one cylinder. At least that is what most people have when they talk about a miss. (like from a bad plug, bad wire, carbon tracking on distributor cap, etc.) But a flat spot can actually cause you to lean forward while driving because instead of you being pushed back in your seat by the power of the V-8, you anticipate that, and instead there is not enough gas per Wrench's accelerator pump theory, and it's more like the engine starves out momentarily. You should be able to go out to your carb and throttle it by the linkage under the hood while you look down into the venturi with the engine off and see if a healthy burst of fuel comes out into the venturi, the way it should if the accelerator pump is working. Let us know what you find out. If that is the trouble or not so we can guess at other stuff.

But if you have a miss going on all the time while low rpm cruising, that would not be it. Because, the accelerator pump is literally a piston plunger that only squirts out gas when that plunger is activated during acceleration.

Have you repulled plugs to see if you can see any signs if it is picking on one particular cylinder?


Is your engine timing correct and have you verified that your spark advance curve is correct?, - as engine vacuum is higher when engine is not under load, such as conditions you mention. What happens if you accelerate (while driving it) and then let off the gas pedal all the way to listen to the engine while it is decelerating under heavy vacuum? Does it sound like popcorn going off, or does it behave fine then? Have you made sure that your vacuum line from the distributor to the vaccum advance is hooked up to manifold vacuum?

 

how about buy a holley or edelbrock carb? I never had anything but trouble from Quads. the edelbrocks are alot like the old carter afb carbs, well they are afb's I think. holley make's a nice streetavenger carb thats more or less plug and play. if your engine is stock a 570 should be enough cfm or for more performance oreinted engines a 670 cfm. or a basic 600 holley would suffice. That is if it is a carb problem. Is it always at the same point during acceleration or is it just a whenever type of miss. Could be a plug wire breaking down or arcing off of something. Go out at night in the dark and watch the engine to see if you can catch a spark from the wires. What I call a dead miss has always been an ignition problem. Being it plugs, wires, cap, rotor, coil, or whatever. if its a miss that happens at a specific point during acceleration or normal highway speed , sometimes that can be a real bear to run down.

 

if I remember my Q-jets, there is an adjustment available for the accel pump. You may have it set for too short of stroke.

and grumpy, don;t go knocking Q-jets. there are many people that spent countless hours making those things work right. I believe a few of them actually succeeded.

the other thing I just remembered. Vacuum secondaries, correct? If the linkage is improperly set, it will allow the secondaries to open up prematuraly( before enough vacuum) and cause a stumble or the diaphram unit does not cause the secondaries to be or remain closed altogether.

To all others; be gentle with me; it's been a long time since I worked on a Q-jet.

 

What nap said, Q-jets were matched to the engine size, and adjusted to the engine. A manual on Q-jets would be helpful, if it has tuning tips. But it does sounds like accelerator pump tweeking is in order.

 

hey NAP, That was my personal experience with quads, if u like um, thats cool. But u just said it yourself, people spent countless hrs. trying to tune them. Its kind of like anything else. If u get a good one, u might never have a problem out of it, but if u get a bad one, u will never get it right. But hey, its all good man. Personal choice, aint it great?

 

grumpy.......that was sarcasm. ya' know, that line where I said " a few of them actually succeeded" should have been a hint, but that's ok.

actually they were a good carb if you could tune them but that is where the problem came in, the guys that could were far and few between.

 

Like Grumpy says deep six the Q Jet unless you are set on keeping it original. If you are God help you. I got to where I wouldn't work on them cause you were sure of a come back. Much like the flat top used in the 50s. RW