I recently replaced my quadrajet with a rebuilt one from a car parts store. I didn't mess with the air/fuel mixture as the insructions state you should not have to. I adjusted the throttle to about 800rpm in park. Timing is around 12 degrees. Engine is stock. My spring choke is shot so I completely removed it. The car jerks back and forth really bad at low throttle. Its really bad when its cold. The problem is less once the car is hot, but still there. When I step hard on the gas, there's no problem...takes right off. Its hard to keep the car at a steady speed though because of the jerking at low throttle. Will a new choke fix this? The car did not do this with the old carb.

Thanks

You do need the choke when cold. You also need to set the idle mixture. Before you do though, check your timing. Being too far advanced will cause the symptom you describe. So the order would be timing, followed by mixture followed by idle speed. This is normal tune-up stuff.

Concerning the choke...I know the choke closes up the intake when the car is cold, but does it do anything when your driving? For example. If I'm driving at 50mph in 50 deg weather. Will the choke close slightly due to the cool air cooling the engine off? Maybe my thermostat is bad and the car is running too cold?

Once the engine is warm, the choke will remain open if adjusted correctly. The heads and block hold enough heat at that point to permit correct combustion. The exhaust crossover keeps the choke stove hot (and choke open) even if you're running at 150 degrees. If your choke spring is the one that fits into the intake and has a cover over it, they are VERY cheap and available from all of the repop houses. Good investment.

Would the idle air/fuel mixture out of adjustment cause issues at low throttle? When I speak of low throttle, I mean just the amount required to keep the car moving steady at 30-50mph. The car seems to idle fine.

I had these symptoms recently, and it was, no question, that the timing was advanced too far. If the engine is stock, shouldn't the timing be 8 or less? Check the timing, then adjust the idle. (D1-bradley just said this didn't he?)

Update... I dropped it down to 6deg and it ran bad, it was bogging off the line. I upped it to 8deg and it seems to run OK, but it still exhibits the same problem. I think it still maybe a timing issue though as I recently had a friend adjust my aftermarket HEI. Now I'm wondering if its set wrong. He had adjusted it on a machine and came up with these number for mechanical advance:

600rpm 0 deg
800rpm 1 deg
1400rpm 8 deg
2000rpm 13 deg
2600rpm 16 deg
3000+rpm 17 deg

What do you guys think of these mech advance numbers? I'm now wondering if the problem is at an rpm where I still have vacuum advance and the mechanical is kicking in causing too much total advance? Thanks for the help.

Update II...I removed the Vacuum advance hose from the distributer, plugged it and... Problem Gone! So whats causing the excessive mid rpm advance? Excessive mechanical or vacuum advance? See my mech advance curve on previous posting. Thanks!

I don't really know what your friend did on his machine, but I'd say that the next step is still to adjust the idle mixtures. (Leave the timing at 8deg)

I also have an aftermarket HEI and if I recall there are special advance cams that can be put on distributors to "accelerate" timing advancement. Maybe yours has one of these.

You also might have a vacuum leak. Re-check your PCV valve and hose in case they got knocked loose a little while working on the carb.

What he did was change out the springs and set the advance limit. Originally, when I got the car, it was only mechanically advancing to 6 deg at 3000rpms. He increased that to 17deg. I cannot adjust the vacuum advance on mine but I've heard of adapters you can put on to make these HEI's vacuum advances adjustable. I'll probably look into one of those. Does anyone think 17 degrees mech advance is too high (for stock 350)? This would put me at a total of 25 degrees at WOT. I checked for vacuum leaks. I didn't find any. I had put a vacuum gauge on the ported vacuum output of the carb and I don't recall the reading, but there was definitely vacuum when I increased the throttle.

Thanks

Pretty soon you're gonna have to get someone else to give you advice, I'm just doing the best I can. :confused: However, I'm sure that if you had overadvanced your car, the most obvious symptom would be detonation (loud knocking) at higher RPMS. If your car doesn't knock at high RPMs it's probably not advanced too far. Try the idle mixtures, and make sure you got the floats adjusted right.

this is what we tell our customers for the crate engines..... 10 degrees initial timing, 34 degrees total advance at WOT and make sure the dist is not 1 tooth off. when someone calls saying the car runs rough, we ask if they adjusted the carb (after the timing question).... when they say the carb doesn't need adjustment, we just ask them.... is the air the same in Denver as it is in New Jersey?...... we usually do not get a call back so it must fix their problem.