Here's what I know: I don't know squat about carburetors....... Small block chevy. 4 barrel Quadrejet. Runs great going down the road. Idles great in park, although occasionally will change RPM on its own. Intermittenly stalls when idling in gear. Almost constantly sounds like she's ready to stall while in gear and stopped. At idle in park, I can see a small amount of gas spilling down the venturis which I believe is not correct. I've pulled the carb a number of times now and readjusted the float thinking that the fuel in the venturis means the float level is too high. Each time I've forced the float level down a little bit to where I should be at 1/4" and I'm now more like 3/8" and still dribbling gas down the venturis at idle. Spraying starter fluid at the outside of the carb does make the idle increase. I have new gaskets all the way around and cannot find any vacuum leaks at the vacuum hoses or connections. Any suggestions on a plan of attack to pinpoint my problem area(s)? thanks
Stock cam or something a little hotter? What kind of manifold vacuum will it hold at idle. If spraying carb cleaner around it picks up the idle that would indicate that it's still lean, even with fuel dripping from the main nozzles. Gaskets could be OK but the throttle shafts might be worn that badly, or the secondary throttle plates may not be closing fully. Does it respond to adjustment of the idle mixture screws?
Sounds like too much fuel pressure over riding the needle and seat. Usually if you spray starting fluid around, some of it gets pulled into the airstream of the carb and affects the idle speed. Something less volatile is better for checking for vacuum leaks. You are correct, no fuel should be leaking out the venturas at idle.
Many earlier Q jets had floats that got heavy with gas saturation. A new float is available for cheap. I'd start there.
Idle screws have no effect on the idle. All the way in or all the way out, there is no change in RPM. She holds a pretty steady vacuum of 19 / 20 while idling in park. I don't know what's in it for a cam. It's a crate motor that the previous owner put in 8 or so years ago. Supposed to have 40,000 miles on it. I do know that the previous owner just replaced the fuel pump (mechanical pump) just before I bought it. What should the fuel pressure be for this carb?
If you see fuel dripping it may be the main wells leaking. Take the carb off a look at the bottom, Don't flip it over just look up and see if there is fuel on the gasket next to the protruding wells in the oval hole. Leaking wells will cause erratic idle and your idle screws will not respond. Fix it by using epoxy on the lead plugs.
QJets are notorious for worn front shaft area that creates major vacuum leaks with idle and low speed headaches. The cure is installing bronze bushings with a special tool. I agree with the fuel pressure recommends...good luck. Frank
I've rebuilt a few in the past.. The only problem I was having with them was re-attaching the choke lever... Man, those things drove me nutz!!
It's surging as it trys to idle too, right? That suggests the idle speed screw is set too high and it's trying to idle on the midrange circuits above the idle circuit. Screw the idle screws in all the way then back them out about 3 turns then start it and screw the idle speed screw out (lower rpm) until it starts idleing on the idle circuit. Then adjust the mixture screws as needed. Shoud idle around 650±RPM (in gear if automatic) Check to see if the (7?) screws holding the top of the carb on are tight too. If they are loose it can have an internal vacuum leak affecting a lot of things including idle.
The very first thing to do is determine exactly WHICH quadrajet you have! Without this information, answers that are spot on correct for some models are totally wrong for others. If your vacuum gauge is correct, and you have 19~20 inches of vacuum at idle, I would suspect the carburetor float. As others have posted, the later Q-jets have a closed-cellular poly nitrophyll float that will, over time, have its seal broken down by modern fuel. The fuel will then permeate the foam, and cause the float to be heavy. While special scales are available for weighing these floats, we don't use them. Any Q-jet with a foam float will automatically get a new float if we redo the carb. The earlier carbs had brass floats. Do not think you can automatically change to one of the earlier brass floats and eliminate float issues; as several different float designs have been used, and many are NOT interchangeable. One other possible problem could be an incorrect fuel valve (float valve), as the cheap generic FLAPS kits contain a "one size fits all, and works for none" fuel valve. Even with a good float, one of the high volume fuel valves will give fits except under race conditions. Many different orifice sizes for the fuel valves. A proper street carb with correct internal parts will easily take 6 psi; although 4 psi is generally enough for street use. Jon.
So I threw a pressure gauge in line and she's reading 8 PSI. Someone above said that too high of a pressure could force fuel past the needle and seat and into the venturi's. Do you guys think that 8 PSI could cause that condition?
Yes!!!!.... Get a regulator for it and set it for 4 psi!! May as well get a new needle and seat for the carb because the old one is wasted!
Quadrajets are very good carbs and easy to screw up if you don't know what you're doing. Some of the rebuild kits have a plug that fits into the well to stop leakage but sealing the plugs works better. You can distort the body by overtightening the wing nut for the air cleaner.Bushing kits are available to repair the wear around the throttle shafts. 4 PSI is plenty for fuel pressure.
The two screws on the base aren't idle speed, they are idle mixture. The idle speed is a different screw. On mine the idle speed is on the side near the throttle linkage stuff.
So I've been tinkering and tweaking and have gotten her to settle down quite a bit. I did have the idle settings all goofy, and those seem to be better now. Occasionaly and intermittently, she'll run rough at idle and stall unless I keep gas pedal down a bit. The newest sympton is when she's hot and I shut her off and pop the hood, I noticed gas dripping from behind the screw where the pencil is pointing to in this picture. Note the black area on the intake where the gas is boiling. Is this indicative of worn butterfly shaft bearings and would installing the bushings in a good rebuild kit fix this?
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