I have a 1968 Chevelle SS with the original 396. I am finishing a total restoration and can't seem to get the carburetor to stop flooding. It has a new float, needle and seat, the carb base and top are not warped, all new gaskets. I have a mechanical pump that I measured 6 PSI with a gage. Is 6 PSI too high? Can fuel be leaking down past the piston that actuates the primary metering rods? I am out of ideas.

Forgot to mention that I epoxied the wells beneath the main jets to make sure that there was no leakage.

Hey John, define flooding?!? Is it backfiring? Did you pump the pedal on start up?

After I start the car I can tell that it is running rich and you can smell the excess fuel out of the exhaust. When I shut off the engine a lot of vapor comes out of the carb for quite a while.

After I start the car I can tell that it is running rich and you can smell the excess fuel out of the exhaust. When I shut off the engine a lot of vapor comes out of the carb for quite a while.

Who's to say that the engine is firing on all the cylinders? Or do you have a couple of bent valves?

I would start with a compression/leakdown test, to see if the valves are sealing and compression is adequate. If it is in range for your given engine, I would check plugs while you have them out for signs of misfiring and then trouble shoot those results.

Is the float adjusted correctly? Did you change the fuel filter? Could be a small piece of trash under the needle, I mean small, it don`t take much.

Are you sure the needle is seating properly when the float reaches the shut-off height? If it's flooding as bad as it sounds then you may want to double check the needle and seat and also reverify the float setting.

The float is set correctly. It's probably set a little lower than spec just to help stop the flooding. I am curious about the piston that actuates the primary metering rods. If the ring that gets pushed into tower in the carb doesn't hold securely, can it pop up with spring force and allow fuel to get sucked down from the top? Does the top of the carb hold that ring in place once it is assembled?

Float and a chunk of junk was my first answer. You can check your metering rods with a pocket screwdriver (unless you have a fancy emissions tester). The tube at the air horn on top of the carb. sits right above the primary metering rods. With the engine off, you should feel the spring pressure on the metering rods when pushing down on the metering rods. You should be able to push the rods down with little ease. Start the car and safely do the same thing with the pocket screwdriver. The needles should be all the way down at idle. They should not move until you rev the motor. Are you sure that the spring is the correct spring? I've seen a lot of stretched ball point pen springs on previously rebuilt Q-Jets. Easiest part of the carb. to loose when apart. Good luck.;)

John,

The primary metering rods, power piston, and primary jets have no influence on the idle circuit. When the rod tips are all the way down into the jets fuel still gets through the jets because the rod tips are not the same diameter as the jets. In fact, the idle circuit is fed fuel through the primary jets. If the rods seated completely into the jets the idle circuit would be closed off.

So, while it's correct that the primary rods should be down at idle, they don't have anything to do with the carb flooding or running rich at idle. It's certainly possible that you have a bad float.

Have you adjusted the idle mixture properly? At what RPM is the engine idling? What's the vacuum at idle? What's the timing set at?

First thing I'd do is get the engine warmed up and idling then, with a flashlight, look into the primary bores to see if you have any fuel dribbling from the primary circuit or from the booster holes with the engine idling.

Dan

I found something interesting. I also have a 1968 Firebird 400 and I measured the fuel pressure from the fuel pump on it. It fluctuates between about 4 and 6 PSI and immediately goes to zero when the engine is trrned off. On my Chevelle I measured 6 PSI of pressure during cranking only, so it is probably going higher with higher rpm. But the most interesting thing is that the pressure does not go to zero when you stop cranking the engine. It will stay at 6 psi indefinitely. This is very interesting that the Chevelle fuel pump must have a check valve in it. Has anyone noticed this before? This mechanical fuel pump is probably going higher than 6 psi and then holds it there even with the engine off. The needle and seat can't handle it and the carb floods when the engine is turned off. This pump is brand new from NPD. The carb has everything replaced in it, twice.

I would think that if the pressure gauge holds at 6 psi with the engine off that the needle and seat are OK. If they were leaking the pressure wouldn't stay at 6 it would fall.