We have two Camaros with carburetors on them. I have a 84 with a mild 350 fitted with a 3310 Holley and Dad has an original 86 IROC with 305/Qjet. Both cars, after being driven long enough to reach operating temperature, will be flooded when trying to restart them. My 84 even has a lift off cowl induction hood that I have not yet sealed to the cowl, so there is plenty of room for heat to escape. I was wondering if anyone had any advice for us on this, we've never had this kind of problem from carbureted vehicles before.

 

Have you removed the vapor canister?

 

No, both cars still have it intact and properly attached. The IROC is completely factory, nothing but the muffler and tail pipes changed. My car is as close to "compliance" as you can be without having the SMOG stuff on it.

 

You say they flood after having been run. How do you know that?

Have you removed the gas cap after shutting it off to see if there is pressure on the tank?

 

After shutting it off, the throttle plates will be wet from fuel pushing over through the boosters. Also, when you start either of them, you have to hold (just hold, not pump) the accelerator petal to the floor and you get a cloud of black smoke.

Yes, we have pulled the gas caps off both vehicles after being run. Stopping to buy gas gave us the first hint of what might be causing it, but we soon began doing it to test and see what was happening.

 

What kind of fuel pumps are you using? Maybe a bad float or dirt in the needle/seat

 

Check the float levels. With the q-jet, the float can take on gasoline over time and require a higher fuel level to shut off the fuel flow.

With too high float levels, you can have the fuel spill over with boiling that can occur when the engine is shut off.

 

If when you remove the gas cap there is a rush of air, then you either have a bad vapor canister or the purge valve isn't working or it's not getting vac to allow it to operate.

 

I assume you pulled the gas caps as soon as it was shut down to rule out overpressure from the tank forcing fuel past the needle. The tank is going to vent when the gas cap is removed even with everything functioning properly. The purge vent should only open when a pressure limit is reached. You should have covered this failure possibility with the gas cap loose.

A common problem with the older q-jets is the well plugs on the bottom of the main body leaking fuel out of the bowls into the manifold. Don't think that applies here.

Both vehicles likely have the engine mounted mechanical pump with return line? The fuel in this pump and line can boil out when heat soaked, especially if the return line is restricted. The pump should move a significant amount back through the return line while the engine is running (you should be able to feel it pulsating), which will also keep it cool. I'd check here.

 

The IROC has the factory mechanical fuel pump and my 84 has the Holley "Blue" electric pump mounted under the tank with a return line style regulator mounted in the engine bay.

We know the Q-Jet doesn't leak. The purge vent of which you speak is the little piece under the rear of the car near where the brake/fuel lines end behind the back seats, correct? I know the return line in my car is not stopped up or restricted because it will allow me to drop down to the minimum 4psi my regulator is capable of when the pump is running and the engine not.

We're fairly good with carburetors. My Dad was a drag racer and GM certified mechanic in the 70's and 80's and I grew up helping him. Rebuild my first Q-Jet at 12 and now build them for off-roaders for extreme angles. I was mainly wondering about the purge system and if this was a common problem as we have 2 cars, with different carburetors, that have the same problem. I know my 89 TBI car also has a "woosh" of air when you take the gas cap off.

 

Yes that's the purge valve.

I wouldn't consider purge valve failure common but it does seem to crop up now and then.

I thought, however, that you had tested these by keeping the gas cap loose between a hot shut down and a later attempt at re-start? This would definitively rule out the purge valve because any excess pressure would vent through the loose cap.

Keep in mind that the purge valve only opens when pressure exceeds the amount that can be vented to the charcoal canister. If you find a problem I would thoroughly check both systems.

There is little else similar between the two with different fuel pumps, other than fuel/return line routing and the tank vent systems.