Quadrajet and intake flooding

[Patrick H.]

We have tried 3 different fuel pumps on the car (original, used original, new replacement) with the same results. We have checked that the return line to the tank is open, and leaving the gas cap off has no effect. Same result with all 3 pumps.

We have tried 3 Quadrajets on this car (original, new Edelbrock 1901 Q-jet replacement, restored carb from another Corvette) with the same results. In the course of this I meticulously have gone over the original carb several times, the last time installing a new rebuild kit from Cliff Ruggles. No change. I did the same with the other restored carb; no change. I have been playing with Q-jets for 25+ years so none of these were my first time looking inside one.

We discovered yesterday that if you prop open the throttle so that all 4 barrels are open, then you do NOT get gas in the intake. You can spin the starter and engine as much as you want with the throttle open and get no fuel accumulation in the intake. If you then close the throttle, the fuel accumulates immediately.



Any suggestions or discussion would be appreciated at this point.

[Edward J.]

Re: Quadrajet and intake flooding

Pat, when the butterfly is closed on the choke fuel is drawn into engine.The engine is nothing more than a vacuum, give this a try, clean the plugs while the plugs are out crank the engine over a bit to expel the gasoline from cylinders,MAKE sure that the coil wire is of so no stray sparks can ignite the fuel. Then install plugs,then take a screw driver to hold choke butterfly open. Then hold acelorator pedal to floor. Once started run the engine at 2500 RPM,s. Good luck

[Patrick H.]

Re: Quadrajet and intake flooding

We've tried similar actions, but I will try this one.
We've gotten very good at removing and reinstalling spark plugs. Usually if we leave it open to air, the intake dries over the course of 24 hours. We also soak up the gas we can see using paper towels. It's a LOT of gas.

[Patrick H.]

Re: Quadrajet and intake flooding

Ed,

Any idea what is actually happening, or why 3 different carbs would be affected?

[Edward J.]

Re: Quadrajet and intake flooding

My take would be that in the beginning the car flooded severely, and when this happens continuely cranking the engine to start makes things worse, washing the cylinder walls with fuel. Also you can take a look into fuel tank and make sure no water or debris from a bad tank of fuel.I cannot think of anything else.

[Patrick H.]

Re: Quadrajet and intake flooding

We've dried it out several times in between all of these swaps, so it would seem unusual that whatever happened on Day 1 the gas from then - and the subsequent episodes - is long gone. We're talking verifying that everything is dry by inspecting it, then reinstalling the plugs, and instantly having gas in the intake the next attempt at restart. It doesn't take any time at all for this to occur, so it's not like I'm hitting the accelerator 40 times to make this happen. No foot on the accelerator at all many of the attempts.

[Edward J.]

Re: Quadrajet and intake flooding

Pat when your cranking the engine with choke fully closed more than the normal amount of fuel is drawn into engine. And once the engine cylinder walls get washed it make the compression drop because the walls have lost there coating of of oil. That’s why I say to hold pedal to floor while cranking that’s the clear flood mode.starting a flooded. Engine can be a bear once this happens.make sure battery is fully charged and the engine cranks fast.

[Patrick H.]

Re: Quadrajet and intake flooding

It cranks great when there are no spark plugs in the cylinder.

We also drained the oil last night, and will replace that before the next start.

[Mark E.]

Re: Quadrajet and intake flooding

Does the engine fire up, backfire or stumble at all while cranking? Or just constant cranking? With that much fuel, I'd expect some reaction from the engine if the ignition was working.

"It has spark." Did you check for spark at the spark plug end of the cables? If there's spark, check timing while cranking to make sure the distributor didn't shift.

"" I initially thought stuck float or leaking/missing fuel bowl plug. But this happened with three different carbs, so that's a mystery.

Even though you replaced the pump, check fuel pressure. High pressure will overwhelm the needle and cause flooding. This would explain the flooding with different carbs. But even flooded, it's odd the engine shows no tendency to fire up. That takes me back to checking spark.

This isn't a compression issue.

[Patrick H.]

Re: Quadrajet and intake flooding

We have had it "run" for very brief moments of time on about 4 cylinders. However, it does not seem repeatable.
This may happen when you first attempt to start it, but it seems like the barrage of fuel then "extinguishes the flame," or drowns it out.

Yes, checked at the ends with a plug, and also checked with a spark tester.

It is NOT easy to check fuel pressure without a full custom gauge setup, unfortunately. But, that's something we may still end up doing.
And, one would think that if the pump made excess pressure it would do it even when I had the throttle completely open, i.e. it's still making pressure no matter what position the throttle blades are in. The fact that it spills zero fuel when the throttle blades are all wide open leads our thinking away from the pump, but maybe that's just us.

[Mark E.]

Re: Quadrajet and intake flooding

"We have had it "run" for very brief moments of time on about 4 cylinders. However, it does not seem repeatable. This may happen when you first attempt to start it, but it seems like the barrage of fuel then "extinguishes the flame," or drowns it out... Yes, checked at the ends with a plug, and also checked with a spark tester."

Well, this confirms spark. Are you confident about proper timing?

"...one would think that if the pump made excess pressure it would do it even when I had the throttle completely open, i.e. it's still making pressure no matter what position the throttle blades are in. The fact that it spills zero fuel when the throttle blades are all wide open leads our thinking away from the pump, but maybe that's just us."

This is mysterious. Why does it flood only with the throttle closed? The only thing I can think of is fuel being pulled from the idle air bypass (which is below the throttle plates) because of the high vacuum condition. But it's not likely this abnormal condition would happen with three different carbs.

To eliminate fuel pressure as an issue, pinch the fuel inlet hose closed and disconnect it from the pump. Then with a full fuel bowl, crank the engine and see what happens.

[John S.]

[Bob W.]

Re: Quadrajet and intake flooding

John. Good idea or may be some other part of the vacuum system , just a thought he has tried ever thing else, just a thought.


Bob

[John P.]

Re: Quadrajet and intake flooding

Hello Patrick, Older cars that are driven short trips do not allow the engine to develop enough heat to burn off residual gasoline that has bypassed the rings and diluted the oil. Most often they are driven short distances that will not developed enough heat to even open the choke. This causes gas dilution of the oil, reducing its viscosity. This results in lifter bleed down causing issues you have described. Remove and clean all spark plugs and change oil and filter. Crank engine with plugs removed, choke and throttle tied open to purge raw fuel from cylinders. Install plugs, keep choke tied open, and attempt to start holding throttle open till engine starts.. Once the engine starts, allow it to run at about 1000RPM until it is fully warmed up. Again, your problem probably began with diluted oil causing hydraulic lifter bleed down. Recheck proper choke adjustment and operation after engine cools off. One other possibility could be a restricted exhaust including the heat riser. If the engine cannot breath due to valve function or exhaust restriction, it will puddle fuel in the intake as you stated. After the engine has been run and problem corrected, I would highly recommend changing oil and filter again.

[Patrick H.]