Quadrajet and intake flooding

[Patrick H.]

Re: Quadrajet and intake flooding

Ken-------


Yes, that's a very common problem with Q-Jets. However, in this case, when the carb was installed on another engine there was no problem. Also, a different Q-Jet was previously installed on the engine in question and the same problem occurred. So, all this pretty much rules out the carb as the source of the problem.

And, any of the carbs can sit on the engine for any length of time, and the intake is dry.
In addition, they've all had the plugs epoxied already.

[John P.]

Re: Quadrajet and intake flooding

Hello Patrick. The interesting thing is the fact the engine did not attempt to start when the choke was partially open. Even when the choke was closed and fuel added, the engine did not want to start. I know you have checked for spark, however is the spark taking place at the correct time? Remove the distributor cap and turn the crank. Make sure the rotor is turning and then align the timing mark on the balancer with TDC on the indicator on the compression stroke and see if the rotor is pointing to #1 cylinder. Something could have happened to the distributor drive gear, shaft or advance plate. Attempt to start the engine with the choke tied open using the accelerator pump to supply fuel. Does the engine attempt to start? The pictures of the fuel in the intake does not look out of line for a fully choked engine. The choke allows additional fuel to be drawn from the idle circuit by using engine vacuum to draw the additional fuel, enriching the mixture to enable better cold start. Again, eliminate the choke from the problem and try starting the engine. Also, what were your compression readings?

[Mark E.]

Re: Quadrajet and intake flooding

Well Patrick, your troubleshooting at least eliminated several causes, including the carb itself and my favorite by John Pelkofer that the vapor canister was full of gas.

I'd still like you to eliminate fuel pressure as the possible cause. Pinch and remove the supply hose to the pump, fill the fuel bowl, start the engine and see what happens.

[John S.]

Re: Quadrajet and intake flooding

Patrick, I go back to my original comment regarding a possible problem with the PCV system that could cause an excess vacuum condition resulting in the fuel to be pulled from the carb as you described.

The most common PCV failure is a valve that is stuck open allowing excess air into the system causing a lean condition. What you may have could be an extreme case of the opposite, a clogged or closed system causing an extreme vacuum and rich condition.

Since you have another ‘72 that runs fine with the carb swapped, I would swap as much of the PCV system (PCV valve, hoses etc.) from that car to the car in question to see if there is any improvement. If anything you can eliminate one more possible cause to the problem.

Good luck !!

[Patrick H.]

Re: Quadrajet and intake flooding

We had the PCV system disconnected during all of the tests yesterday on the trouble car.
We had the PCV connected previously.
I tested the system (see start of yesterday’s post), and it flows air to the carb readily, whereas the EEC line does not. The EEC line is much smaller, though I could test another car and see how much it does or does not “flow” in comparison.
If you can think of another test for the PCV system let me know. I might be able to swap PCV valves, but given that the lines are taped together it is difficult to swap entire systems. I can look at it in a few days. Having “less” air from the PCV system resulting in “more” having to come through the choke is about as logical as anything else I’ve yet heard.

Any information I can find about a cam failing doesn’t mention anything about increased vacuum.

The photos show the 1/4 inch of gas that is ABOVE the ridges in the bottom, if you look through the front left port. That reflection? It’s the puddle of gas. Gas comes out of #2 plug when you remove it. Note the puddle extends more than halfway down the photo of the lower left port, covering the ridges. This is not a “normal” choked engine. I can use 3 paper towels to soak up the gas laying in the bottom of the left side of the intake. Yes, the right side of the intake looks much more normal, though it too has more gas in it than the other car did after testing this carb.

Ignition system is working fine. That was the first system I went through when the car would not start 2 months ago.

Continued thoughts appreciated.

[Joe L.]

Re: Quadrajet and intake flooding

We had the PCV system disconnected during all of the tests yesterday on the trouble car.
We had the PCV connected previously.
I tested the system (see start of yesterday’s post), and it flows air to the carb readily, whereas the EEC line does not. The EEC line is much smaller, though I could test another car and see how much it does or does not “flow” in comparison.
If you can think of another test for the PCV system let me know. I might be able to swap PCV valves, but given that the lines are taped together it is difficult to swap entire systems. I can look at it in a few days. Having “less” air from the PCV system resulting in “more” having to come through the choke is about as logical as anything else I’ve yet heard.

Any information I can find about a cam failing doesn’t mention anything about increased vacuum.

The photos show the 1/4 inch of gas that is ABOVE the ridges in the bottom, if you look through the front left port. That reflection? It’s the puddle of gas. Gas comes out of #2 plug when you remove it. Note the puddle extends more than halfway down the photo of the lower left port, covering the ridges. This is not a “normal” choked engine. I can use 3 paper towels to soak up the gas laying in the bottom of the left side of the intake. Yes, the right side of the intake looks much more normal, though it too has more gas in it than the other car did after testing this carb.

Ignition system is working fine. That was the first system I went through when the car would not start 2 months ago.

Continued thoughts appreciated.

Patrick------


So many things have essentially been eliminated as the root cause of the problem I think we're down to considering the obscure and unlikely. Another thought is this: I know you've swapped out the fuel pumps without curing the problem. However, all 1970-81 Corvette fuel pumps have a fuel return provision. Have you checked to be sure that the fuel return line to the tank is clear of obstruction?

[Patrick H.]

[Tom L.]

Re: Quadrajet and intake flooding

Patrick, Your condenser comment rang a bell. When I was a teenager on mom and dad's boat running up the Hudson river we had a similar issue, engine started loading up, would barely run, raw fuel out the exhaust, running VERY rich.

Dad always had spare parts on the boat and on this trip we had a mechanic on board. They, like you went through everything, rebuilt the carb, swapped the carb from the port engine to starboard, changed points, wires, you name it. Just like your experience. just because it was that "last item that surely couldn't be the problem" they changed the condenser. Varooom! I try to never forget what I was once told by an old timer, "Most carburetor problems are electrical". Engine ran up to Albany and back to long island not missing a beat.

As sort of a joke, and for the memory of that day, Mom decided to save the condenser and use it as a Christmas tree decoration. Wish that memory was triggered sooner. Good luck!!!

[Patrick H.]

Re: Quadrajet and intake flooding

Thanks Tom.

Thankfully I have a good selection of parts here so I'll try the condenser too.
Next update may be a few days since I have to bring my daughter and horse away to college this weekend.

Any other comments are still welcome.

[Mark E.]

Re: Quadrajet and intake flooding

Any other comments are still welcome.

See my prior post. It's not clear fuel pressure is eliminated as the cause.

[Patrick H.]

Re: Quadrajet and intake flooding

See my prior post. It's not clear fuel pressure is eliminated as the cause.

3 fuel pumps later -> no change

multiple attempts at starting Wednesday with no choke -> no fuel spill. This suggests (proves?) to my thinking that there is not excessive fuel pressure blowing the needle off the seat. Or, why would there be a fuel pressure issue only with the choke fully closed?
Last week as well, multiple starting attempts with the throttle open had no overflow.

attempt at starting with fuel line disconnected -> no fuel spill


Realize that any test that causes the fuel spill/gush eliminates all testing for the rest of the day, as the intake then has to dry out.
So, how would fuel pressure from 3 pumps plus the current one point to a fuel pressure issue?

Just trying to follow the line of thought as any test that I think "might" cause a fuel spill gets done as "the last one."

[Mark E.]

Re: Quadrajet and intake flooding

Is this what you've tested?

1) Fuel Pressure + No Choke = No flooding
2) Fuel Pressure + Choke = Flooding

3) No Fuel Press + No Choke = No Flooding
4) No Fuel Press + Choke = ??

I understand concluding fuel pressure isn't the problem because of #1. I'm suggesting verify this by running #3 (again) and #4.

If no flooding with #3 and #4, it suggests fuel supply is at least an intermittent problem. Why it would only happen with the choke closed, I don't know...

[John S.]

Re: Quadrajet and intake flooding

Patrick, one thought just came to mind regarding the PCV system. On your next starting attempt you may want remove the main PCV hose that connects to the front of the carb base and leave it open.

It should create a good size vacuum leak and the engine probably will not run, but if fuel does not appear in the intake when the throttle plates and choke are closed while cranking, it may confirm that vacuum is pulling fuel in from the carb. If the hose is disconnected and the fuel still appears it may indeed indicate a fuel pump/pressure problem.

Good luck !!

[David H.]

Re: Quadrajet and intake flooding

Patrick

When this car initially broke down, what happened? Running fine, then sudden quit like someone switched it off? Running fine, then chugging to a stop? Any details that might help identify car's initial failure?

Dave

[William F.]

Re: Quadrajet and intake flooding

You SURE you're getting full 12V to coil when key is in start position?