Quadrojet and fuel pump vent

I have a problem that I cannot solve and the workshop manual did not help me either.
1981 Crusader 270 engine. I just bought this boat and on the port side engine I had a carburetor problem (Rochester Quadrojet). I decided to replace it with a remanufactured one for $ 349 (exactly the same, but with a small difference, read on). I installed it, I also have a new gasket under the carburetor, new gaskets under the intake manifold. But when I connect the vent pipe from the fuel pump to the carburetor, the engine runs for 10-15 seconds and stops.


My old carb didn’t have a fitting for this tube and the clear tube just lay on the intake manifold. Now the new carburetor has this fitting, but with the tube connected, the engine cannot work for a long time. In general the engine runs fine at all RPMs but only for 10-15 seconds, then it just stops. The ventilation pipe is clean, no fuel inside. The fitting on the carburetor (as far as I understand) goes straight to the intake manifold because when the engine stops and the tube is disconnected from the carburetor there is a little exhaust smoke coming out of this fitting. Also, this tube has a strong vacuum when the engine is running.


If the tube is disconnected - the engine continues to work perfectly at all RPMs, yesterday we checked this and were at sea for 1 hour. All perfectly.


I have a suspicion that the fuel pump needs to be replaced but I'm not sure about that. I decided to ask you. I would be grateful for your help.

The tube from the mechanical fuel pump is for “diaphragm burst protection“.

This tube does not connect to any vacuum port on the carburetor.
It will connect to an area that would allow fuel from a diaphragm rupture to enter the upper area of the primary throttle bores.


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I would say you need to explore your theory a bit further and see if the fuel pump is delivering fuel with the sight tube disconnected from the carb...

Being as you stated the original (to you) carb didn't have the fitting for the sight tube, I'd assume the carb wasn't equipped with the matching bore and was the wrong carb.

You mentioned the sight tube fitting on the carb has "strong vacuum" when running but didn't state the RPM where this was observed - I'd suspect this was NOT at idle...

It sounds like neither of the carbs you had is set up like a regular marine Quadrajet, on these the safety tygon hose from the fuel pump connects to a fitting that is on the left side of the carb as you face it, just to the left of the fuel inlet fitting. With the air horn (top cover) off it looks like it leads to the primary bore on that side, not a manifold vacuum fitting source. The way you had it hooked up it might be creating a vacuum leak.

What is the setup on the starbd side engine? Does it have the fuel pump to carb tygon hose?

click to enlarge

Yep that’s how mine is....

That is a dead end....just a chamber where the choke rod is located. It will be open at the top.....this is not a good place to direct fuel from a ruptured fuel pump diaphragm. If a hole has been drilled to evacuate this chamber into the secondaries, then fuel will spill there instead of out the top.....vacuum would be relieved from the fuel pump to some degree alongside the choke rod, but no vacuum would exist if the secondaries are not in operation. The safest place to dump the fuel pump would be back to the fuel tank or outside the boat. I have rebuilt hundreds of Q-jets. The carb pictured in post #6 is typical of the 60's and early 70's.....it's quite obvious that fitting was "added".

I'd suggest you take another look....

Here's mine....

as you can see on the Marine version it doesn't dead end into the choke linkage area but winds up on the starboard side primary venturi...

Excellent....my comments were based on the "cheesy" fitting that was clobbered into the carb. depicted in post 6. I'd love to see that center carb section with the air horn/top removed. Furthermore who wants to direct raw gas directly into the intake manifold in case of diaphragm rupture? I realize it's acceptable engineering for many of you, but for me, it is inacceptable, unsafe, and unwise......spill off should go back to the fuel tank and any fuel in this line should activate a warning.

what you labeled as 'cheesy' is Rochester OEM, and was used "as is" by all of the OEM marine engine builders that used a quadrajet on their GM production engines....and the airhorn looks like the pic louc posted...

Beg to differ with you, but that carb in post 6 has been "modified". I have rebuilt hundreds of these and still have 20 or 30 on the shelf, not counting what's still on motors around here. Rochester would not drill and tap a barbed fitting like that. I demand to see the chamber behind that fitting on this carb pictured....or I will not back down.

That how they come from mercruiser

Okay, so then the carb is "modified". The photo in post 10 is a Rochester original.....post #6 carb is not. I will bet that the post #6 "Mercruiser" mod DOES NOT contain the solid passage to the starboard primary bore...so where will the fuel go???? That is not a fitting used by Rochester, where did that photo come from? Has anyone here seen how much fuel can pass through a mechanical fuel pump diaphragm that ruptures.....particularly because of storage having ethanol fuel?

I have witnessed what can happen.....not only engine fire, but internal engine damage. If this tube is simply laid on top of the engine, then I hope you have good insurance....and/or quick reflexes.

I’ve never had the fuel pump diaphragm leak but from what I have read the way this is supposed to work is if the pump diaphragm ruptures thecfuel is drawn into the engine and the excess fuel causes rough running so the operator knows something is definitely wrong.
However, what I did find was this, I took off my old fuel pump because it was so corroded and also I found something in the tube (not fuel but old congealed oil) I took the pump apart and found that the port in the pump that the tube attaches tube all clogged up. So if the diaphragm leaked the fuel would have no where to go. Fortunately original Carter fuel pumps are still available for these.