Thread: carb leak question

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My engine has been sitting for a couple months and when I uncovered it I saw old looking gas drip from point as arrow indicates. There was a ooze of old gas on the manifold.
Why would gas drip from this point ? bad gasket I assume.
Can I buy just buy this gasket/diaphragm alone ?

Carb = Holley 390

Navy, my guess is that ethanol in your fuel ate up the rubber diaphragm, and likely anything else rubber in the system. I can't answer definitively on buying the diaphragm alone, but I'd think you'd be looking at a carb kit for it.

That is the vacuum pot for the secondaries, and that is the atmospheric side (non vacuum). Are you sure it fuel and not condensation? Remove the four screw on top pull the cover and look at the top side of the diaphragm, you should be able to see damage if it is bad. But again, are you sure it fuel, shouldn't be any fuel at this diaphragm.

Oh crap, correction!
Just realized you have the top of the carb sealed, this prevents the carb from venting causing fuel vapors to pressurize forcing vapors into areas they would not be normally. I believe Roger has nailed this one.

I noticed the top of the carb is sealed with plastic (thus the oh crap correction). The plastic is covering the float vents, when the temperature rises the fuel in the bowls expands and vent vapors that can no longer escape to atmosphere are forced into vacuum and other passages not intended for fuel. As the temperatures cool the fuel vapors condense to liquid again, deteriorating parts and plugging small vacuum and air passages. Most likely this carb will require complete disassembly, boil out and rebuild.
I believe Roger caught this first.

Thanks 36 sedan, but you're analysis went deeper than my thought process when I suggested the problem. I've just seen so much damage from ethanol in systems that I've come to really dislike the whole idea of mandated gasoline additives, and it's the first thing that comes to mind when someone says that they have had fuel sitting in a vehicle for an extended period.

No disrespect intended, but the secondaries vacuum pot pulls vacuum from above throttle blades through the main body. I may be wrong, but I can't see any other way for fuel to get to that diaphragm?
JMHO, at the very least I would blow all the passages out and rebuild the carb. However, often when fuel condenses back from vapor all the LP is gone leaving the heavier resins behind that solidify and plug passages, especially small passages. Failure to remove the resin (unplug passages) by boiling the carb (chemical soak), is the most common reason for unsuccessful carb rebuilds.

I agree and understand how the vacuum secondaries operate. The bottom side of the pot is atmospheric (open to air), so you're correct the fuel has to get through the diaphragm to come out at the shaft. This is why questioned it being fuel and possibly water condensation instead, until I saw the plastic cover over the carb horn sealing off the carb from venting any gas trapping it in the manifold system. This is the only way I can see the fuel entering the vacuum port leading to pot, once the diaphragm deteriorates allowing the fuel to vent, even more fuel will be pushed in due to the lower atmospheric pressure.
All do respect, I too am only trying to help.

Might not be fuel-he needs to check the drip to see if it matches charictris with gasolene-----

Match Check-----a type of test where you hold a lit match in the area of a drip to see better in case its a flamable liquid

Careful Jerry, not everyone has sick humor. Lol!

Well after the fire dept. left .........

I had run the carb (ran the engine til it stopped) out of gas when I pulled the engine, but I'm sure there had to be some gas left. The carb had just been rebuild at the beginning of this build but it's been 1.5 years. Every time I ran the engine I always ran it dry. I'm putting it back in the truck for good now that I've completed the suspension. I'll see what happens when I add gas . THANKS for ALL the inputs !

Originally Posted by Navy7797

My engine has been sitting for a couple months and when I uncovered it I saw old looking gas drip from point as arrow indicates. There was a ooze of old gas on the manifold.
Why would gas drip from this point ? bad gasket I assume.
Can I buy just buy this gasket/diaphragm alone ?

Carb = Holley 390

UPDATE I reinstalled the engine and ran it ran fine next day I checked to see if there was any gas drippings. There was a drop on the phillips head screw that is screwed it to the shaft of the secondaries
butterfly plates, just below the red arrow in the picture. It came through by the shaft. Any idea why ?

Yeh
Seconary float level high? any way--if your needle / seat assy won't hold the fuel pressure(do you have a pressure guage?) holleys have some various size seats and the bigger ones are diffently pres sensitive, if the bowl vents over it sets on top of the throlle plates/shaft and leaks out the shaft end-will form a drip on the screw every time.

want to verify it????? after you shut down the engine, drain the float bowls by pulling a corner screw and drian into dixie cup-no smoking, and engine will be hot so careful and know where fire ext are-------

Thanks Jerry, I'll try this tomorrow night and see what happens.

I don't have a pressure gauge, but I'm only running a Mr. Gasket Pump and its only suppose to put out 3-7 lbs. Heck I'm not even sure what maximum should be ?