Fuel Leak After Warm Shut Down

[Jim B.]

1965 L78 car idles well and runs well at speed. After the engine is fully warmed up, and driven for 10 miles or so and then shut down, a small amount of fuel will spurt in periodic bursts out of the accelerator pump nozzles. This behavior slowly dies down after 20 or 30 minutes. This does not happen when the car is not warm. The way it periodically spurts out this fuel would seem to suggest that pressure is building up somewhere in the circuit and then drops off as the intake manifold and carb cools down.

Checked both floats (with engine cold) and the levels look to be about right (just below sight window level). Found many links in this forum regarding modern fuels and percolation etc. and perhaps thats is what it is, however is there anything i can check to further isolate what might be causing this so that I can attempt to eliminate it?

Thanks in advance

[Edward J.]

Re: Fuel Leak After Warm Shut Down

Jim, I'm thinking that under the Nozzle there may be a check ball that is not seating correctly or a piece of debris, and sometimes there is a light spring to hold the ck.ball down, But I Am not overly familiar with Holley's

[Timothy B.]

Re: Fuel Leak After Warm Shut Down

Jim,

What is happening is the fuel in the accelerator pump is boiling because it's just a small amount and it is sitting just above the hot intake manifold. When this happens the vapor pressure is pushing fuel up it's discharge tube and the pressure builds until it overcomes the top check valve and releases it's pressure through the nozzles.

If you have a old school Holley with a check ball in the accelerator pump cavity this should not be happening because the check ball should be off it's seat allowing the vapor pressure to vent back to the float bowl.

If the Holley has a umbrella check valve in the pump cavity (new style) you can drill a small hole in the cavity .015+- to allow the vapor to vent back to the float bowl. It's not a uncommon problem.

[Jim B.]

Re: Fuel Leak After Warm Shut Down

Hi Tim
Thanks for the note. I can actually hear the fuel boiling in the bowl after about 10 minutes post shut down. Never noticed this happening before. Is this "venting" fuel enough to worry about cylinder wall wash-down and/or oil dilution?

What is the best way to access the accelerator pump cavity to inspect? Do I need to pull the carb or can this be done top side? The carb is a 3310 which is part of the 4150 family of Holley carbs.

Jim

[Ed S.]

Re: Fuel Leak After Warm Shut Down

Jim
I had the same problem with my 64 L76. Switched to non alcohol gas - problem solved immediately

[Gene M.]

Re: Fuel Leak After Warm Shut Down

Plug the exhaust heat passage thru the intake manifold is a common practice on small blocks.

[Duke W.]

Re: Fuel Leak After Warm Shut Down

The first and easiest thing to do is wire the heat riser valve open. Also, the OE 201 15 VAC doesn't pass the Two-Inch Rule unless you raise the idle to about 1000, so a lack of total idle advance can raise EGT which will heat up the carb. A "B26" 12" VAC might help - NAPA VC1765 or equivalent in another brand.

Duke

[Timothy B.]

Re: Fuel Leak After Warm Shut Down

Jim,

IMO, nothing to worry about, that little bit of fuel will evaporate as soon as it hits the hot intake manifold plenum. If the engine has a heat riser you may try wiring it open as this will help the temperature under the carburetor. Do you notice any fuel drip from the booster venturi's looking into the primary side of the carburetor.

I will try to find some pics showing the situation inside the accelerator pump cavity.

[Timothy B.]

Re: Fuel Leak After Warm Shut Down





These two pics show the different pump check valves, the ball bearing valve allows vapor venting back to the float bowl where the umbrella valve does not. If you have the umbrella valve a small hole .015 will allow hot vapor to vent back to the float instead of building pressure in the discharge passage hence your problem.

If you search on the internet you will find more info and may give you a direction to proceed if the other suggestions don't work. I know there are heat shields that you can buy to go under the carburetor and they extend out beyond the float bowls to keep heat down in the areas around the carburetor bottom.

[Jim B.]

Re: Fuel Leak After Warm Shut Down

Thanks Duke
Will look into the heat riser valve mod you suggested. What are the "VAC" parts you mentioned above?

[Jim B.]

Re: Fuel Leak After Warm Shut Down

Hi Tim
Thanks for the note and the helpful photos. The fuel spray does seem to mostly evaporate in the throat and I am sure furthermore if it runs down into the hot intake manifold. I actually kind of like the smell in my garage but my wife is beginning to object so I am getting more serious about addressing it. The date code on my carb would suggest it was one of the "old school" flavors so I guess the second pic you posted is what would be typical in my set-up for the accel pump check valve system. I am looking at a insulating gasket between the manifold and the carb first and then if no joy will proceed another direction.

Thanks again

[Timothy B.]

Re: Fuel Leak After Warm Shut Down

Jim,

Try to get a little more timing in the car at idle speeds and with a open heat riser and the heat shield under the carburetor you may find some joy.

[Richard M.]

Re: Fuel Leak After Warm Shut Down

Thanks Duke
Will look into the heat riser valve mod you suggested. What are the "VAC" parts you mentioned above?

Jim, He's talking about the Vacuum Advance on the distributor. You local NAPA can get them.

http://www.napaonline.com/napa/en/p/...65_0168354959#

Rich

[Jim B.]

Re: Fuel Leak After Warm Shut Down

Thanks Rich

[Duke W.]

Re: Fuel Leak After Warm Shut Down

What are the "VAC" parts you mentioned above?

Vacuum Advance Control - the official GM nomenclature for that device.

Duke