66 L-72 Holly flooding, power valve??

[Glen C.]

Several weeks ago I attended a Corvette meet which was about 50 miles from home, with all most all freeway driving. It was hot, 90 plus, not unusual for Tampa. I did not encounter any overheat problems, mostly because of my new Dewitt direct fit radiator. After 3-4 hours at the show, plenty of time for a complete cool down, we left, but took the scenic route with lots of traffic lights and stops. After only three miles my Holly (4150) started to flood, I could see fuel dripping down inside the carb, as if it was boiling over, or the float was stuck. I did a quick front float level adjustment and struggled to get home without flooding out. The next day I rechecked the front float level and the back float level. I use the 93 octane 10% ethanol, and the timing advance is modified to non ported with the appropriate vac can. Reading on this site, I saw a thread about back fire causing Holly power valve failure. I get the occasional backfire, and have to really pump the pedal on a cold start. The thread also noted that maybe after stopping I should remove the air cleaner to see if fuel is leaking in the engine. Also does the original L-72 intake manifold have a heat rise port or passage that should be modified or disabled? Are the Holly power valves sold individually or only as part of a rebuild kit? Any other possible things I should consider in solving this problem?

[Ken A.]

Re: 66 L-72 Holly flooding, power valve??

PV's are available separately & you need a # 65. Also, you should purchase a PV blow out preventer (made by Holley) & a couple of new needle & seats. Ethanol is bad on these old carb parts.

[Timothy B.]

Re: 66 L-72 Holly flooding, power valve??

I agree with Ken, replace all the parts he mentioned. Keep in mind if the power valve is leaking the fuel will leak into the motor under the throttle blades. What you report sounds more like a needle and seat or high float.

Stop and go traffic could make the engine warmer because there is not much air flow around the engine/carburetor and the gasoline with ethonal can boil and expand causing fuel to drip from the booster nozzles. Try to lower the float three flats and see if that helps.

I think all the carburetor engines use a heat riser so wire it open, keep in mind also that the spark plugs may be gas fouled and need to be removed and cleaned.

[Jim T.]

Re: 66 L-72 Holly flooding, power valve??

Holley power valves for me have been available in a single package from Auto Zone, Oreillys, and Advance Auto Parts.

[Glen C.]

Re: 66 L-72 Holly flooding, power valve??

Thank you all for your input. Re: the heat riser, yes it has one on the passenger side between the ex manifold and the side exhaust pipe. I'll wire that open. I had lowered the floats to just below the opening, maybe I should go lower? What's is the flats? not familiar with that term? I looked up the power valve check ball kit for the 4150 and other Holleys it shows #125 500. I think the 4150 has two PV's, correct? I think I'll just get a rebuild kit with both PV's an both needles & seats & gaskets. Thanks again!

[Jimmy G.]

Re: 66 L-72 Holly flooding, power valve??

Key word in your problem was ethanol. I have had to rebuild 2 Holleys in the last 3 months due to ethanol problems

[Glen C.]

Re: 66 L-72 Holly flooding, power valve??

Jimmy, Thanks for your response. Unfortunately, Florida is one of ten states with mandatory E-10 ethanol requirement, although E-0 is still sold at some marinas. There are several marinas within a few mile of my home, I'll check them for availability, octane, and price.