Holley fuel bowl level/power valve

[Rick C.]

After my '71 LT-1 sits for a while, the fuel level in the primary bowl drops ( about 1/4" to 3/8" in 24 hours). Is this because the power valve is bad? If so, what size valve do I need to get as a replacement for a stock engine? There is nothing stamped on the valve that is currrently in it. Thanks again.

[Michael H.]

Re: Holley fuel bowl level/power valve

Rick,

The most likely cause of fuel level drop after the car has been shut off hot is evaporation. This has been a common problem in the last several years because of the newer fuel blends used today. It's actually common for primary fuel bowls to go completely dry after an engine has been shut off when hot. Especially true with a Holley with an external bowl vent at the top of the bowl.

If the power valve was leaking, it would also leak fuel when the engine is running and this extra fuel would go into the engine, making the mixture extremely rich. You would certainly know if this were the case. The engine would probably idle very slow, or not at all, and you would see black smoke from the exhaust.

Not sure on the power valve number but I'm sure one of the C3 guys can help you with that. Most likely a 6.5 valve.

[Jim T.]

Re: Holley fuel bowl level/power valve

Rick the rule of thumb for your LT-1 is when it is at normal operating temp and idle measure the manifold vacuum. Use a power valve that is at least 2 numbers below what your idle vacuum is. More than likely a 6.5 will work on your LT-1. I have a Holley 650 double pumper on my ZZ1 crate engine in my 70 and I use a 4.5 because the idle speed is brought down to much by the turbo 400 and factory air. I use a 6.5 power valve on my Holley spreadbore double pumper equipped 68 327/350.

[Rick C.]

Thank you! *NM*

[Jon #40768]

Re: Holley fuel bowl level/power valve

I have never seen a power valve without a number. A backfire will often blow out a power valve. A bad power valve will cause a stumble at off idle or on acceleration. Ups, come to think of it, my 67 has developed a stumple on a quick acceleration.

Jon

[Michael H.]

Re: Holley fuel bowl level/power valve

Jon,

If a power valve diaphragm tears or leaks, it will cause more than just a stumble at off idle. The mixture will be VERY rich and the idle speed will drop or the engine will not idle at all.

I've seen power valves without the number. Must come in cheap overhaul kits.

[Jon #40768]

Thank you, Jon *NM*

[Gary Hoffman]

Re: Holley fuel bowl level/power valve

I just went through this with my 71 LT-1. It calls for an 8.5 power valve in both primary and secondary locations. 72 jets in front and 76 in the rear. My idle vacuum on a totally stock engine was 11 inches so the 8.5 valve worked just fine.

Gary