Thread: Holley 780 Carb

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Charlie,
I'm not in any way a Holley expert, but I recall the Holley forum talking about power valves and jets -

The purpose of a power valve is for it to add the needed extra fuel when you are under wide open throttle. It is not intended to be an enrichment circuit during normal throttle cruising. If you are lean or rich during normal driving conditions, you need to tune the jets, not the power valve. The reason you check the vacuum at idle and take that reading and divide it in half, is so you do not have a power valve that will open at idle as this will cause an excessively rich mixture at idle. Under WOT the vacuum of the engine will drop to 0 or near 0 and the power valve will then open adding the extra fuel needed during WOT.

Here's the link to the source I used for info - Power valve selection It sounds to me like your jets are too big, and you should be back at the "6" power valve as a starting point (1/2 of your idle vacuum) but you said you installed new jets. A puzzler, for sure.