what would the symptoms be for a blown power valve in a holly/demon? my uncle was talking about it happening to his '71 Camaro with a 650 holly. I was gonna ask him but I forgot at the time. anyway...just wondering. ;)

no clue...sorry

pigrich idle,,,hard to clean out, adjusting mixture screws makes no difference in idle.

what's the differance in power valves? like 6.5,8.5???

it takes 8.5 inches of vacuum to keep the 8.5 closed,drops below that and its open. Same with the 6.5 power valve...Holley says you want the power valve at least 2 below your vacuum reading,others say half of what vacuum is at idle.

So could we say going from a 6.5 powervalve to an 8.5 powervalve makes the air/fuel mixture run richer?

All that number is for is to tell us when that valve will open. How low or high the number is has NO effect on HOW much fuel we get. It only effects WHEN we get it. But a power valve does change the air/fuel ratio but not at all times. Under normal driving it does nothing, absolutely nothing. But when you go to WOT (wide open throttle) your engine vacuum drops almost to zero which in turn allows a spring to open the power valve. The purpose of a power valve is simple. Your motor needs more gas at WOT and near WOT. So one of two things have to be done in order to 'feed the horses.' One is to install bigger jets in the carb to allow more fuel to flow. The problem with this is that you can have horrible gas mileage and foul out your spark plugs. The second thing that can be done is to run a power valve. The power valve allows you to run smaller jets. And to make up for the loss of fuel from the smaller jets, the power valve is actually tapped into the fuel bowl. So when you need the gas, (at WOT) it provides it. I run a 4.5 primary power valve and a 3.5 secondary power valve in my 850 Holley because I run a moderate cam and therefore have very little vacuum. If I was to run a 8.5 pound power valve my power valve would be open at all time, including when I'm sitting at a stoplight. Furthermore if you have a blown power valve you will run rich obviously because it is providing enough fuel for WOT. The most likely cause for a blown power valve in is a backfire. The backfire puts pressure against the rubber diaphragm of the power valve and causes it to tear. But it should be said that newer Holleys have a pressure check valve that doesn't let this happen. So all in all Holley and the power valve is the best way to go. (Boy do I know I’m going to hear it from the Edelbrock guys.)