Originally Posted by Tech @ BG
A blown PV would make it run rich, but it would not allow you to screw the mixture screws all the way in and still have the engine run. There is something else wrong causing that. There isn't anything you can see that will indicate a PV being blown or not, you'll need to remove it and actually check it.
The way the idle circuits are designed if everything is working properly all of the fuel the engine gets at idle should come from your curb idle discharge ports. These are controlled by your idle mixture screws. If they are shut the engine should not be getting enough fuel to idle. If it does you have a problem. It can be that the butterflies (front or rear) are exposing too much of the transfer slots pulling fuel into the engine. It could be the float level being set too high, and pulling raw fuel out of your boosters at idle. It could be a bad PV gasket causing raw fuel to leak into the engine. A blown PV would leave it in the "open" or flowing position adding fuel to the main fuel well which is where the idle circuit gets its fuel from. This would cause a rich idle, but would not allow you to completely shut your idle mixture screws.
Dear Mr. BG Tech
are there idle circuit slots on the secondaries of my Holley 650? I thought they were only on the primaries. They are not open to far. I checked that. they are one turn of the idle speed screw open - barely cracked open.....so it shouldnt be drawing air/fuel from the transfer slots. There is no visible raw fuel being delivered from the main metering boosters. I looked and did not see any while the engine was idleing. shall I go get a new power valve and throw it in there? It's probably only a 1 hour job to do right? I realize I will need some new gaskets also.