I have a Speed Demon 750 carb on a 350. I have started tuning the carb alittle and need to know the best way to fined out the correct power valve. I have heard several different way to do this but just wonder what seems to work the best. The car seems to have a little bog then picks up. What does the power valves do?

 

The PV enrichens the main circuit under load.At a preset vacuum level it opens up adding fuel to the circuit so that is does not lean out as engine speed and load increases.How you choose which one you need depends on your motor and its usage.Go to the sticky at the top from Jester and in there there is a link to Holley's site that gives specifics directions to help aid you in choosing the correct PV.If you have a healthy street strip motor you use a PV that is around 2" below your hot idle vacuum in nuetral or park.On a mild street motor you pick one that is around 2" below your high cruise vacuum.On my Speed Demon I run a 9.5"PV as my hot idle vacuum is 11.5-12" for a healthy street strip motor..increased lift and duration,low gears,increased compression and so on.

 

Power valve selection really doesn't become critical until you run a cam big enough to pull single digit vacuum at idle. Above that all that really matters is that it's rated sufficiently low to keep from opening under idle or cruise....the one it comes with will suffice most of the time. Even then, it's about the last thing you should tune.

Tuning it does become useful however, if once everything else is tuned, you wind up with a lean spot inbetween the primaries and secondaries. You can put in a PV that opens a bit earlier and covers it.

an off idle bog would be an accelerator pump that needs to be tuned.

 

How would I go about tuning the accelerator pumps? What do I need to start with? I am purchased the accel. pump cam kit but have not changed anything as of yet. Thanks for the help.