Air/ fuel tuning

The power valve never flows fuel at idle. Unless you have a bad diaphragm ( and this not a clutch or birth control device...lol). If you do have a bad diaphragm it will leak back into the PV hole in the main body.

The reason for using slightly less that cruise vacuum instead of half of idle vacuum becomes a big deal when you have low vacuum at idle.

As an example I have 9.5 inches of vacuum at 950 RPM idle. Conventional wisdom says use a 4.5 or maybe a 3.5 PV. At a cruise I have almost 16. With a 4.5 PV that is a huge gap to fill. That late opening PV will require a much richer T slot setting to keep from getting a tip in stumble. Some guys try and cover it with the accelerator pump, BUT if you barely touch the throttle, the accelerator pump does virtually nothing.

That's why I never set PV opening by idle vacuum. If you do it by cruise vacuum, you can clean up the T slot circuit and lean it out where it should be, and the quicker opening PV will cover the tip in stumble.

Mark Whitner has an excellent video on YouTube showing how the PV does NOT open at idle. I put a link to it in this post but I don't know how.


Damn technology.