quote:According to this article, by Holley, the power valve opens at the manifold vacuum referenced by the valve size....
http://www.holley.com/data/Tec...cal/power_valves.pdf
So:
65 = Opens at 6.5" or below
85 = Opens at 8.5" or below
Therefore the 85 power valve is actually opening at a higher manifold vacuum (earlier) than the 65 valve.
The Power Valve is SIZED by using the max vacuum at idle, then dividing by 2. That value is a "rule of thumb", not when the valve opens (as far as I understand it).
Rocky, you're right. Chris, the 2x8.5=17 is not the correct way of looking at this. In short, if your engine idles as X, get a PV named X divided by 2.
I doubt any carb has ever backfired from being too rich when floored. When opening the throttles quickly you create a need for a lot more gas, which the carb desperately tries to cover with acc-pump and power-valve. Looking at a Air/Fuel ratio gauge, any engine goes leaner just when floored. Backfires in the exhaust on the other hand can happen from too rich, but usually when letting go of the throttle.
Normally backfires in carbs (not exhaust) are due to too late timing. So set the timing first.
In all humbleness, I'd suggest reading my book, it deals with tuning the ign/carb and the symptoms of when your engine needs adjustment. Or let me know directly of any questions?