Originally Posted by wixomhead
I have a ford 460 with an 8-71 and dual 750 Barry Grant Mighty Demons with boost ref pwr valves. Cam = Comp Xtreme Energy 262/270 with 110 lobe centers.
Car idles excellent (900RPM) when all 8 idle screws are all set at only 1/2 turn out. Manifold vacuum shows 16+ inches steady at idle. I did NOT connect manifold vacuum hose to boost referenced power valves on base of carbs because .... the engine has STRONG vacuum at base of carbs at idle.
If I back off even ONE idle mix screw only 1/4 turn...the motor begins to surge immediately. The car won't even run if I set the mix screws to 1 turn out or more.
When I DO connect manifold vacuum source to base of carbs (like BG Tech suggests) no difference at all is observed (para #2 still holds true).
Hope I'm adding (not subtracting) to this discussion with above info and the following ?s:
1. Why such strong vacuum at base of carbs? I thought roots type blowers at idle has little/no vacuum, hence the need to connect manifold vacuum at base of carbs to prevent pwr valve from pouring fuel at idle?
2. Does 1/2 turns out all around sound "proper and correct"? Or am I masking something else up.
Plugs look light brown...
Sounds to me your power valves are removed from the picture if not sourced under the blower.The way you have them set up..they are just along for the ride.The pv chamber is normally sourced thru the carb base.When referanced for a draw thru system like your roots..this source is blocked and the pv chamber is routed with a tube in to the blower manifold.You will not feel this at idle.but the lack of pv enrichment will lean out the main ckt delivery when most needed.Keep in mind..as long as your throttle angle at idle covers up the transition slot correctly..the pv has no effect at idle.It only will fatten up if the diaphram is ruptured and leaking fuel into the pv chamber.