between circuit miss?
Just checked a few things out this afternoon. Currently at 1200 RPM idle, my primaries are ONLY open by 1/2 turn of the idle screw. In other words if I back off the idle screw 1/2 turn, my primary blades are completely closed.
The main reason for only being open a 1/2 turn is because the timing is so far advanced. The engine will produce more torque at idle with less advance, a richer mixture, and it would allow the primary throttles to be a little more open. Then it would idle stronger with a little less rpm.
Since that's something your probably don't want to do... yes, you can try backing off the secondaries and opening the primaries a bit.
On older Holley's, the guidelines were .020 to .040" of transfer slot showing beneath the primary throttle blades (2 corner idle). With your HP, the slots might be longer or wider than those old Holleys, so while concept is true, the range may be a little different, maybe closer to .020" - .030"
When the carb is off the engine, take a look at how much of that slot you can see under the throttle blades with the 1/2 turn in, and how much it changes with another 1/4 turn.
The higher number power valve will open sooner so the higher your cruise vacuum the higher the power valve opening you need. The idle vacuum setting is wrong and always has been.
Exactly. ^
Power valve should open when the load on the engine gets high. When you rig the vacuum gage, you'll see opening throttle from cruise causes vacuum to drop. The more the throttles open, the lower the vacuum. At some point, more opening does not result in more acceleration or power. This is when the power valve should be open so the mixture gets richer instead of leaner. Its somewhere's between 1/2 throttle and full throttle. So if the vehicle cruises with 16"Hg, an 8.5"power valve is a much better guess than one based on idle vacuum - especially with a hot rod.
There is a cool video by Mark Whitner where he proves the power valve cam be open at idle and not change the A/F ratio
When a carb is set up as it ought to be...
Very large idle restrictions and other things can mess that up. But basically yes, shouldn't set PV based on idle.
The T slot restricters are the holes that let fuel get down to the T slot in the base plate. You can thread the main body and use some brass set screws to change how much fuel you get while on the T slots.
I still think its not enough fuel in the transfer slot rather than too much because IronMike wrote
"miss
right around 1800 to 2000, only at cruising speed(
very light throttle)"
That's why I suggested:
1. Stick your finger on an IAB and see if it has effect on idle. If not, then changing them won't do any good for sure.
2. If it passes test (1), drop the IABs, readjust idle mix if neccessary, and take it for a road test.
And I would do this before messing with the initial throttle positions. But which ever sequence of changes you choose, do one change at time.
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The transfer slot is the "idle" circuit's main fuel outlet into the engine.
The idle port with the needle valve (mixture screws) is below that.
At idle, the portion of the transfer slot below the throttle blade sees vacuum and contributes fuel. The portion above the slot contributes air into the mixture.
As the throttle opens the transfer port supplies more and more of the fuel from the idle circuit. Sometimes its useful to restrict this amount.
