I'm Confused! Carburetor Related?

[horseballz]

Well Folks,
I just finished neutering my 5.0 with GT40"P" heads from a Ford B303 cam to a stock Explorer cam. I was told that I would hate the cam and that it would lose all performance. NOT TRUE! While certainly not as stout as the B303, it still performs quite well up to/thru +4,500 RPM and fuel mileage should be greatly improved, etc.

Now for my confusion:
A) Throughout life with the B303, I played and played with the idle air bleeds on the QuickFuel HR-580 vacuum secondaries carburetor to try and tame the overly stinky rich exhaust at idle, all with little success. Current air bleeds are considerably larger than stock at .081 in the primaries and similar (can't read the numbers) in the secondaries. The car ran and drove quite well and was pretty quick and the idle mixture screws were at about 5/8-3/4 out. Vacuum was between 15 & 16.
B) Now with the new cam, with vacuum in the 20-21 range, I had assumed that I would need to (at first, before changing any air bleeds) turn the idle mixture screws out a bit farther to richen up the mixture a bit to compensate for the added vacuum and air. The surprise and confusion comes from needing to completely close the primary screws and slightly open the secondary screws to reach smooth running maximum vacuum. I also suspect that it would be even better if the primaries could be "closed" even more! This is also leading me to think that I need to put in even larger air bleeds, at least in the primaries.

What Thinks You Guys?
Gene

 

[rbohm]

you may have the wrong power valve, jets are too large, etc. but one thing that has been used by racers everywhere, and that is to drill a small hole the the primary throttle plates, about 1/8" to start, to add more air into the system. that will lean out the mixture.

but before you do that, you need to tune the carb to work with the smaller cam. that means starting with the idle circuit and going with smaller jets, change the emulsion tube, etc.

 

[TommyK]

Put the carb back to stock calibration and start tuning from there.

 

[horseballz]

I did go back to base one/original stock calibration. Now, at least I can get the primary mixture screws out a tiny 1/8 or 1/4 turn from seated. Not optimal but on the right path. Still a little confused as to why reducing the idle air flow allowed me to "open" the fuel mixture screws! Recommendation from several folks, including a tech at QuickFuel, was to open the secondary throttle plates just a little bit. This caused the idle speed to be too high and no longer controlled by the regular idle speed screw. I adjusted the secondary plates closed to just the point where the idle speed screw controls the idle at about 550-600 rpm. Car drives pretty nice at this point, but haven't driven enough to study the cam change's effect on fuel mileage.
Thanx Guys,
Gene