BurnPgh wrote on 05/25/10 at 19:20:19:Well....the pilot circuit is actually alright now. I still cant tell a discernable difference tuning with the air screw but if I idle for a few minutes then check the plug its a nice toasted brown. Its from about 1/8th throttle and up were Im obviously lean.
I still think a functioning pilot jet will help with the transition between 1/8th throttle and 1/2 (or so) where the main jet becomes the overwhelming factor.
I haven't exactly run a finite-element analysis on it (and could well be talking outta my a$$ -- really, I would not contradict Lancer's advice without trying it both ways) but I imagine this is happening: When the slide is all the way down, the UFO funnels the stream of air right on the needle jet, just above the main. So if you just install a UFO and don't touch anything else, that concentrated stream causes too much fuel to be sucked through the main circuit, thus the idle to 1/8 mixture is crazy rich. This part I experienced directly and joked about it on here while I was still trying to sort it out.
Then when the throttle is cracked above 1/8, that concentration of air is no longer on the needle/jet. There is a more uniform distribution of air within the open cross section, and it goes back to a good mixture.
One can fix the low speed richness by blocking off the pilot, and then fix the 1/8 throttle mixture with a bigger main jet, but by sizing the main to fix a 1/8 throttle lean condition, it will turn out too rich at large throttle openings.
That's why I think it's better to leave the pilot circuit working, and do other things to lessen the UFO's effect at idle to 1/8.