I have an older Magnum with an air bleed carb. I could take the screw out of the carb ( the one parallel with the throttle barrel) and the idle was still to rich. I drilled out the air bleed hole 2 drill sizes bigger and then I could adjust the mixture to get a good idle. There was an article in one of the R/C mags back then about making the hole in the barrel oval to do the same thing.

Yes the screw in the back of the carb that sticks UP is the idle speed and does nothing for the mixture. It also hold the barrel in place.

Dru.