The air bleed screw is whichever screw that happens to block the air bleed hole in the front of the carburetor when it's turned in. Turning in the screw blocks the air bleed hole and richens the low end adjustment. Turning it out opens up the air bleed hole and leans the low end. This effect isn't as precise as a low speed needle, so you have to kind of "eyeball" how much your adjustment affected the air bleed hole's open area. It's more like setting the crescent area in the carburetor intake for your idle than it is like turning your high speed needle, if that makes any sense.

Air bleed carburetors rely on allowing extra air into the mix to lean out the idle setting. The more you turn out the air bleed screw, the leaner the low end gets. This effect is limited by the size of air bleed hole drilled into the carburetor in the first place, however, so it's almost impossible by design to set an air bleed carburetor too lean on the low end. With some engines, users have reported obtaining best tuning results by actually drilling out the air bleed hole in the carburetor to the next size up to allow for more maximum air intake and to widen the range between full rich and full lean.

Interestingly, like plain bearing engines, only the best engine manufacturers still make air bleed carburetors. You can get air bleed carburetors on new Enya, O.S. Max, Thunder Tiger, and K&B engines. You cannot get air bleed carburetors any longer from GMS, Super Tigre, Evolution, Magnum, Aviastar, SK, JBA, or most other glow engine makers.

I am currently flying a Thunder Tiger GP-61 plain bearing air bleed two-stroke on my Goldberg Protege very regularly. The engine is a pleasure to fly and is very powerful and reliable. It was easy to tune and it seems to require less fine tuning as the weather changes than many of my twin needle carburetor models. It has been my experience that air bleed carburetors make terrific engines for the casual sport flyer because they're not sensitive to climate changes, don't require much in the way of fine tuning, and are very hard to get tuned so out of whack that they won't run at all.

In my opinion, ball bearings are a great option if you can't get fuel with real castor oil in it from your local hobby store. Also in my opinion, twin needle carburetors are handy if you really want to fine tune your two stroke for maximum performance. Neither is necessary for a great sport flying experience, however, and many pilots enjoy season after season of terrific, reliable flying with their O.S. Max LA engines, Thunder Tiger GP series engines, and K&B Sportster engines.

If you need heavy, sensitive, and prone to corrosion in your engine, then twin needle ball bearings are the way to go. For float planes, trainers, and general sport flying however, plain bearing engines with air bleed carburetors are simply a smarter value.