luken wrote on 12/31/69 at 16:00:10:Im at 155 and 55. It has been running great but lots of smoke. I put on the sportster muff but didnt to the neck anlge peice so the brake lever on the rear is black from the exhaust. Not good, the brake calbe rubs against the sprty too, will be fixing that soon though. I have been thinking of going back to 152.5 because my air screw has no noticable affect when I turn it.
Your main jet #152.5) has no connection with the pilot air screw adjustment. When you turn it there is NOT supposed to be any affect at all. The main jet, for all practical purposes, is responsible for 3/4 - full throttle.
The needle (and it's white spacer) is responsible for the midrange ... 1/4 - 3/4 throttle. If the midrange is running lean then file the white spacer to richen the mixture. If the midrange is running rich then use washerson the needle to lean the mixture.
The pilot jet is responsible for idle - 1/4 throttle. The pilot air screw is used to adjust (fine tune) the pilot circuit. When adjusting the pilot air screw the rule is that the adjustment should be between 1 and 3 turns out from the fully seated position. Ideally, if you have the best size pilot jet installed then the engine should run best at 1.5 turns out. Notice I said IDEALLY.
When doing the adjustments, if the engine runs best at less than 1 turn out then you need a smaller pilot jet. If it runs best at more than 2 turns out then you need a larger pilot jet. The goal is to get the best size pilot jet so that when you finish the fine tunig and the engine is purring, the setting will be between 1 & 2 turns out.
Backfiring when rolling off the throttle is primarily caused be a lean pilot circuit and/or an air leak in the exhaust system.
Changes in altitude, temperature, humidity and barometric pressure all affect a carbureted engine. If the changes are significant then rejetting may be necessary, depending on how much time will be spent in that condition. Some carbs automatically adjust the mixture for changing conditions ... the Edelbrock does that to some extent. The dial-a-jet can also do that to some extent.
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