backfiring but stops when i turn the pilot screw all the way off

[tideman7469]

i realy dont know were to start but my 2003 venture started backfiring out of my left exhaust if your looking from the back so i tried plugging the ais system that did not work so ive been trying to adjust the pilot jets on each carb and while i was doing that for some reason my right exhaust started blowing out white smoke and black oil. i pulled all spark plugs which i just installed new ones less then a month ago for black oil on them and they are fine. there is no knocking or sounds like loss of power. so i kept on adjusting the carbs and i set all of them at 1.5 turns to start out but the backfiring was the same or worse. so while it was running the carb closes to the seat on the left side i turned the pilot screw all the way in and backed out just a half a turn and the backfiring stopped and so did the smoke and black oil. can anyone explain this for me. on the same carb if i go a full turn 1 time it starts to fackfiring and i go to 1.5 it gets worse so i go back to half a turn and its fine again. what puzzles me is that it started smoking and the black oil from exhaust. could i have caused this to happen just by adjusting the pilot screws.?

[FaceDeAce]

Popping in the exhaust, backfire, is a symptom of a rich mixture. Unburned fuel in the exhaust ignites in the pipe when it reaches enough oxygen, towards the end of the pipe.  Adjusting the screws richens or leans the mixture.  So what is happening while you are doing this is exactly what is expected to happen as it is adjusted from a rich condition to leaner setting.

The smoke and/or black oil … is just the build up of soot and ash from a long term rich condition being burnt off and blown out as you lean the mixture and the pipe gets hotter.  Also exactly what is expected to happen as you lean it out.

all good.  Carry-on.
 

Edited July 12 by FaceDeAce