I run a 39 PWK carb on my 87 zilla, it is stock w/ pual turner pipe, air box lid removed. I was having problems with my carb to, it would not clean up until I was in 4 gear wide open. The local shop I deal with said that carb on the 500 stock should have a 187 main jet, our elevation here is 1400 ft. I changed my main to a 195 from the 210, that did help some. I moved my clip on the needle numerous times till the clip was all they way to the top with no change, finally I noticed the fual air mixture screw on the side of the carb (first pwk I have delt with) and after checking the setting I found it turned all the way in. I screwed it out two turns, set the clip in the middle and the bike runs great now. Not sure what in your case but thought I would give you an idea if you havent looked at that already, good luck