Probably has a little to do with the fact that you're using a short conrod. Not sure if those cyls are setup for D or ABC cranks.... but you'd have more compression with a long rod D crank - chikabowow ;)
You should have a chocolate brown color to the plug... my money is on jetting... but you should check your timing and compression too. Buy a jet range and get it over with... there is no magic jet calculator... JUST BUY SOME JETS BRAH.
Also, I love you already because you got rid of the POS bing carb... so you're on the right track.
What's your compression?
What's your timing?
If you don't know what those are, go search for "compression" and "timing" in the wiki, do it and post the results.
Both of those are important - don't say the timing is stock - that means nothing.
You shouldn't need to change gearing... the 11x44-45 gearing on those bikes is pretty low for a pipe and carb (the athena 50 cylinder is basically a little better than the D cylinder) you should be gearing up - AFTER you figure out what's up.
I did 38 on an A with gearing pipe and carb once, but the take off sucked... Keep at it... you should see at least 42 out of that setup with a 13t up front... probably more if you're getting to 35 and the tuning sucks. (edited)