While chasing a fuel leaking onto airbox issue I ran into a rough running situation. I replaced the carb with a rebuilt cowled up the plane and ran it up. Everything seemed fine other than the the idle stop screw needed to be adjusted (RPM was below 500 and starting to stall). Removed the cowl made an adjustment to the stop screw and and tested again, this time without the cowl on. Started immediately but ran rough, guys monitoring on the ramp said there were black puffs of smoke from the exhaust pipe. Shut engine down and pulled it back in the hanger. Cyls 1-5 were hot but cal 6 was only warm, pulled plugs and check compression 74/80. Both plugs were carbon fouled, cleaned and tested plugs reinstalled and this time cowled the plane. This time the engine started and ran fine. I'm stumped why cowling the plane would make such a difference.

I can't tell you why they run rough with the cowl off, but I think it could be erratic air Flo to the carb from the propeller. I have experienced the same issue and other mechanics have also. If it runs fine with the cowering on don't worry.

Bty, concerning the fuel leak. Are you using car gas?

Adjust the idle mixture at the same time?