I run 15X4 and 15X6 so I guess I better go smaller, well, maybe not, I like the way my plane fly's and my engine runs just fine with them?? I do open up most of the bottom of the cowl for air flow/cooling. Here is how I tune mine, I have a good tach so I used it, not really needed though. New OS#8 plug, fire up the engine, run it up to full throttle and lean out the top end to max then back it off about 400 RPM, cut the throttle to idle. Here is where it gets fun. I put the glow driver on the plug and let it idle for about 20 seconds then pull the driver, if the engine dsrops RPM then I put the driver back on the plug and lean in the low end then remove the driver again. I do this until there is no drop in RPM then I fatten up the low end until there is a slight RPM drop. That should do it?? If the engine boggs down on take off roll then don't take off, bring it back and fatten the high end a click or two. The Pitts muffler may not be giving you enough tank pressure so you can close up the holes in the pipes a little with JB weld but if the engine ran for someone else it seems like you are just out of tune.