Motor was rebuilt 1 year ago. My friend says its been doing it from then but he's never messed with it. Idles good. When revved up under load we got blow back thru carbs. No power ran rough. I replaced reeds and reinstalled everything. No more blowback but still runs really bad under power. Idle is ok. Compression is good. Timing ok. No misfire, backfire, etc. I'm thinking a piston could be in upside down?... I encountered this problem on a yamaha many years ago. Similar symptoms. Weather is bad so I thought I'd get some second opinions.
