Well my HS50 67181c Tecumseh 5hp engine runs and starts, but at mid to high throttle (not full throttle) the engine can surge up & down. What's weird is at times it smooths out and at other times keeps surging. The idle is fine, full throttle is fine, what's up with this engine? I went thru the carb last week and cleaned it and it seemed to run fine then, but now today it's doing this surging thing... :(

Sounds like there's dirt in the carb somewhere. When you cleaned the carb, did you thorougly clean the fuel tank and your gasoline storage container? That's about the best I can come up with right now. If I think of anything else, I'll post another. Good Luck.

Wanted to update this thread to mention that I bought a new replacement carb (eBay) w/ fixed jetting and she's running good again. :p