81 Marathon suddenly not running right

[theboz001]

The cart was running perfectly until a few days ago. Now it won't run at all with the exhaust connected to the muffler and the clutch belt installed. It will run, but won't rev up enough for the clutch to engage. The engine actually starts but just barely. When I put my hand in front of the carb the rpms drop back down to whatever speed the starter turns.


The engine was supposedly rebuilt just before I bought it. Has good spark, compression right at 125, timing appears to be correct, tried a brand new muffler, gone through the carburetor. I pulled the exhaust and confirmed the piston is in correctly and there doesn't appear to be any restriction in the pipe. I pulled the reed block and confirmed there wasn't anything holding the reeds open. Pulled the fuel pump and made sure it wasn't leaking. While it still has points, it does have the HEI conversion. I put it back to just points and that made no difference. While running I did spray carb cleaner at the front and rear seals. No change there either.

I even tried swapping the carburetor from my 86 that runs perfectly.

I would appreciate any input as what to try next.