Diablo is right. It is a carburetion thing.
It can be anything from the carb itself, to the phenolic block that it's mounted on. I have an old 140 myself, and it runs like a clock.

It always comes down to the big three, air, fuel, and fire.

I actually installed a DA carb block on mine. I didn't care for the characteristics of the phenolics. Mine runs consistent and strong.
If you stay with it you'll get it, and everything will be ok.
These engines take some time and patience to get right, but it's worth it.

Hopefully this thread won't go the "ford" "chevy" "DA" "3W" route.
Don't take this the wrong way, but it's not the engine, it's the user.