Sounds like the gas/air mixture is too lean.
I would start by overhauling the carb and clean
it, especially the jets and bowl. You may not
have to rebuild it and the gasket might still
be serviceable -although at $5.00 a pop, it's
not very expensive.
While you're at it, change the primary gas filter,
and clean the sediment bowl and screen of the mechanical
fuel pump -if so equiped. Adjusting the iddle screw might
also help for final adjustment.
If this doesn't work, check your dipstick to see if there
is any gas in the oil (by sniffing it) -could be the
diaphragm of the mechanical pump that has a small leak.
Go ahead ands take this carburator apart first!
Hope this helps.