You need to pull off a plug wire when the engine is at an idle. See if there's a hot blue spark
when at an idle. You may have a weak coil .
I would also close the main jet adjustable screw
to lean the engine some. Also do a compression check. If the carburetor is set too rich you may be getting too much fuel on the cylinder walls washing the oil off. If you have the old points and condenser I would install new points and a condenser. If an electronic module is used for ignition it may be getting weak. Hal