It could be a sticky intake valve.
It could be one of your carburetors is dumping raw fuel into the intake and then into a cylinder or two, causing a delay in combustion, whereas combustion is occurring while an intake valve is open.
Disable one of the carburetors and see how it runs. If it runs the same or worse, disable the other carburetor (enabling the first carburetor, of course) and see how it runs.
You could make a carburetor base gasket without a center hole in it for this test.
Carl
Last edited by 52Carl; 10/20/2019 11:34 PM.
