Is there an insulator, with another gasket on the other side of it, between the carburetor flange and the intake port?

The carburetor flange gasket looks rather woeful, Paul. If it is of uneven thickness, it will leak air. It can be difficult to tell without running fingers over both sides. I think I'd make a better one, partly because it might leak, and partly because it is hardly something to be personally satisfied with.

Here is the workshop manual's advice on hunting:
[Linked Image]

It sounds rather defensive to me: I suspect they think their governor tends to be a bit unstable. However without having actually heard your engine, I think lean mixture is a more likely issue than a governor fault.

Your carburetor is a Series 7, as shown here:
[Linked Image]

Please check the illustration and description against your carburetor to confirm that identification. It seems we have to face up to the unlovely task of cleaning a Tecumseh carburetor.