It has the symptoms of a vacuum leak.

Not sure what accessories on a stovebolt could leak vacuum, but prior to disassembly of the carb, try snugging the carburetor mounting bolts, and all of the carburetor body screws.

Does this engine have a distributor vacuum advance?

Jon

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop