If the carburetor works well in first and second (also the timing) then it is also good in third.

I would check:

(A) fuel filter
(B) possible clogged tank vent
(C) leak-down test on fuel lines from tank to pump
(D) fuel pump volume test
(E) (stretching here) possible worn fuel pump lobe on camshaft
(F) minor kink in fuel line restricting volume

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