We need to know some details of your engine, Rob. If you post the long string of numbers stamped on the cooling air cowl, we can get most of what we need, but a picture of the carburetor and linkages would also help clarify things.
You probably know that the Flo-Jet carburetors tend to have two mixture adjustments, one for idle and one for high speed. If you have only been adjusting the idle mixture you would be likely to find it made no appreciable difference to the way it runs at high speed or under load. A picture of your carburetor will tell us which type it is, and we can then talk about potential problem areas.
I am not ruling out ignition difficulties, but let's begin with the mixture. Engines with weak ignition usually show it first in hard starting.
