MIke,


With any and all engines I had run, the mixture strength changes only a little, between full tank and empty.
no more than three clicks of the needle are needed to compensate for that.

Do you happen to have your ST.40 inverted and fully enclosed inside that nice cowl the Contender has?
I had the .40 model and selected to mount the engine upright, under the wood mounting beams, to get the carburettor and the fuel tank center-line at the same level.

If you have it cowled, make sure there is sufficient air intake and at least twice as much exit area AND that the air actually flows around the head and cylinder fins.
That is in case your engine is simply over-heating, with effects similar to a lean condition.

This is a ringed engine and its piston-ring will not tolerate too many lean-runs.