scoot only starts after draining carb

[bull]

Skyrider, you are correct that a petcock can starve an engine for fuel, especially if it is stopped up. the filters in some fuel control valves are inadequate, especially in light of some of the additives people use that are up to 30% ethanol, which can destroy the filter in it and attack the tank coating.

I prefer testing, and repairing rather than just swapping parts, so with the latest information bluefire shared about his SD BBK, the next step for me would be to check the compression and valve adjustment.