ORIGINAL: Tired Old Man
In most cases I do my tuning with a running engine but there are some that just can't be done that way. Those start and stop engines are a real PITA but that's the only way to do them. If you have a carb with needles that angle towards a prop do not EVER try to adjust them with the engine running. You're life depends on not tuning with a running engine under those circumstances. Those that elect to tune an engine with a bottom carb with needles pointing straight out the side must excersize extreme care in remaining clear of the propeller with every part of their bodies, clothing, hair, whatever. Rear carb engines are easy if some planning went into the engine installartion to permit needle access. Side carb engines can be easy or dangerous depending on the way the needles face and prop clearance.

Liability Disclaimer:
With any running engine, you tune at your own risk, and make sure you always have someone else nearby. Like within a few feet.