When should the ppetock be set to prime i always use the on position and reserve. When would be a good time to run it on prime?

whenever the carb bowls are empty after doing service, hose removal, etc. and also when you're jamming down the freeway and the bike starts acting like its running out of gas.

If the bike's been sitting for a while (many days) without being ridden. Switch to prime before starting (to Prime the carbs ). Start and then switch back to On once the bike is running.

Michael