Originally Posted by Baedarlboo

If you're referring to test the bottom end (low speed needle tuning), you pinch the fuel line while idle, and count how many seconds it takes for the engine to die out. If it dies immediately low end is too lean, if it doesn't effect the engine for a while, it's too rich. If the engine revs up and dies all within a 4-6 second range, you're close on getting a rough tune for bottom end.