hav a muccolloch 335. keeps stalling. how do i set timing or is it dirt in ?

It is not timing, the symptoms you describe are fuel starvation, check for dirty or stale fuel in the tank, check for blocked fuel filter in tank, remove carb and check filter screen in the pump side of the carb ( cover held with one philips head screw ) richen the L screw on the carb, 1/4 turn anti clockwise.

Husqvarna man
November 2009