Chainsaw Repair

With a chainsaw or weed whacker with a regular Walbro/Zama carburetor, the low speed circuit has to be tuned richer than best running so that the engine can accelerate to high speed. Therefor the magic tuning for the low speed normally is to make the low speed as lean as possible and the engine still able to accelerate when you jerk the trigger.

With a accelerator pump carburetor, the idle needle can be tuned for best running at idle and the engine will still accelerate to high speed. Probably a EPA feather in the hat thingy?

I experienced a problem with a new chainsaw and it took me a moment to figure out what was wrong because I didn't know in the beginning that the carburetor was different. I have a Husqvarna 357XP with the HDA-199 carburetor and this is a accelerator pump carburetor. When I first bought the saw, it would quit on you in the lower mid range speed area unless you wiggled the trigger a little every now and then. I then decided that the chainsaw had a accelerator carb and that for some reason the midrange was lean and the accelerator pump would keep the chainsaw running. I finally decided to make the low speed a little richer and the midrange (poor lean) running problem went away.

With this knowledge of the operation of my 357XP I feel that I would know if the accelerator pump would not work? But I also see the possibility that a bad accelerator pump carb could be tuned to run the chainsaw with a bad pump by making the mixtures richer but the overall running characteristics would not be as good as original new condition.