How do you set the H & L screws on a small carb?

I'm only familiar with Stihl and Husqvarna saws but on one side of the body where the screw holes are they have little embossed diagrams with arrows telling you what the 'factory' settings for the screws are. On my Stihl saws they say "¾" meaning turn the screw all the way in, then turn it out three-quarters of a turn. Both High and Low screws may not use the same setting so have a good look or use the method below.

If you're feeling more confident and/or you have a digital revcounter handy you can find your own settings. Ideally you need the saw at it's running temperature with the chain correctly tensioned.

Once it's warm adjust the Low screw by turning it anticlockwise (making the mixture richer) until the motor picks up cleanly and accelerates properly, but no further. Once you've adjusted the Low screw, you need to reset the LA (idle) screw. Turn this clockwise until the chain starts creeping round with no throttle applied, then back off half a turn or so. Basically you want it to idle reliably but not so fast that the chain creeps round or takes a long time to stop once you release the throttle.

Tuning the High screw can be risky without a revcounter or a very good ear as too high a speed will damage your motor. 13500-14800 rpm are normal speeds on Stihl saws, depending on their size. Basically turning it clockwise will lean out the mixture and make it run faster at max throttle, and anticlockwise; richer/slower. Set it so it screams at the same pitch it did when it was new or working properly. You can check the spark plug for signs of rich or lean running.

Good luck!

javelin10
August 2010