I have a Stihl ms390 recently it has started acting up. While cuting it seems to lose power like maybe it has taken to big of a bite and bogs down sometimes dying I have cleaned airfilter mixed new gas drained tank and inspected in tank filter, without blindly tearing it apart or taking to a sawshop I was just wonfering if maybe I was missing something simple. The saw idles fine so I have not messed with the idle screws.

Mac, February 2009

have you polished the CANOOTER VALVE?

LOPPY, March 2009

sound like the HIGH speed airscrew on the carb needs adjusting. if saw screams too high at full throttle adjust it SLOWLY ANTICLOCKWISE. if it doesnt rev as high/ smooth as you expect turn it CLOCKWISE SLOWLY. SAW MUST BE HOT to get setting right, and ALWAYS REMEMBER SCREW POSITION that you start with. the screw will have an H next to it and another will have S(that is for slow speed tuning) the idle screw is another screw. if you don't know much about engines DON'T ATTEMPT DOING IT as you'll not understand how to get it right.

muleman, February 2009

Does it throttle-up normally when not under load? If it idles and throttles-up ok when not cutting, then it is a fuel starvation problem. Have you checked and cleaned the fuel filter. A cheap and easy fix in the first instance.

pleasant, February 2009

Do this and see if it will work after. idle and high speed mixture.
I would start with them both backed out from the closed position to 2 turns open, for starters. One should be idle mix and the other should be high speed. Crank the motor, get it warmed up. You may have to fiddle with the throttle and or choke until its warmed up. Then slowly turn in the low speed jet, until it starts to die, then back it out another 1/4 to 1/2 turn. Then hold the throttle wide open, and slowly turn in the high speed, until it really starts to smooth out and rev high, and start screaming, then back this out until it starts to run rough or slow down, and then turn it back in to midway between these two positions. Keep playing with the low speed needle until you get rapid immediate response from the throttle, and good idle with the idle adjustment. Then play with the high speed needle, but always back it out from the setting about 1/4 turn or so, from where it runs the smoothest. This setting of backing it out will allow more fuel when under a load, and keep your 2 cycle motor from running too lean. Put a load on it and just tweak the settings just a hair at a time. If you're in the ball park it won't take much adjustment either way to make a difference. Better on the rich side than lean side. Also make sure your oil / gas is mixed at the proper ratio.

Bobby, February 2009