Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

I have a Stihl string trimmer with the KM90R engine.  I think this is a 4 stroke engine which uses mixed gas.  It is about 5 years old, and I use it weekly during the grass growing seasons.  Recently, I've noticed that there's a reduction of power and the engine does not rev up and down as quickly.  The throttle response has gotten sluggish and there's a noticeable power loss at the top end.

Before I open things up or pour in some magic elixir (fuel additive), is there anything I can check or try to narrow down the problem? 

My amateur opinion: 1) clogged spark arrestor in muffler 2) clogged air filter 3) carb out of adjustment 4) carb diaphragms need replaced 5) bad gas 6) clogged fuel filter 7) bad fuel lines 8) impulse line cracked 9) head bolts loose/gasket leaking 10) piston rings worn/cylinder scored I don't really work on these much or well so look up donyboy73 on youtube and search through some of his videos. Just noticed it was a 4 stroke. Never even used one of those, sorry.

Thanks Shryp, the engine is a Stihl 4-mix.  It has lots of torque and good fuel efficiency.  I think it is most likely that the spark arrestor is clogged, so I'll clean it or replace it.  I'll check the air filter also.
I like how you numbered the likely causes, very logical order of severity based on the age/use of the engine. 

I was just ordering them in the order I thought of them from things I have read or videos I have seen. :) It was formatted nicely, but it decided to auto format when I posted it.

I checked the spark arrestor screen and it doesn't look clogged at all.  The 4-mix engine seems to burns clean with very little soot on the spark arrestor screen.  I tried putting in new gas and see it runs any better.  Same symptoms, lack of top end power and sluggish throttle response.   In fact, the engine died out on me after extended high throttle use.  It's probably both air and fuel starvation since I ruled out exhaust blockage.

I see Stihl has a tune up kit which consists of a spark plug, fuel filter, and air filter.  I'll pick up this kit and replace the parts.  That should be all it needs. 

I installed the tuneup kit. The engine is much more responsive and has good power.

I've got an FS90 with the same (or very close) engine. Great machine - I use mine for brushcutting. It's been trouble-free - with one exception.

Spark plug kept fouling out. No big puffs of smoke or anything, but the plug was glazed. No, I didn't add any magic potion to the gas.Tried thinning out the mix, that helped a little. 2 years ago. I switched to synthetic 2-stroke oil at the recommended mix. Haven't had to replace the plug yet. Also using synthetic in my chainsaw, which virtually eliminated exhaust smoke. YMMV, but I'm sold. Wish this stuff was available when I thrashed dirt bikes ages ago.