How long have you owned/ used this saw? have you checked the condition of the plug and gap?
What type oil have you been using? and what is the ratio mixture?
Sthil products are temperamental and need the stihl brand oil with a 50:>1 mix in order to run at best performance,
Most every 2-stroke engine I have will use the cheap oil mix 32:>1 but the stihl pack blower I have has to be feed with the "special formula" stihl brand oil and @ 50:>1 trying to mix the bargain oil @ 50:.1 can result in Galling the piston and eventually lower the compression of the cylinder, which could also be a factor in your case,
I've tried using the mix I use for my other tools Poulan,Homlite,and crapmans, into the stihl and even though it will run in it, is as you stated seems to loose power, Which does make since
when it is trying to burn off more oil then should be,
Just a thought!
Examples of the mixing ratio
For a mixing ratio of 1 : 50 you need 5 litres of petrol and 0.10 litres (100 cmｳ) of STIHL two-stroke oil.
Petrol quantity STIHL two-stroke oil
1 : 50 Other brandname two-stroke engine oils
1 : 25
Litres Litres cmｳ Litres cmｳ
1 0,02 (20) 0,04 (40)
5 0,10 (100) 0,20 (200)
10 0,20 (200) 0,40 (400)
15 0,30 (300) 0,60 (600)
20 0,40 (400) 0,80 (800)
25 0,50 (500) 1,00 (1000)
