Stihl RM 248 T

SRTsFZ6
Did you run it empty last year and now fill with fresh fuel?
 
Stale fuel WILL cause all kinds of running issues..
 
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SRTsFZ6
If being stored for a long time, I'd drain it.  
 
I don't know if you have Ethanol fuel over there but if so, it'll go bad usually around 2 months.
 
As of right now, I gather it has some old fuel in it.  DO NOT add fresh fuel to it, you'll just get more bad fuel in the tank.  
 
Dump, suck out, run all the old stuff out.  If you can, catch what comes out of the tank and inspect.
You'd be surprised sometimes what you'll find.
 
Put in fresh fuel, check the spark plug for cleanliness or replace with a known good one.
 
 
BTW, easily 90% of the machines I work on (mowers, small engines, chain saws, concrete saws, riders, etc), are what the issue is.
 
The smaller engines, with the small carbs go bad and clog up easily with varnish, water in the fuel, etc..
 
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