Stihl FS 45 wont stay running

Not open for further replies.
Mar 20, 2014
So I bought this FS 45 back around May 2010 and used it on 3-4 lawns for 2 seasons, then on 2 lawns since end of last year. Well it hasn't run since last fall and I am stumped. After i install a new plug, and clean the spark arrestor, and say it doesn't help, should I just buy a new carb? I have found Zama's for $20 new and the OEM for $37.99 via ebay. Anyone have suggestions? I am very tight on money and we only have 1 other running trimmer that is 8 years old. Thanks for your help guys!
Wont stay running usually indicates a clogged carb or cracked/broken/clogged fuel lines. Put in new fuel lines and fuel filter and clean out the carb. Also check your air filter. And make sure your Fuel cap is still breathing. And for the future, TruFuel is your friend.....
Last edited:
Your symptoms are common, This model (I own one) is affected by E10. The fuel line cracks, and the carb sucks air. Try replacing the line first. After the line was replaced on mine, I switched to trufuel and I've had no problems for the past three years. Starts right up and runs great now...even after sitting all winter. Started right up this morning, in fact....
Last edited:
'Won't stay running' is very non-descript. What does that mean? If it runs until you squeeze the throttle, which is common with 2 strokes, then you need to richen the fuel mixture by tuning your carb. That is still a stab in the dark but is usually the issue with most 2 stroke OPE these days because they are set lean at the factory.
I had same issue on 2 Stihl (blower and trimmer) the E10 definitely corroded up stuff. I took the carbs apart and cleaned everything with a toothbrush and carb cleaner. Sprayed through all jets/ports from both sides and reassembled. For some reason the blower had issues after and would not rev up all the way. Youtube always a friend and showed exactly where to adjust full throttle mixture. Like you I searched and figured at worst case IF I couldn't get everything back together and running correct, $30 and 2 days later I'd have the new carb installed. Much cheaper than shop time just to look at it and replace carb anyway sonce it's only 2 screws to hold it on.
I have a km-90r 4mix. Don't think it is the same engine type, but I had similar issues, none of which had to do with the engine type. I replaced the external fuel lines from the tank to carb, tore down carb and cleaned all the diaphrgams w/ seafoam (iirc) then reassembled, and swapped tanks thereby replacing the internal fuel lines. Reassembled and all is well 3 years later. I run only premium fuel w/ startron and synthetic G-oil or Stihl oil (mostly for the 4mix valve cleanliness). Combination seems to work well so I'm not going to try to fix it til its broke again.
If it's a Stihl, the gas line should last a long time. Before you spend any money, empty the tank of fuel. Blow out the tank with compressed air. Push the primer bulb several times and blow out the tank again. Do this three or four times, then put fresh fuel in and try it. Worst case, is you'll have to add a little Sea Foam or some other additive to the fresh gas to finish the clean job after it starts running.
FWIW my small engine shop got mine running for $65. It was a Stihl with a similar issue. They replaced part of the carb, but not sure which piece. Hasn't messed up since and that was about 3 years ago. I would go with the new factory carb and install myself.
One thing I do with small engines that seem to be 'rough running'. Give it some either(starting fluid) and spray 'carb cleaner' treatment. Get it running then alternate spraying both into the intake, vary throttle. Do it enough to clean the inside of the carb, not so much at any one time to damage anything. I also burn non-ethanol fuel in all my small engines, easy to find in WI.
Not open for further replies.