I have a Stihl FS55 trimmer that I am fixing for a friend. It wont start. The first thing I did was check for spark, it had none. I narrowed it down to the ignition module and replaced it. Still not starting. Then I took the carb apart and cleaned everything up with no luck there either. I put the carb on my personal FS55 that works and it worked fine, so that rules out a bad carb. I took the flywheel off to make sure the key was not sheared and it wasnt. So as of right now, I dont know where to look next. It has plenty of compression and the cylinder walls are smooth. What am I overlooking? I just noticed the primer bulb is cracked but I think that is just from me pushing on it a million times the past few days. Could that be the problem? Any help is appreciated.

 

I forgot to add that when I do try to start it up, it will backfire through the carb when it is on full choke.

 

Have you put a new spark plug in? What type of plug does it have? Is it a resistor type of plug? If so, they have been known to look fine on the outside, but inside the little resistor assembly isn't fine and it can cause crazy backfiring. Went through this with a chainsaw about two hours after Hurricane Ike.

It sucked big time.

 

Yes, I have put in a new spark plug already with no help. It seems to be a timing problem but the key on the flywheel wasnt broken, so I am kind of confused. Every time I pull the plug after trying to start it, it is wet. So it is getting fuel, its just not igniting at the right time I guess.

 

Only thing I can add to the mix is check the muffler. There is a screen between exhaust port and muffler on a lot of trimmers that clogs up from burning oil and will screw them up big time. I've fixed 5 or 6 that were headed for the trash heap due to that simple thing.

 

sure you didn't put brake fluid in it? /K ( I hope you know what this is reference to)

I was going to vote the fuel mix. But what do I know lol. I hope you get it fixed. I am suffering chain saw woes as well. will soon turn into withdrawls.

 

I'd check the muffler like it was said, back firing out the carb could indicate the exhaust has no where to go

 

put a lil gas in the cyl and pull it, see if it fires

 

Man I know what your going through. Hope you get her going.

 

No, I didnt put brake fluid in it, lol. I read that one too. The muffler isnt the problem either. I took the screen out of it earlier , it was clear too, and still nothing. I even tried starting it without a muffler, and still nothing. This one has me completely stumped. It is getting a good blue spark and it is getting gas, so it should at least fire up, right?

 

I'm working on an fs66 right now.
have you checked the fuel filter?
Also play with the choke, I have a blower that only works on half choke.

 

Also does anyone have any idea why my pickup coil won't make spark I've replaced it so I don't believe it to be the coil itself I'm wondering if it's a bad ground out something but all connections look good just can't get it to make any spark at all

 

https://homeguides.sfgate.com/adjust...er-102856.html
Most likely you need to get the adjustment right. The rotor needs to spin fairly close to the pickup.