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I have a Stihl FS 55 trimmer. It always started easy in previous years. It sat over the winter, but I did drain fuel before winter. Now for the first start in the spring, it did not fire at all.

- I changed spark plug.
- I checked for spark and it has spark. Checked with putting plug against case and using spark tester.
- I took carb off and cleaned carb. I did not replace any diaphrams or other parts in the carb. Carb looked really clean before I cleaned it.
- Checked spark arrestor and it was clean.
- Tried higher octane fuel mix.
- Checked compression and it was in the 80-95 range.
- I tried putting fuel and carb cleaner in piston, still no fire.

After sitting the winter, I can understand if there was a fuel problem due to something in the carb. But I am thinking it should at least fire when I put fuel or carb cleaner in the chamber and air filer inlet. I did not test the kill switch, but since I get spark, I am thinking that should not need to be tested.

Any ideas on what to check next? Thanks so much.

For small 2 strokes usually if the unit is capable of running you can get it to start by holding the throttle wide open and pulling it as quickly as possible several times with the choke off. I suggest that you try that if you haven't already.

Ok, it started with choke open and throttle pulled all the way in after a few pulls. But it will immediately stop when I let go of the throttle. I tried several times, and it will start but immediately shut off when I let go of the throttle. What problem does that point to?

mikulskn said:

Ok, it started with choke open and throttle pulled all the way in after a few pulls. But it will immediately stop when I let go of the throttle. I tried several times, and it will start but immediately shut off when I let go of the throttle. What problem does that point to?

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Sounds like something in the needle valve took up residence, try another look in the carb, THOROUGHLY .........

Well I took the carb off and gave it a good cleaning again. In doing so, I noticed a small crack in the primer bulb. I replaced it with a primer bulb from a different old carb I had laying around. After installing the carb again, it started up and ran fine. So I am assuming that the problem was the bulb, even though it could have also been due to another cleaning as well. Having a cracked bulb though seems to be the logical problem since I was wondering what could went wrong just sitting over the winter. Usually it is a fuel clogging problem from sitting over the winter, but I can see how the rubber bulb finally just became brittle enough to crack over the winter such that it started to crack when I first primed it after the winter.

Thanks for the help and advice!

The cracked primer bulb could let air in. & with it empty the bulb sounds like it dry rotted.

I never drain my stuff for storage, I use 100% gas & I still add Seafoam*

FWIW :- The primer bulb has a check valve in it which keeps the fuel tank pressurised ( 5 to 7 psi ).
This pressure is necessary to get the fuel to the fuel pump under the carb and for the unit to work upside down.
A crak or leak in the primer, or the return line to the tank will cause the unit to stall out soon after starting.

I know you got it fixed. Glad to learn how.
My neighbor dropped off that same trimmer off for me to look at. Hard to start, and no power. Turned out to be a clogged spark arrestor. Just FYI .