At Wits end - Stihl FS 55R

Wannabe...

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My son did some weed eating a couple of weeks back and took a break then couldn't get the trimmer to start. It's been getting finicky and I figured needed a tune up. I could get it to crank but not stay, so I ordered a carb, plug and filters.

Put it on and I could get it to start and idle, but not rev up. All adjustment on the high side did not help. Did some reading and found out about the duck bill under the cap. I figured that couldn't be it, but on a lark, I removed the cap and cranked and it ran like new. So, removed the cap, slit the duck bill, re-installed and I haven't been able to crank it since. With or without the cap. Back to reading. Put the switch on an ohm meter, good. Checked spark, good. I'm ready to toss the thing and get a new one....except my pride won't let me surrender.

Here's what I've done:
Fresh fuel - 50:1
New air filter
Removed spark arrestor and cleaned - tried with arrester removed as well
Replaced carb - amazon aftermarket
New gaskets for carb
New air filter
Plug (from kit and a cross reference from the original NGK)
Checked switch - good
Checked spark - good

What am I missing? Someone save my wallet, hide and pride!

Wannabe...
 

Wannabe...

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Oh, I left out a couple of things:

New fuel filter
New fuel lines
Have tried a quick shot of starting fluid too.....yeah, I know.


Wannabe...
 

bertsmobile1

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What you have done is 100 % the wrong way to go.
Now you have 5 things that might be wrong.
The only way to fix these hand helds is to diagnose the problem first then rectify that problem.
So your problem or multiple problems could be anywhere.
Get a neon in line spark tester and some starter fluid and a fresh new spark plug.
Install the spark tester remove the air filter and give it a little and I mean little shot of carb cleaner down the throat and give it a few pulls with the throttle wide open.
It will be a lot easer if you hold the trimmer in a vice , or if you have one a folding work bench where the whole table top is a vice.
If it does not fire in 5 or 6 pulls, pop in the fresh plug & try again.

If not joy there, remove the muffler and repeat.
No joy again, get a strong torch & carefully inspect the piston & bore.
If it is scored then the engine is toast

Compression on these engines is really critical and very deceptive.
Anything under 80 psi is not going to work but because the flywheel is so small , it feels like you are turning over a deisel and the old thumb over the plug hole just does not cut it.
I have had units "with plenty of compression" that nearly give you a heart attack pulling the handle test out an 40 psi

By far the best treatment of 2 stroke diagnostics is the series by Joe Pace, called 2 stroke diagnostics, what else. Find it on EWE Foob.
You have to be methodical with these things or you just end up going around in circles .
If your cap is bad , replace the cap as the fuel tank on some hand helds is pressurised just like the old outboards.
Cube carbs are troublesome little critters and most of the stuff you buy from the web is junk rejected by the original purchaser.
I could not tell you how many time I have swapped the "brand new " carb a customer bought from the web on the engine that would not work, for the original carb and had the unit singing in no time flat.
Or how many times a customer had fitted a "new " fuel line that bought off the web, that was delivery line, not supply line so it simply collapsed where it went through the grommet as soon as the engine started to run.
Then there is carb cleaner which attacks the vitton tips of the diaphragm needles and the check valves or even worse the clowns who "blow out" their cube carbs with compressed air at 120 psi thus embedding any debris so hard it can never be removed.
 
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