fs 90 won't run

[jorgy]

I have stihl FS90 weed trimmer with lots and lots of hours on it. It always started and ran very good. I used unoxygenated gas for mixing always. This summer it would start and when switching off choke it would die and then would not restart no matter what. I would wait a week and same scenario.
Compression is 85 PSI. Spark is good and consistent. Plug would always seem wet with gas. I replaced new Carb, spark plug, fuel filter, air filter, both plastic caps from fuel tank to carb with associated "o" rings. I did use same rubber lines, but couldn't see any defects in them. It started today and ran about 5 sec and then nothing.
I'm stumped. Any help would be appreciated.
tks,
Gary

 

[StarTech]

Well you are a little designed spec of 6 bar (87 PSI).

From service manual....

If the compression pressure is below 6 bar, check valve clearance
and readjust if necessary. If this produces no improvement,
check the cylinder, valve seat, piston and piston rings for scores or
other damage.

The design specification is 6...7 bar.

Also check for a partially spark arrester on the muffler.

 

[jorgy]

the spark arrester was dirty but not bad, also I'm not sure what compression below 6 bar means pounds per square inch was right around 85. I forgot to mention that I did adjust valves.
Gary

 

[Fish]

How did you adjust them?

 

[jorgy]

I cut a feeler gauge with a scissors to fit they were originally pretty loose. This was done before new carb and all other new parts. I watched you tube video for clearance of valves, which I can’t remember right now.

 

[Fish]

Yeah, I modified a feeler gauge to do it myself.

Plug would always seem wet with gas.

Sounds like it may be "floody".
Are you using fresh fuel?

Fuel older than a month or two should be discarded.

 

[StarTech]

the spark arrester was dirty but not bad, also I'm not sure what compression below 6 bar means pounds per square inch was right around 85. I forgot to mention that I did adjust valves.
Gary

Well you take the bar spec and multiple by 14.5 to get PSI spec. Your gauge can off some too so your reading can be plus or minus the actual PSI it reading.

As the plus being wet I would check the carburetor metering diaphragm for stiffness as when get stiff they allow the carburetor to flood the engine.