Stihl 4-mix FS-110 Low Compression - Barely Runs

[SteveSr]

I received this from one of our volunteer trail crew with the complaint that it would barely run. New air filter, sparkplug, OEM carburetor, cleaned spark screen made no difference.

Valves were a bit loose but adjusting them also made no difference. Pulling the rope feels weak even for a 4-mix.

Pulled the muffler and the exhaust port had some black carbon in it but didn't look too bad. Spark plug looked oily and black. Top of the piston through the spark plug hole looked oily and black but could see aluminum piston in spots.

I improvised a leak down test with my Mityvac tester and spark plug adapter. I could hear air leaking out the exhaust valve. When I covered the exhaust port with my thumb I could hear gurgling through the carburetor.

So obviously the valves are leaking somewhat, probably due to carbon buildup. Is there any way to fix this without a complete teardown of the engine? Is it too late for Stihl's engine decarbonizer? Anything else worth trying?

Extra Credit:

1. What is wrong with the photos below?
2. Did/does it have anything to do with the current problem?

 

[dmb2613]

Nay not help, but will not hurt anything

 

[67L36Driver]

Got this on the cam lobe?

Its a compression release built in.

They stick. Remove that crap.

 

[SteveSr]

Got this on the cam lobe?

Its a compression release built in.

They stick. Remove that crap.

I haven't pulled it apart that far yet. I have read that they can be problematic. However, the compression release is out of the picture when performing the leak down test. I don't know if a sticking compression release could have caused the initial problem but the current issue is a carboned up and leaking exhaust valve.

BTW, I spoke to the primary user last evening and he said the trimmer went from "working fine" to dead in a couple hours of run time if this provides any clues.

 

[legdelimber]

Bonus question? Line cutter missing from guard?

I'd expect most trail work has enough rocks to keep the line length beat back.

 

[PissRev]

Comment to bonus question:
Ditch the guard and switch the Stihl 26-2 head to a SpeedFeed.
I can't diagnose the problem with this engine, but I won't have any more 4-Mix trimmers ever again.

 

[SteveSr]

Ditch the guard and switch the Stihl 26-2 head to a SpeedFeed.

The guard is there to cut the line to keep from overloading the engine. I did put a speed feed on one of the trimmers that I rebuilt. However, for a non-profit trail club replacing otherwise working equipment is not economically viable. BTW, the volunteer end user doesn't know any better !

I can't diagnose the problem with this engine, but I won't have any more 4-Mix trimmers ever again.

So I am guessing that have you had issues with 4-mix engines... What kind of issues have you seen?

 

[SteveSr]

I finally got some cooler weather and mostly time to tear into this engine. The main issue found was that the exhaust valve was crusted with hard carbon and leaking like a sieve. The exhaust passages behind the valve and to the muffler were also caked in fairly soft carbon. The inside of the engine was also covered in plenty of mix oil. I suspect that this accumulated due to poor running after the exhaust valve started to leak. The inside of the engine and valve train was covered in soft black carbon and oil that will wash away with fuel mix. Other than this the engine looks almost brand new inside.

Unfortunately, the root cause is still unknown. At this point I am speculating that the engine and cam gear got hot and caused the decomp cam to get stuck and allow carbon from the poor combustion to get hammered into the valve seat. @67L36Driver does this look like your sticking decomp cam results?

I am sort of loathe to remove the decomp cam as it will put excessive strain on the recoil assembly and rope.

 

[SteveSr]

The remaining photos...

 

[RKoger]

Clean it up, lap the valves and then put it back together. Set the valves at .010 MM. Make sure to use new crank seals. The very next thing to do is see how and what oil they're mixing in the gas and be sure they're using 89 octane only. A high quality synthetic oil mixed at no fatter than 50:1 is required. I use Amsoil mixed at 80:1 in nothing but name brand gas and I have zero problems with any of my Stihl tools, both 2 strokes and 4 Mixes.

 

[Gaston]

In my experience, I take the valves and clean all the carbon off with a wire wheel. Then lap them good. I have seen piston rings wear and have pretty excessive gap. Make sure you check the crank bearings and the large end rod bearing. Have seen a couple with large rod bearing failure making for a noisy engine and varying compression readings.