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Possible Low Compression Effects?

SteveSr

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2-Stroke Gurus,

I am having issues with a Stihl FS120 (4134 series) 30cc brushcutter. It acts like an intermittent sticking main nozzle check valve. Sometimes won't idle or stall at idle. Sometimes won't rev up. Tried a couple of different carburetors but all from the same source so may have multiple bad carburetors. These carbs are marked as Stihl C1Q-s161 but are NOT from a Stihl dealer. I am begining to think that they came out of the Zama scrap bin in China!

The other issue is that the compression on this unit is about 10-15% less than a couple of other similar units that appear to function. Now, these units don't seem to have a lot of compression to begin with so I am wondering if all or part of the issue with this one is the somewhat lower compression. It does seem to start easy which I wouldn't have thought if it was too low compression.

So the next question is if the compression is deemed too low should I just re-ring it or also replace the piston?

Thanks,
Steve
 

AlfA01

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Hi Steve,

Before you tear it apart, I'd do some pressure/vacuum testing. Some erratic running can be caused by airleaks. Check the intake boot, fuel lines, impulse line and all seals on the unit. If it consistently runs crappy, I'd especially check everything downstairs in the crankcase, ie., crank seals. And, don't be afraid to throw a new plug at it.

I answered on your compression tester post as well. Is that a single ring or two ring piston? I'm not sure on the brush-cutters. You can normally get away with re-ringing, as long as the piston tolerances aren't completely out and there's no scoring or other damage. You'll need to get a hold of a shop manual, or a Stihl smart dude to give you the tolerances for that model/piston/cylinder setup. Make sure you check for what I call 'ghosting' wear on the piston skirt. This can happen when the piston to cylinder tolerances are high and the piston floats a bit in the cylinder when cycling, which causes wear on the skirt more than other areas of the piston. Depending on the condition of the ring, this doesn't occur at the top of the piston as the ring compensates for the higher tolerances between the piston and cylinder.

I'm having a similar problem on an Echo top handle saw. It has compression (128psi) and actually runs well from cold to about 10 minutes. After that it idles fine, but stumbles every time you hit the trigger. And, if you actually try to throttle it up hard it dies. I'm planning on re-ringing it and going from there. I would go ahead and check compression when its hot, but I've already torn it apart. Its a friend's saw, and he wants it re-ringed, so I'm doing it. Planning on checking all the problem areas as I put it back together.

Good luck.
Dan
 

Wonkydonkey

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It does sound like air being sucked in the wrong place, ie a cracked fuel line/carb diaphragm or fuel filter blocked , if it’s a little low on compression, it would not make it run erratic.
Or it could be just a dirty carb,

As for reringing it , and a new piston, it depends on where the wear is. Ie if there’s lots of wear in the piston ring seats. Or if there is just to much ring end gap while seated in the cylinder.
 
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