stihl ms 261c troubles

[plotthound]

this is a saw from work that has an issue,it was taken to the stihl dealer and was diagnosed with poor compression. damn thing wont rev out has no power.so I took it home and put the gauge on it,i get 75 psi with a dry cylinder and almost 100 psi when I oil it up.removed the spark arrestor and it was nice and clean,checked the spark at the plug and it was weak and intermitant.just wanted to confir with you guys before I order a new $152.00 ignition module. im not sure what the psi should be but it seems good to me.what should I try next???

thanks

[bgsengine]

Nope. probably have to pop the jug and get a closer look at rings and cylinder, or better yet crankcase pressure/vacuum leakdown. most more modern 2-strokes won't run well until they hit at least 120 PSI 100 is just too low and the fact that you get 75 "dry" and 100 "wet" leads me to think you got bad ring, possibly a broken ring, or at the very least, worn out ring. Should not have to do a wet vs dry compression test on a 2-stroke - if it has run within the last 30 days or so, you'll want it at 100 at an absolute minimum, without doing anything like adding oil. typically they'll be 120 - 150 PSI to be in spec. Just be sure you are doing your compression test right - Throttle wide open, choke open, and pull over until needle on gauge stops moving.

[plotthound]

yes bsengine and thanks for the reply.this saw has been sitting for quite some time.have not had time to look into it.might as well pop the clylinder off and take a look.i found some aftermarket top end rebuild kits fairly cheap.might just do it since ive never had one apart before.
cheers

[RoyM]

Removing the muffer to have a peek at the piston skirt and cylinder wall should tell the tale. Was the carb throat reasonably clean? If dirty somebody neglected air filter maintenance which could be the source of the problem. Do yourself a favor and use a good quality top end, if you are going to that much work do it right the first time.

[tuffnell]

Think twice before ordering a cheap rebuild kit.
Before you do, have a look at "Small Engines Q&A #156" by donyboy 73.

[bobodu]

Bought a cheap kit for my Yamaha G1 Golf cart.....broke the bottom ring because the groove was too narrow..

[KE4AVB]

Most cheap cylinders will not have quality plating even if they are plated. On top of that most are Chinese knock offs which have poor quality controls.

Most of the 2 cycles I see with cylinder damage will not even start due low compression but some will.

But before you take things apart check to see if this saw has a decompression valve that maybe stuck open. Some have them and some don't. A stuck decompression valve can cause one to start but not produce the full revs. This kinda sound like the case since the you indicate it will start and run but just won't rev up fully. If does you can temporary install a M10x1 screw in its place for testing purposes.

[Arkie]

Do you know for sure that the compression gauge you are using reads accurate on a small CC 2 cycle engine? (check your compression tester on another small 2cyle small cc engine that is running good. (note that I did not say check the gauge, check the tester) Some people will think just because the gauge itself reads ok on a air compressor tank or the tester reads ok on a automotive engine all is ok. I've seen people overhaul a chainsaw and when they got through their tester read less than when they started and the saw still not run due to only a minor issue that was the main problem in the first place.

The Stihl saw will easily produce 150 psi or more when new using a correct reading tester and pulling the recoil rope until you get a max reading (usually with about 6 pulls of the rope. If it's 75 psi it has cylinder/piston issues usually) and you do not have to hold the throttle wide open on a chainsaw to get the correct compression readings.

Sometimes you can completely remove the muffler and slowly turn the engine over and look at the cylinder and piston and rings with a strong flashlight to see if you see any scoring. If you do see such and you replace them with new the same issue can re-occur fast if the saw is running lean and overheating.

[plotthound]

thanks for all the advice.the decompression valve works fine and this saw is for sure low on compression.i pulled the muffler off and the piston is scored.watched a couple of donyboy videos and they are quite helpful.looked at a few sites to order a rebuild kit,but having trouble identifying the Italian made kits.will keep looking.thanks again.

[Gerry]

Don't know your part number but Stens number 632-540 is a kit under Stihl part number 1141 020 1200. Its 44.7 mm, Piston, rings, pin and clips