Causes of low Compression

[fidiro]

0.006 or 0.007 to 0.009 is good...no tighter than 0.006 though. If it is you will have to file the ring ends lightly but equally on each side to get to 0.006 to 0.007. In the 0.020's is way to much and very likely the reason for your low compression.

Thanks. I think I will be OK then with the aftermarket gap. If I have to pull saw apart again after that it's OK too. I had to order some more gasket maker as the tube I was going to use now is a bit old and silicone is coming out a little lumpy. I am using Permatex MotoSeal on this saw to see how it holds up.

I have a pic of the ends of the aftermarket rings I bought and the oem I pulled from saw. I don't like the way the aftermarket ends ramp out so far. OEM definately has more meat at the ends and rounded over a lot nicer. I have a spare Caber ring for my 7311 and the ends on that ring are better than these cheaper ones I bought now. I should have ordered Caber but didn't want to wait longer and these were $5 less, I'm a cheapo so I get cheapo. As long as it works. I may just go ahead and order caber to keep just in case.

 

[Eccentric]

Looked at piston again from exhaust side and pushed both rings and saw flexing on both. Took Carb out to see the same perfect piston, no scoring. Intake side only shows bottom ring and I push it and saw some flexing as well.

Don't know what to make of it at this point. Looks like I'll have to pull it all apart. Could it be a leak in crank seals that will cause low compression? Don't know if the bottom end has anything to do with topend compression so I can only see a ring issue at this point.

Piston looks perfect from both sides. Cylinder looks perfect from exhaust side but can't see much from intake side. Does this still look like it will be a worn/split ring issue?

Anyone have anything else I should test before opening up this clam? I should've tested the impulse line to rule that out but that was already mentioned that will not affect compression testing.

The rings are probably worn off on top where the ELKO identification number was, and yes they are OEM Stihl.

See Elko Demoversion

Use the piston to push them in evenly, making sure the ends are where they would normally ride in the cylinder. New OEM ring gaps are about 0.008-010", but have seen over a 1/2mm on worn out ones or crappy aftermarket rings.

Yep I was going to say the same thing....measure the end gap on the old and the new...that's the only way to tell how much wear has actually occured. Use the above procedure as that is the correct way...Good luck...

0.006 or 0.007 to 0.009 is good...no tighter than 0.006 though. If it is you will have to file the ring ends lightly but equally on each side to get to 0.006 to 0.007. In the 0.020's is way to much and very likely the reason for your low compression.

There's the answer. I'm late to this party. I kept saying to my monitor "check the ring end gap" as I read through all the posts here. The cylinder, piston, and rings can all 'look' great (and the rings can be 'springy')...................but if the rings are worn out then you'll still have low compression. For example, my McCulloch Super 250 has a P/C that look fantastic through the exhaust port. The rings look great too and have good 'springback'. However, the ring gap was almost .125"! Compression was a tad low....

Make sure you get an accurate measurement of ring gap with the new rings before you put it back together. You really need to put the rings into the cylinder & measure the end gaps to tell. Buy a new feeler gauge set. You'll need one anyways, and they're cheap. If the ring gap is too tight (and it can be, especialy with cheapo aftermarket rings), then the gap will close once the rings heat up and the saw will sieze. It may break loose right afterwards (so it won't "feel" siezed when you pull on the rope)......................but the damage will already be done. Don't skimp on a few bux by not buying another feeler gauge set............only to burn through a c-note or more on a new jug...

 

[fidiro]

This diagnosis has come a long way. There would be no other way other then to pull saw apart to check and measure everything. I just wish I had my tools in order so that I can use the gap gauge I have somewhere to get a more accurate measurement on the new ring end gap.

I will have to get something to read it better even if I cut up an aluminum can for now and measure the layers of the can with the caliper.

 

[fidiro]

I tore strips from a couple different aluminum cans(different thicknesses) and measured the one that fits best in the end gaps of the new rings. Measured the aluminum strip with caliper and came up with .008 - .0085 I ordered another gauge to get a better reading but it is pretty close using the digital caliper.

If I get the gauge at the same time as the MotoSeal I'll recheck the gap. I just want to get it all back together again before I lose any parts and if sealant gets here and I have time to do it that day, It's going back together.

Thanks for all the tips and help. I learned quite a bit about rings on this thread. The best part is learning how to take the dreaded clam saw apart and getting it back together, I'm so good at it now that once this one is back together the 390 is getting new rings as well.

 

[Cantdog]

Well it doesn't really matter what you use as a feeler gauge as long as the fit is correct...not loose and not so tight you have force it in. If you know the "feel" of a "feeler gauge" fitting properly and it measures 8-8 1/2 thousanths I'd say you're good to go. Remember a "Correct" tool doesn't change anything just makes it easier to do the job....if you know what the tool is for and how to use it most anything can be adapted to work.

 

[fidiro]

Well it doesn't really matter what you use as a feeler gauge as long as the fit is correct...not loose and not so tight you have force it in. If you know the "feel" of a "feeler gauge" fitting properly and it measures 8-8 1/2 thousanths I'd say you're good to go. Remember a "Correct" tool doesn't change anything just makes it easier to do the job....if you know what the tool is for and how to use it most anything can be adapted to work.

Absolutely. I just ordered the feeler gauge to just have it around and if it gets here before I have time to put saw back together I'll use it. The feeler gauge I had had about 15 or more leafs(don't remember exactly) and the one I just ordered has only 8.

At first I was hesitant to take this saw apart and was always hoping it would be something simple because of all the horror stories I read about clam saws and how much of a PITA they are to work on to take apart and put back together. I started looking for an easy fix first to avoid pulling it apart and there was no easy fix. Now that I pulled it apart 3 times, I am comfortable working on this type saw. Pro saws may be easier to do a top end but if I ever had to do a bottom end I hope it's a clam.

Hope when I get around to rebuilding the 440 it doesn't take three teardowns to make it perfect. But practice makes perfect.

 

[SeanNJ]

Did you ever get to do a final compression test after you installed the new rings? Just curious because I'm in the process of rebuilding a 029 Super right now...

Thanks!
Sean

 

[fidiro]

Did you ever get to do a final compression test after you installed the new rings? Just curious because I'm in the process of rebuilding a 029 Super right now...

Thanks!
Sean[/QUOTE

I guess it would help for me to keep this updated. I recently finished getting it all together and tested compression, didn't like what I saw. 115 psi.

I don't know if it was the super cheap aftermarket rings I went for or if the landings on the OEM piston are too tight. When I threw rings on, they kinda felt a little tight in the landing as they did not move freely enough, but I threw it all together anyway. End gap was good but they felt like they were rubbing a little. Now I'm probably going to tear it down AGAIN and order a different set of rings. It starts but can't get it to idle/run properly with that low comp. I'm actually going to try and run a couple tankfulls (if it stays running) and cut some wood to see if it fixes itself.:cool2: first and if that don't work then apart it comes again.

Now, another story of low compression on a different saw. My ms390 was only putting out 130psi when I first tested it when I got the tester. With the issues I was having with the rings on the 029 I automatically thought the 390 was going to need the same thing and yesterday I started taking it all apart and got down to just about tearing open the cylinder and forgot the damn thing has a decomp:msp_mad: Guess what happens then. Well I'll just say it, I cleaned the carbon off decomp and reinstalled it. Threw the tester on and grabbed the drill with a 13mm socket to turn the flywheel, 162psi. I just wanted to tear that one down to give it a good cleaning.:bang: Yeah. At least I stopped just before I removed the cylinder bolts and it did get a really good cleaning. Also cut off the limit tabs on the carb, still have to set the high needle out in the field.

How many 1127's, even if it's the same one numerous times, does one have to take apart to be a clam pro?

I just have a feeling if I get any time cutting with the 029 that it will still need to come apart once again. I have lots of wood to be cut so if this one don't cut the 390 is ready.

 

[SeanNJ]

Hi Fidiro, thanks for the reply and the update.

It sounds like we are having fairly similar experiences. I just put my 029 back together and put some fresh gas in it just a couple minutes ago. I got it to start but it's not running right. It will idle and it will rev up when I hit the throttle but it won't rev down. It dies. Some times it will rev down to an idle but most of the time it will just die when I let up on the gas and even if I hit the gas again, it will just bog to a stall. Not really sure what that means.

I originally had a scored piston and cylinder and I got a good deal on an original used Stihl piston, cylinder, rings, etc.. All in really good shape. After the rebuild the compression is about 120psi. Does that sound low? I'm thinking I should have put in a new pair of rings instead of the used pair.

Yeah, I think I may be opening up the clam again too. I'm not really sure what to do at this point. How many times will this be for you? You can probably do it with your eyes closed now. Haha! I was intimidated at first. All the shops that I talked to made it seem like a nightmare. It wasn't too bad. I took a lot of pictures along the way to help me put it back together.

Did you have to reallign your ignition? Just curious.

Wow. That's funny about your 390! You got lucky. My luck isn't so good. Seems I always have to do everything twice. How do you like the 390?

Thanks for the info. This is my first post. I think this is a great forum. Lots of knowledgable folks here!

 

[fidiro]

BTW, welcome to AS.

Even if you didn't take pics you would have plenty of help here to get that thing back together.

The 029 will come apart for the 4th time. The 390 was not completely apart but it was the first time to get it mostly apart, I didn't break loose the cylinder as that was the next thing to come off. Remember always do the easy stuff first, like the decomp.

120 psi is not good enough for these clams, although my 440 tested at 110 and it runs pretty good.

Other then the 390 being a clam saw I have nothing bad to say about it. Don't know if you know but the 390 and the 029Super are identical saws other then the piston/cylinder. 390 has 49mm 290/029S have 46mm. When you replaced that cylinder/piston you could have searched for the 390 and it would have bolted on just the same and you would have a 64cc saw, you would also have to change the jets on the carb of the 290/029S.

Nothing changed on the ignition.

 

[SeanNJ]

Thanks for the welcome to AS.
I don't think I would have done the rebuild if it weren't for some of the other posts that I've read lately. There was one where Fish (I think) posted a bunch of pics and I printed them out as a guide. I'll have to thank him one day.

I did read about the 390 and 029/290 match. I did a quick search on a 390 cylinder and didn't find anything. Plus, I didn't look to hard because I didn't want to get in over my head with tweaking the carb and jets. Bit it would be nice to have a 64cc saw!

So, if 120psi isn't good enough for the 029, then I guess I'm back to the drawing board. Would you know if that's typical behavior for a saw with low compression or weak rings? I'm wondering if the crank seals may be part of the problem too. If I'm going to open it again I may go ahead and do those too along with a new impulse line.

I see you're in Jersey. I'm up in Sussex county. Burning lots of wood these days. I just installed a large wood burning insert this winter. It's already paying off. I'm saving lots of money because oil is so expensive. Cut my oil bill by more than half! I grew up in Middlesex county.

 

[fidiro]

Yep, Fish knows these saws.

I'm in Middlesex County. I've had my gas furnace switch off fulltime for the last 4 seasons since I went full time on the wood stove. I've also added the pellet stove on the other side of house at end of last winter to help with those really cold days. I must have burned about 5 cords of wood and 1 ton of pellets so far this season.

I have about 120 yards of 20' long logs to cut and split and may just bring out the 029 and the 390 tomorrow to see what else either may need. I'm saving the 440 for the bigger diameter logs.

Low compression definately plays a part in your problem. We both really need to see these saws at 150 or better with new rings. Have you rebuilt the carb. If fuel line and impulse line look cracked I would replace them.

 

[SeanNJ]

I've spent most of my life in Spotswood until I moved up here about 10 years ago. We always heated with wood back then and I'm doing it again now. Well, I'm trying to at least. Haha..

Lots a lot of wood you're burning. I'd like to turn my furnace off fulltime but I can't get the heat to the back of the house so I'm supplementing with the furnace at night. Luckily, we broke our heat out into 3 zones when we remodeled. So I only have to turn on the back zone at night if I want to.

Let me know if you make any progress with the 029 this weekend, if you don't mind. I'd be curious as to what is happening.

No, I haven't rebuilt the carb yet but I definitely thought about it. I think I may start with new rings first. How the heck did you get your rear bar stud out? I FUBAR'd mine in a vice trying to get it out. Had to get a new one to replace it.

The fuel line and impulse line looked good. I even blew into the impulse line with my finger over the other end and it seemed good. But who knows. There may be a pinhole.

 

[fidiro]

I would just throw in the carb kit if you don't know if it's ever been done. It's only about $8 shipped and easy to do. Should be kit K10-HD by Walbro.

I chose not to remove the bottom end and did not even attempt to remove the stud. I've had no problem removing and reinstalling the jug that way. Just slide the piston in and move it to TDC to bring the crank weights up into the cylinder so they clear the bottom case. Try and get that position of the crank and dry fit it into the case bottom a couple times so you get the idea where crank needs to be before you apply the silicone sealant, this way you don't get it all over the place. But since you already removed the stud recently it may be easier to remove now, don't know. Try it first without removing the bottom.

 

[SeanNJ]

I'll take a look for the carb kit. Thanks for the info.

I was trying to do the rebuild without taking the bottom end out like you did. But I had a heck of a time getting the piston and rings in. I couldn't see the rings and couldn't get them started so I decided to go all the way. If I open it back up again, I'm going to try and leave the the bottom case. I may order the rings from Bailey's and they also have a ring compressor for a couple bucks. That may being one of those tools that's worth it's weight in gold. Ha.

Good luck with the saw today. I'm going to go and take another look at the carb and fuel lines and see if I can find anything. But I'll still have the 120 compression and that still bothers me...

 

[fidiro]

Yep 120 is not good. It should start but run poorly.

I install the piston into the cylinder first and then hold both the crank and cylinder together with weights up into the cylinder and slide them down at an angle into the bottom case. With the weights up it's actually BDC not TDC like I said in above post.

To install piston into cylinder I have the cylinder opening pointing up on table and drop piston in until the first ring rests on the two lips of cylinder that are opposite of each other. Then I gently push one side of first ring until it slides in a little(piston will be a little slanted until other side of same ring slides in) then I do the other side of same ring until the second ring touches and piston is straight again then I do the same to second ring. I use a tip of a drywall knife to gently push the rings being careful not to break a piece of the tip of knife, making sure it doesn't leave the tip of the knife blade in the jug. I always checked the tip of blade to make sure it didn't break off before I completely seal jug. Need to add that I push the ring in right where those cylinder lips are where ring rests, the lips are about 1/4" wide, don't know what they are called but they separate the channels that run up and down on cylinder.

Pushing the ring on first side is easy but if end gap is not lined up with the little pin in piston, the opposite side of same ring will not push in so don't force it just pull it out again and make sure it's lined up close. I also use a clean paper towel or rag and soak some oil on it to wipe the inside of cylinder to leave a film of oil. Weight of crank and piston alone will almost drop into cylinder when rings are pressed.

 

[fidiro]

I had the 029 out and got it to start and ran a fuel tank through it. If I let go of the throttle it would die so I kept triggering it to stay running and was able to buck about a face cord. I refueled it but could not get it started, so I quit. Brought it home and tested comp. and was only able to get 85 psi.

I'm going to tear it down again soon to recheck end gap with feeler gauge and check to see if rings are getting stuck in landings. I may have to lightly sand the landings of piston to see if rings free up, if the stuck rings is the case.

The OEM piston is in great shape and would hate to go buy an aftermarket. Hope I can get compression up by loosening up the rings some in the landings. Don't know if it's the aftermarket rings either as I didn't like to see the finish on the ends of them, they looked like it would let compression sneak by some.

 

[a. palmer jr.]

I bought an 021 Stihl the other day and it has low compression. I took the muffler off and expected to see the piston and cylinder all scarred up and ugly but to my surprise it looks to be in terrific shape. I'm kinda at a loss now, other than maybe having a stuck ring or something like that. Mine doesn't have the de-comp. I'd guess the compression at under 80, no gauge. I hate to tear it down just yet until I'm sure that's what it is. I looked inside the port with a flashlight and it's better than most of my saws that run. Any ideas?

 

[SeanNJ]

Fidiro, sorry to hear the 029 will be back on the work bench. It gets frustrating after a while. You'll be able to do a rebuild with your eyes closed soon.

I actually have some good news to report. My 029 is running pretty good now. All I did was mix up a fresh batch of 50:1 fuel. And now it runs good. Friday I mixed a batch of 40:1 and it didn't like it. Not sure why. I tried to cut with it but the chain is a little dull and I didn't want to strain it so I stopped. I'll do another compression test to see if anything changed.

Funny thing; I went to a Stihl dealer to get a couple spline screws that I misplaced and asked him what the compression for the saw should be and he told me at least 80psi.


A. Palmer, that's good news that your piston and rings look good. Most likely means your cylinder is probably good too. You may want to try and look at the piston and rings from the carb side which means you'd have to remove the carb.

Poke the rings and see if they're stuck in the piston. When you have it open, pull the starter rope and listen for excessive air leaking past the rings. Sometimes it helps to put a couple drops of premix oil into the cylinder so you can hear the air hiss. Maybe the rings are just too worn out to hold enough compression. Just a thought.

It may even be an end gap issue like Fidiro. The end gap is where the two ends of the ring meet.

Does it run at all? If so, how does it run?

If you decide to open her up and never did it before, there are some other threads here where people posted some pics. Ya just have to do a search.

Also, check out these videos of a MS250 rebuild on Youtube.

This one is called "Stihl MS250 Chainsaw Teardown":
YouTube - ‪Stihl MS250 Chainsaw Teardown‬‏

This one is called "Repair of Stihl MS250 Chainsaw Part 1 of 4" and has 4 parts:
YouTube - ‪Repair of Stihl MS250 Chainsaw Part 1 of 4‬‏

Hope this helps.
Sean

 

[SeanNJ]

Fidiro, sorry to hear the 029 is going back to the work bench. That is frustrating. You're going to be a pro with these saws.

I have some good news to report. My 029 is running pretty good now. All I did was mix up a fresh batch of 50:1 fuel. I doesn't cut too good because the chain is dull so I stopped cutting before I strained it.

On Friday I mixed up a batch of 40:1 fuel and it didn't like it. It kept stalling.

I'm going to do another compression test to see if anything changed.


A. Palmer, that's good news that your piston and rings are good. Probably means your cylinder is good too.

Try checking the piston and rings from the carb port. A little harder to do since you will have to remove the carb. Try poking the rings to see if they're stuck in the piston or if they move.

Another thing you try is to slowly pull the starter rope while you have the muffler or carb off and listen for air leaking past the rings. Sometimes it helps to put a couple drops of premix oil down the spark plug hole so you can hear the hiss or even see little bubles around the rings. If it's a lot of air leaking, it may be that the rings are worn or too weak to hold good compression.

It may also be an end gap issue like Fidiro. The end gap is where the ends of the ring meet.

Here's some helpful videos if you decide to start tearing it down:

YouTube - ‪Stihl MS250 Chainsaw Teardown‬‏

YouTube - ‪Repair of Stihl MS250 Chainsaw Part 1 of 4‬‏

Hope this helps,
Sean