fidiro
ArboristSite Guru
Thanks again for the info. I'm going to try and take the carb off tomorrow as right now I'm getting ready to push some snow(about 8" and meteorologist say the worse is coming later)
Causes of low Compression
- Thread starter fidiro
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check your spark plug. you coulda fouled it when running rich.
fidiro
ArboristSite Guru
It was fouled when I took it off to check compression but that will not make it read only 90 psi. I dried out plug and put it back in to just get it to run really bad and when it dies it will not start again. It has to be a stuck or broken ring.
fidiro
ArboristSite Guru
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.
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.
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Warroad50
ArboristSite Operative
If the saws in question have a lot of hours of running more than likely just needs a new ring to bring it back to life. I would think also theirs nothing wrong with your compression tester as long as it's getting a good seal.
andrethegiant70
Addicted to ArboristSite
Your problem is not uncommon.
Compression is a reflection of how well the piston seals in the bore, nothing else, with the exception of a leaking spark plug or decomp. I've had 2 Dolmar 112s that had nice-lookng pistons but the rings were just tired. The dang things didn't even pop. Maybe they just sat too long and the rings lost their tension or maybe they were worn or both.
How much time do you have on this saw? If everything looks good but rings are tired, it's a pretty sure sign the saw has been treated well in it's lifetime but it does need attention. It sounds like its time for rings at least, but you might want to pressure test it to check seals and find any air leaks first. Also, seeing the machining marks around the TOP of the piston doesn't mean all that much, as the most significant wear will be found in the piston skirts. Good Meteor pistons are only $35 delivered, you might want to look at doing that.
These are easy saws to work on. Provided you have the parts with you and do your homework, the whole job for a newbie is a fairly leisurely afternoon. Replace all the rubber bits while you're at it.
Compression is a reflection of how well the piston seals in the bore, nothing else, with the exception of a leaking spark plug or decomp. I've had 2 Dolmar 112s that had nice-lookng pistons but the rings were just tired. The dang things didn't even pop. Maybe they just sat too long and the rings lost their tension or maybe they were worn or both.
How much time do you have on this saw? If everything looks good but rings are tired, it's a pretty sure sign the saw has been treated well in it's lifetime but it does need attention. It sounds like its time for rings at least, but you might want to pressure test it to check seals and find any air leaks first. Also, seeing the machining marks around the TOP of the piston doesn't mean all that much, as the most significant wear will be found in the piston skirts. Good Meteor pistons are only $35 delivered, you might want to look at doing that.
These are easy saws to work on. Provided you have the parts with you and do your homework, the whole job for a newbie is a fairly leisurely afternoon. Replace all the rubber bits while you're at it.
boutselis
Addicted to ArboristSite
Bad crank seals won't change the compression. That will only cause a lean circumstance in the mixture. If the rings wore over time without damaging the cylinder I doubt you would be able to tell without some type of measurements. That is probably the cause. Rings are cheap (after market ones are at least). Pop new ones in and see how you do.
fidiro
ArboristSite Guru
Thanks warroad, problem is I bought them used and know nothing about their usage. My main concern is trying to figure out the 029, the others can wait they run fine. I know it's not going to fix itself but I was just hoping to have a simple fix but it looks like I need to prepare an operating table to start pulling things apart and hope I know how to put it back together.
fidiro
ArboristSite Guru
The whole piston still shows the machining marks, including the skirts. For as old as this saw is, the original owner took really good care of it, it is near mint with just a scuff on top plastic that can probably be buffed out. I bought it just as powerhead needing a carb kit and I mounted a brand new 18" B/C on it and never had it into any wood.
Thanks for clearing up the seals question. Looks like there is no easy way out. I already looked for just rings and found a cheap set for $8 shipped but don't know who or where they are made. I recently installed a Caber ring in a 7311 that seems to hold good comp at 180, the Caber's will run $14 shipped but I know they are a brand and not just called AFTERMARKET.
fidiro
ArboristSite Guru
Thanks to this thread http://www.arboristsite.com/chainsaw/117444-4.htm I was able to take the saw apart and have the cylinder and piston out. Left the bottom end of case attached. Thanks to FISH and his pics I'm pretty sure I can put it back together.
What I found when I saw piston/cylinder was that they are both flawless. Piston still has machining rings throughout, a little less on intake side but still very noticeable. Rings still have machining lines as well with the top one being a little less visible but still noticiable. Top ring seemed to be a little stuck in the channel ,but not much, and I found some carbon in both channels so cleaned both out and am planning on reusing the rings. Was about to put it back together when I noticed I was missing a cylinder bolt, can't find it anywhere so it will show up when I don't need it. Now I have to wait to get one to get it back together and see what happens with comp. I also switched the rings around.
This saw has probably been sitting for many years and when I had it running at high rpms last week it must have sent some carbon into the ring channel locking the ring enough. I'll have to wait for a head bolt, put it back together and test comp again.
What I found when I saw piston/cylinder was that they are both flawless. Piston still has machining rings throughout, a little less on intake side but still very noticeable. Rings still have machining lines as well with the top one being a little less visible but still noticiable. Top ring seemed to be a little stuck in the channel ,but not much, and I found some carbon in both channels so cleaned both out and am planning on reusing the rings. Was about to put it back together when I noticed I was missing a cylinder bolt, can't find it anywhere so it will show up when I don't need it. Now I have to wait to get one to get it back together and see what happens with comp. I also switched the rings around.
This saw has probably been sitting for many years and when I had it running at high rpms last week it must have sent some carbon into the ring channel locking the ring enough. I'll have to wait for a head bolt, put it back together and test comp again.
fidiro
ArboristSite Guru
It's alive. The best part is I accomplished tearing apart a clam and was able to get it back together. Other then 300 screws and bolts to take off before you get to the cylinder it's not too bad to work on like some say. I can see if a saw needs just a top end the clam will turn out to be more work, but still doable without complaining.
Haven't checked comp. but got it started and have it idling great again. Max rpms right now is set to about 12k. I'll set it better when I'm out cutting with it. I still need to figure out the low setting and idle speed difference, they both adjust rpms and don't want to run it lean on idle. Currently have it set to about 3/4 out. I just need to read some more about carb adjusting.
I'm just happy to see it run again. Must have been that little bit of carbon stuck in the ring channels and there was no easy fix that I was looking to find.
Haven't checked comp. but got it started and have it idling great again. Max rpms right now is set to about 12k. I'll set it better when I'm out cutting with it. I still need to figure out the low setting and idle speed difference, they both adjust rpms and don't want to run it lean on idle. Currently have it set to about 3/4 out. I just need to read some more about carb adjusting.
I'm just happy to see it run again. Must have been that little bit of carbon stuck in the ring channels and there was no easy fix that I was looking to find.
:msp_thumbup: There it is. If it ain't broke, don't fix it. According to some experts, compression testing is something you do if a 2-stroke is hard to start or isn't running right. Emphasis on the starting part.
Else, you don't waste time on it, since it may trigger unnecessary procedures.
fidiro
ArboristSite Guru
It did it again.
Two more times, actually, I've had to take it apart. So, it's still apart, for the third time now, waiting for new rings. I'm now a clam professional.
OEM Stihl Piston measures 45.99 and looks great with wear lines still visible around most of piston(wear lines mostly gone on intake side but still slightly visible). Cylinder is super smooth, absolutely no scoring.
Got questions on OEM piston rings and crank bearings that should be on these saws.
These piston rings I pulled off still have the wear lines left in some areas of the ring, so it wore uneven, I measured them around and it comes up anywhere from 1.79-1.86mm. I can see the letters KO B8 lasered on the rings and wonder if these are OEM or if the previous owner replaced them, can anyone tell me?
I had switched the bottom ring to top and it ran for a little while but didn't last long when cutting so I switched them back again and again did not last long, so now I'm waiting on the new aftermarket rings which should be here today.
The crank bearings I just realized are made in Turkey, is that where stihl gets them from or did they get changed as well by previous owner?
When I first opened up the cylinder it had grey silicone sealant so don't know if that makes a difference.
This is the 029 Super. Thanks
Two more times, actually, I've had to take it apart. So, it's still apart, for the third time now, waiting for new rings. I'm now a clam professional.
OEM Stihl Piston measures 45.99 and looks great with wear lines still visible around most of piston(wear lines mostly gone on intake side but still slightly visible). Cylinder is super smooth, absolutely no scoring.
Got questions on OEM piston rings and crank bearings that should be on these saws.
These piston rings I pulled off still have the wear lines left in some areas of the ring, so it wore uneven, I measured them around and it comes up anywhere from 1.79-1.86mm. I can see the letters KO B8 lasered on the rings and wonder if these are OEM or if the previous owner replaced them, can anyone tell me?
I had switched the bottom ring to top and it ran for a little while but didn't last long when cutting so I switched them back again and again did not last long, so now I'm waiting on the new aftermarket rings which should be here today.
The crank bearings I just realized are made in Turkey, is that where stihl gets them from or did they get changed as well by previous owner?
When I first opened up the cylinder it had grey silicone sealant so don't know if that makes a difference.
This is the 029 Super. Thanks
The rings are probably worn off on top where the ELKO identification number was, and yes they are OEM Stihl.
See Elko Demoversion
You really need to put the rings into the cylinder & measure the end gaps to tell.
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.
See Elko Demoversion
You really need to put the rings into the cylinder & measure the end gaps to tell.
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.
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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...
thomas72
Addicted to ArboristSite
I would ask if you measured your ring end gap, but I really do not think it would help. Sounds like the ring was partially collapsed from carbon/oil buildup and started to wear the ring unevenly. Just my guess without looking at it.
fidiro
ArboristSite Guru
Thanks for the tips on checking end gap, would have never thought of testing that. I got the new aftermarket rings yesterday and have not had a chance to put back together so I'll check the gap. I did measure the aftermarket rings and they are consistent 1.87-1.88 all the way around but I'll compare the end gap as well to the used oem rings.
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Once you're done measuring and what not make sure to clean ALL the carbon from the ring lands and bottom of the ring groove...A simple tool can be made from a broken ring ground square on the end to scrape with. Make it long enough so that as long a portion of ring as possible can be in the groove to maintain control and alignment...be very careful...no aluminum from anywhere just carbon. Good luck...
fidiro
ArboristSite Guru
Both lands are nice and clean and ready to install new rings. Had a chance to measure the old and new rings. Couldn't find my feeler gauge so used a cheapo one I had around that I used for plug gapping.
The old bottom ring has about .021-.022 gap. The top has about .024
The new rings were too tight to try the gauge on. The gauge starts at about .018 and that would not fit into the gap on ends of new rings.
What is a good gap to get good compression?
The old bottom ring has about .021-.022 gap. The top has about .024
The new rings were too tight to try the gauge on. The gauge starts at about .018 and that would not fit into the gap on ends of new rings.
What is a good gap to get good compression?
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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.
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