Chain Saw Compression

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  #1  
Old 05-22-08, 10:38 AM
jn
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Chain Saw Compression

I have a Stihl 029 super chain saw.
It was running fine for a couple hours of almost continuous cutting and all of a sudden locked up.
After it cooled off I could pull the rope but it never started after that.
I put a compression tester on it and pulled several times. It probably took a dozen or so pulls to get it to max out and it was at 130 pounds.
I took off the muffler and saw really bad scouring on the piston and the rings were severly damaged.
I was suprised that I got so much compression with the obvious damage.
I have come across an exact model for sale (used)but was wondering how to properly test the compression. I would like to know what compression to expect on an undamaged motor with just a few pulls.
Does anybody have opinion on how to test this?
 
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  #2  
Old 05-23-08, 12:20 AM
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With a good engine, a good tester witha tight fit, I would say you should expect 130psi with one good sharp pull.

By your description, your saw is toast, unfortunately. Sounds like it was used without enough oil in the gas, or a lean running condition.
 
  #3  
Old 05-23-08, 08:29 AM
jn
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Gas mixture was correct at 50:1
The problem was that I was way too aggressive and cutting up a huge 40" diameter tree that was 50 feet long (for firewood). I continuously kept on it at high RPM. What I think happened was a piece of carbon broke loose from the exhaust side and caught on a ring. Or maybe a ring just failed from overtemp.
In the future I'm gunna mix my oil a little richer and take more beer breaks!
The cylinder isn't damaged too bad. I may Ebay a piston and ring set and try my luck saving this one to use as a spare.
 
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Old 05-23-08, 11:02 AM
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You might be able to rebuild this engine on the saw. Your fuel mix was too lean in the carb settings or you have the start of a fuel line failure when you were getting additional are in the mix. Also an air leak can occure in the carb boot attachment to the cylinder. There are piston and cylinder kits out there for this model. The high compression on this can fool you with the rings gone you can pump up pressure in the crankcase that adds to the compression readings.
 
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Old 04-07-09, 08:41 PM
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I always run a little richer mixture on 2 strokes (atleast 40:1). You may lack a little power and foul more plugs but your equipment will last alot longer.
 
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Old 04-09-09, 09:30 AM
jn
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Update on this one since I started this thread LONG ago!:
The Stihl Cylinder was hardened chrome. The rings gave out and it looked like the cylinder was shot. There was scoring up and down on the exhaust side. What actually happens is the aluminum piston will rub and build up on the cylinder wall after the ring breaks. I simply sanded off the aluminum until the cylinder looked acceptable and installed a new set of rings ($10). I now run the fuel mixed at 32:1 and have run strong ever since. It has been back in use for months now. I must say those saws are a little tough to disassemble/reassemble but I did it for fun as much as anything.
 
  #7  
Old 04-10-09, 04:56 AM
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With the harden cylinder or chromed, I have rebuild many two cycle engines using the blue Scotch Brite disks for gasket removing, etc. The blue is for aluminum and since the surface I am using it on is hardened and a different metal, the blue disk will remove the aluminum and leave a polished surface. I have also used the blue to remove the deposits off crankshaft journals when a customer has not put oil in the new engine crankcase from the big box stores and broken the rod but not damaged the block. I have saved many pressure washer engines, chain saws, 2 stroke snow blowers and Lawn Boy 2 strokes mowers in this manor.
 
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