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Stihl MS 290 with 75-80 psi compression?

Started by firefighterfox, January 28, 2012, 09:52:43 AM

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firefighterfox

I have recently acquired a Stihl MS 290 that was relegated for parts (low mileage unit). I have a fair amount of experience working on 2 and 4 cycle equipment. I have disassembled the unit with a plan to rebuild it. Initial inspection through the exhaust port did not reveal any scoring but my compression gauge reads between 75 to 80 psi? Closer inspection after disassemby showed a clean and unscored cylinder, piston and rings. The ring gap measures .015 in the top part of the cylinder.
Question: where is my compression loss occuring? I can see no indication that the saw has ever overheated or repaired. Can bad crankshaft seals be the issue?
Thanks for any help,
Greg Fox

AdkStihl

J.Miller Photography

snowstorm

Quote from: AdkStihl on January 28, 2012, 10:25:08 AM
Sounds like a crank seal. Time for a vacuum test.

http://www.arboristsite.com/chainsaw/42768.htm
compression takes place above the exhaust ports. leaking crank seals will cause it to run lean. cannot cause low compression

Al_Smith

You can flea bay a set of rings pretty cheap for those things .Even OEM is about 25 bucks .

If  the cylinder is clean like you said the only thing it could be would be the rings .

Clam77

Quote from: Al_Smith on January 28, 2012, 05:24:19 PM
If  the cylinder is clean like you said the only thing it could be would be the rings .

Agreed.  I would also speculate on a head gasket, but most saws dont have those...
Andy

Stihl 009, 028, 038, 041, MS362
Mac 1-40, 3-25

firefighterfox

Wow,
Thanks for all the ideas. There is no head gasket by the way. I have gone ahead and ordered new crank seals and piston rings. Even though .015 did not seem to be much of a ring end gap? I know it's a little late now, but perhaps I should have tried another comp. gauge? Oh well, I'll put the new parts in, check the end gap on the new rings and then see what my comp. tester is reading. If it's still low I'll try another gauge.
By the way, can someone tell mewhat the correct ring gap is supposed to be and is there anything special about the silicone gasket sealant for the bottom of the crank or will regular automotive type work? Any special instructions for assembling the crank seals or case?
I planned on putting them in after I assembled the crank case.(with a touch of sealant?)
thanks again for all the advise,
Greg Fox

firefighterfox

Hey guys,
It may seem like a dumb question but, how do I get the bottom part of the crank case out of the plastic housing? It appears to be held in place by the rear chain bar mounting stud but I cannot seem to get it loose and I don't want to break anything at this point!
thanks for the info,
Greg Fox

firefighterfox

Ok guys, I was able to get the bar stud out and now I'm awaiting the crank seals to complete the reassembly and see what happens? I'll keep ya'll posted as to what happens in a few days. Hopefully it will be good news?
thanks again for the help,
Greg Fox

firefighterfox

Success!!!!!!!!!!!!!!!
New parts arrived today and after installation the MS 290 is running GREAT!
thanks for all the help guys,
Greg Fox

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