Mac 10-10S - Strange Symptoms - Any Ideas?

[Old Mac Guy]

I got a Mac 10-10S from a local small engine shop. Pull rope would not budge and the lawnmower mechanic, who doesn't even try to work on Macs 'cause he can't get parts, said, "The engine is stuck. I think someone ran straight gas in it."

The saw is complete in all parts and cosmetically minty. Foil decal, paint, bar and chain, sides and bottom of the powerhead show minimal use. Decomp switch appears to function normally. He asked for $20 but quickly accepted $10. Probably just a parts saw...

Got it home and checked fuel tank. Empty and clean inside. Smells like fresh 2-stroke mix. A couple of teaspoons of bar oil were still in the oil tank. Looked like someone ran her until she ran out of fuel. Nothing abnormal I could see.

Removed clutch cover, bar and chain, starter assembly. Clutch and sprocket look fine. Starter assembly pulls and recoils fine. Removed rear handle brace and muffler.

No sign of scoring visible through exhaust ports. Piston shows no scoring whatever. Rings look fine. Removed plug and shined light into cylinder. No scoring visible at all. Turned the flywheel by hand... resistance at first, she turned hard a little bit, but moved. Wiggle-waggled f/w back and forth a couple times and she turned free to compression point. Turned f/w stronger and she popped compression. Engine turned over apparently normally.

Reassembled her and squirted 40:1 fuel mix in plug hole, pulled her over 3-4 times, checked tip of plug and gap... looked OK so re-installed it. The saw pulled over great. Compression seemed good (did not gauge it, but she holds her own weight on rope).

I could tell no difference between pulling her over with decomp switch in or out.

Squirted fuel mix in carb throat, ignition slideswitch in 'run' position, and pulled her over... 18 times... good compression... not a pop... then all of a sudden the starter would not budge not even 0.061 inch in a milliliter. Locked-up. Froze. Seized solid.

Repeated the entire above process two more times. Same result. After priming and pulling a dozen or two times, she seems to lock-up. Upon removing the starter assembly and turning the flywheel by hand, the engine seems tight-up and hard at first, then turns slowly, then easily.

Something jamming the crankshaft?
Boulders in the bearings?
Needles in the nether nubbins?
Rings rubbing on the round-de-rounds?
Palsy in de pawls?
Is a piston pin perpendicular to the post?
Is de rod de-rodded from de rod races??

Engine makes no clunking nor scraping nor grinding sounds. All seems totally normal except she won't fire-up, then, after 18-28 pulls, she freezes-up.

I know a total tear-down is inevitable, but wonder if any of you real chainsaw mechanics have an idea what's happening here... ?

Am I maybe looking at the same problem Rich found with his 10-10A?
http://www.arboristsite.com/showthread.php?t=135449

 

[beavis331]

My guess is the bearings are missing some balls or rollers and every so often it goes into a bind. Let us know when you tear it down. It would be interesting to know for sure.

 

[ric5141]

Sounds a lot like the one I am doing. But mine locked up in about 3 or four turns. Also the piston showed damage.

 

[sefh3]

Pull the plug and pull away, Does it lock up then? If it was a bearing it would lock up by pulling it over by hand.

 

[Modifiedmark]

Ken, I just parted a 10-10S with somewhat the same symptoms. Only it wouldnt pull over as long when it came unstuck.

Here is what came out of this low mileage Mac.

The upside is selling the parts from this one clean saw put a good dent in the total cost I paid for all 40 saws.

Hope yours dont resemble this one. LOL

 

[stinkbait]

I say split the case before you cause any more harm.

 

[Saw Dr.]

I'm with they guys on this one. I got a "locked up" 850 here. When I took it apart, there was a screw jamming up the crank. There are a handful of needle bearings on either end of that rod. It only takes one out of place to ruin your day. It should also be said that the 10-10s uses a proprietary 57cc top end only used on that, the 55, and maybe 555. If you trash it, good luck finding another.

Of course, you could pour some of that magic honey on it and prolly run another 500 cords.