Stihl MS291 repair?

[jiggysmb]

I just picked up a 291 that accidently was run with straight gas for a few seconds. I planned to store it until 3rd party cylinder/piston kits become available but I couldnt let it sit.

The piston was frozen but it was easily freed up with a wooden dowel in the spark plug hole. Looking in the exhaust port, the cylinder doesnt look bad, very little, if any scoring. It sounds like it has good compression but with the spark plug out and connected there is no spark.

I noticed some bad scoring on the flywheel. I used thick plastic to reseat the ignition module but still nothing.


 

Any suggestions would be greatly appreciated.

[dougand3]

Looks like magnets have been rubbing coil fins dang good. Magnets still have good magnetism? Coil fins dinged up? Dunno your air gap but would guess .010"-.015". You may need a new ignition module.

[CTYank]

A few seconds with straight gas on a saw that's been run prior on proper mix won't, by itself, do damage. (Overly lean mixture with no oil could be a death sentence.)

I'd check the main bearings for radial play. A bad bearing could have let the flywheel hit the pole-pieces. When you set the clearance to .012-.016" there's no way they should interfere.

Basic check on p&c requires a good compression gauge, used properly. That will tell the story. If that pans out, you could forget about replacement p&c.

Are you saying that a few seconds on straight gas "froze the piston"? Not likely.

Recently I needed an ignition module for a friend's 029. Listed for $120 + tax from stihl. Honest. I found an AM one from a fellow in KS for $14. Works fine. Bit of a PITA to install for no good reason.

[jiggysmb]

I had the compression tested with a proper gauge, it was 90 psi. There was no spark. Could the ignition module have gone bad due to the flywheel being to close. I dont think the cylinder locked up, I think the magnet was stuck to the flywheel. Im going to order an ignition module off ebay to see where I stand.

[dougand3]

90 PSI is really low. Even if you had spark, it would run with no power, if run at all. I'd pull the cylinder and inspect it and piston and ring(s). You can get an idea of internal engine condition by pulling muffler and inspect piston, ring(s) and intake cyl wall.

[farmguy]

Those 291s have plastic bearing cages. They can go and the the bearing tears itself apart that's probly why your flywheel is scored. It was probly due to exesive heat running it without oil

[jiggysmb]

Those 291s have plastic bearing cages. They can go and the the bearing tears itself apart that's probly why your flywheel is scored. It was probly due to exesive heat running it without oil

Thanks Farmguy! I know about the bearing near the sprocket but is there another behind the flywheel? Can they be upgraded to the metal cages used in the pro models?

[jiggysmb]

I got the saw torn down. There was a small section that chipped out of the exhaust port and got wedged between the piston and cylinder. Luckily there is no sign of damage to the cylinder. I got a new piston kit on the way but while looking inside I noticed one of the crank bearings was destroyed, so I ordered new crank bearings, seals and piston bearing. The new crank bearings are for some pro models and have a steel cage. 

Only problem now is I have no idea how to remove the crank bearings and how to get to the piston/rod bearing? I borrowed a bearing puller from an auto shop but it won't fit in the tiny space behind the bearing. Any help would be greatly appreciated.

[farmguy]

just use a propane torch and heat the inner race of the bearing it should slide right off. put your crank in the freezer overnight and gently warm the new bearings and it should go together no problem. as for the connecting rod bearing its not serviceable i think. if they are they have to be pressed apart and pressed back together with a special jig. the wrist pin bearing is easy and usually comes with the piston

[jiggysmb]

Farmguy, thanks I got the saw rebuilt yesterday and it runs like a champ! It runs as nice is my 260 pro.