Bought a Stihl 291 at a pawn shop. It runs really great--except that when I start it cold, it tends to fire early (I think) and jerks the rope out of my hand. Then, when it fires and runs, it sounds like the coil shorts out a little, then pretty quickly smooths out and sounds real good. Is it the timing, or do I have a partially bad coil? I checked the key on the flywheel, and it doesn't look like it has been damaged or filed, so I can't say that it has been intentionally altered. Can a coil be going bad, then straighten out? My electrical experience says that if it's shorting out, it won't magically stop. Appreciate any help.
Timing on a MS291
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Make sure the starter pull system runs smooth and makes a clean interface with flywheel. Check coil gap to flywheel to within specifications.
Since you picked up a unknown saw go through it and consider replacing spark plug and gap it to specifications. Replace fuel filter, air filter. Check fuel line and impulse hoses. Adjust carb screws. If it does not adjust there could be crud inside. A carb rebuild kit should clean it up.
Check wiring for loose wires or shorts. These are simple first look at making it reliable. Next step would be compression, pressure and vaccum testing and looking at the piston through the exhaust port. Look at the components as they could add to what you are experiencing. If still having the shorting out then address it. Just some opinions.
Since you picked up a unknown saw go through it and consider replacing spark plug and gap it to specifications. Replace fuel filter, air filter. Check fuel line and impulse hoses. Adjust carb screws. If it does not adjust there could be crud inside. A carb rebuild kit should clean it up.
Check wiring for loose wires or shorts. These are simple first look at making it reliable. Next step would be compression, pressure and vaccum testing and looking at the piston through the exhaust port. Look at the components as they could add to what you are experiencing. If still having the shorting out then address it. Just some opinions.
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Is the pull rope too short?
Ok, I checked out the starter rope, the recoil starter operation, checked the key and flywheel, re-positioned the coil. I then put the spark checker on and watched the light as it ran and accelerated. The light flickered when it faltered, I still think it may be the coil. Also tried another plug that came from a 290 saw that runs fine. Can a coil "sort of" go bad?
Yes. I've seen coils do all kind of crazy stuff. Last week I worked on a K760 that would pop and backfire on start up. Once it started it was sluggish than smooth out. Wouldn't idle. New coil and it was perfect. Uncommon but it happens. 3 weeks ago a Shindaiwa hedge trimmer that was sluggish and no power. Almost like plugged exhaust screen. New coil fixed that also.
Thanks Arrowhead, HarleyT and PV Hiker--think I'll try a coil and see. It's a great saw, definitely worth it.
Did you check the grounds and all wire conditions for shorts and breaks? Pull the spark plug boot off and look at the wire pin that pushes into the spark plug wire for proper insertion? Simple checks before replacing the whole coil and wiring.
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Let me know if you are going to try another coil, I have @ 30 of them.
or the inlet needle on your carb is leaking leading to a rich condition on startup. If you have a tester pressurize the your fuel inlet to 7psi, it should hold time the cows come home.
FYI...got a replacement coil, fixed the problem right up...then noticed that the plug wire on the first coil was a lot longer than the replacement, so I am suspicious that someone put a different coil on before I got it.
Google the part number on the coil and find out.
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