Stihl MS391 chainsaw problems starting…Advice please

Bob Hedgecutter

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Just my luck to pick an argument with a bloody theorist! :ROFLMAO:

It is not much to do with size shape nor strength of magnets within any given flywheel, its diameter or replication of another- it is all to do with those magnets and their clock position relevant to the key or keyway within the bore of the wheel.
A few degrees either side of the ideal and you will have issues, issues that can destroy pistons, or cause a saw that is sparking beautifully outside the combustion chamber not to run because it is firing at an incorrect timing.

All you need to do is lap the (original) flywheel to the shaft, position it correctly and torque the nut down (if you didnt booger the threads on either nut or crank stub taking it off originally). No loctite is required.
 

Bob Hedgecutter

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BTW, my ignition coil is the wrong part number for this air filter type but it works fine!!!

Only rings true if the saw is as original- many an old style of filter will be swapped out for newer ones because the old ones are no longer in stock or are an improvement on the former.
What matters is the flywheel you had matched the coil you had- that is the information to follow and continue to pair.
 

Newto2stroke

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Yes of course Bob, it’s all about timing. I told you I over thought the problem. The subtleties of parts matching may be beyond my skill but I’m on the case. I work to nano metres. I will try to spot any small angle difference in the key position wrt to sharft. I’ll make a mark from the old flywheel and see if the new fly wheel matched up. I should be able to see a few degrees.
 

Bob Hedgecutter

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Complete and total success. My MS391 starts first time, and runs smoothly🥳. I’ve started it over 3 days and works every time.

So this is with the Ebay coil not meant for a 391?
If so it is one I will have to add to the memory banks as a "work around"- unless your original coil was also the incorrect model part.
 

Newto2stroke

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Bob, Ebay flywheel did not work so Mr Ebay accepted it back.I rehashed old flywheel and fitted it; ground off concentric fan blade to broken fan blade. Wheel balanced. A good fit and Bingo. So I’m happy man.

ps. there is a very small unnoticeable difference in flywheels as I see. Putting the other flywheel on made the machine run so roughly, back firing, and with no power, must have been a timing issue! I balnaced and had enough key left wiht some locktite for the old flywheel to fit but I will source the same wheel. in the meantime I’m ok but of course the old wheel may fail.
 
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