Need help. I own a Stihl ms250 (9 yrs) which has run flawlessly until now. Recently sheared, not one, but 2 keyways off of flywheels. According to the many posts that I've read, I thought it may be a timing issue. If this chainsaw fires immediately after passing the flywheel magnets, it appears to be firing right at TDC according to my home-made timing wheel. When should it fire?
MS250 ignition timing
- Thread starter RLT
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I think its supposed to fire right before but im not a hundred percent on it. Im interested in what others say. I was going to post a similar thread in the near future.
A lot depends on what type of ignition you have. Older saws with magneto fixed timing were set at somewhere around 25 deg BTDC. Saws with electronic ignition modules will usually have a retard function that can retard the timing at low rpm to as much as 8 deg BTDC for easy no-kickback starting. With a lot of timing advance, the saw WILL tend to kickback on the starter when pulling it over, but it should not be shearing the FW key if the tapered part of the crankshaft that fits into the FW and the hole in the FW are a perfect match and the FW nut is properly torqued.
Yes, I got faked out until I applied the proper torque. Line up the key, or remnant thereof. Apply proper torque. For my MS391 it was 24 FT-LB. I don't know what the number is for your saw.
24 ftlbs for most common Stihls
make both tapers are cleaned with brake cleaner or sim.
your coil may not be reducing timing advance when startin- hence the kicking. may need a new coil. Buy used oem over new aftermarket or better yet new oem.
make both tapers are cleaned with brake cleaner or sim.
your coil may not be reducing timing advance when startin- hence the kicking. may need a new coil. Buy used oem over new aftermarket or better yet new oem.
Well, saws ran for years with a dumb coil at 25 degrees or thereabouts BTDC and didn't shear keys.
Normally key shear comes from the chain brake kicking in at high RPM .
Or, you never had it tight on the taper. Or, the taper was damaged when it sheared the first time.
Not sure whether the key comes out on those or not. But, if it does use some lapping compound and mate the flywheel to the taper.
Normally key shear comes from the chain brake kicking in at high RPM .
Or, you never had it tight on the taper. Or, the taper was damaged when it sheared the first time.
Not sure whether the key comes out on those or not. But, if it does use some lapping compound and mate the flywheel to the taper.
Stihl school said to use a little valve grinding compound to rough up the tapers in and out, then clean before lining up the broken flywheel and crank. Torque to spec.
