Stihl MS 311 misses

[dennis]

I am working on a Stihl MS 311 that had no spark. The ignition coil was shot, and owner wanted to try an aftermarket coil. I installed it, and the saw ran fine.....for awhile. Then it started to miss. At medium speed (maybe 5K to 10K rpms) it runs fine. The final squeeze of the trigger puts it up to 12,500rpm, but then it starts to miss. There is maybe a half second to a second between misses. Of course, I suspect a faulty coil, but before I replace it, any other suggestions?

[bgsengine]

I'd verify the coil's application spec, as well as the unit's top no load RPM spec. Modern electronic coils, many of them, have built-in RPM limiters, so they will cut out the ignition when the RPM gets too high, reducing the revs. It might be one of those "will-fit" coils but was designed or copied from another unit that used an identical coil , but had different (lower) RPM specs to it. (such as a chop saw, perhaps) You may find you'll have to get an OE coil for that specific model and serial of unit.

[KE4AVB]

The MS311 specs at a top no load speed of 13,000 with chain and bar in place when new, slower when worn. Note spark plug should be gapped at 0.020"

[dennis]

Thanks, bgs and KE. According to the application specs, it is the right coil. I should have immediately thought about the spark plug. Must have had a brain fart. Again, thanks to both of you for the good information.