MS391 ignition problem? Or no? What's normal?

[junglebiker]

I've got a MS391 in the shop, nice looking saw, has some time on it, but compression seems good. Owner says that it is incredibly hard to start, but once started works alright. I checked it out and found that it had no spark at all unless you really pulled the rope very fast--much faster than other saws--and then the resulting spark is thin and weak-looking. It's blue, (not yellow) and is just barely visible, to the point where I'm not sure if it really is intermittent or just so hard to see that I only see it once or twice for each pull of the cord. I assumed there was a problem with the coil, so got a new one. When it arrived, I checked both coils with a multimeter, which showed some difference, so I assumed the old one must be breaking down and installed the new one. Pulled the rope and found the new one behaves exactly like the old one. On other saws I've worked on recently, (like the MS720 I talk about in a different thread), I don't have to pull the rope very fast at all to see a fat, blue spark at the plug. What (if anything) is different about the MS391 ignition? Could it be that the flywheel magnets are weak? I'm sure I'm missing something.

 

[SmoothOperator1]

Wel the magnets can loose strength over time, so that could be one possbaility. The second one can be the gap between the flywheel and the ignition coil. If I'm not mistaken the gap should be 0.2mm, but double check just to be sure. Another thing I can think of is to theck the ground and the plug gap.

 

[junglebiker]

I set the gap at 0.010" (0.25mm), which I think is about where it was before. Maybe I'll experiment with reducing that gap and see if that makes a difference.

 

[Leon Couper]

Most people juat use a business card me included and that's about 10 to 12" for flywheel
gap and plug is 20". Also check where the coil bolts to the motor it is not all dirty and if so give it
a good clean. Other than that check your on/off switch is not a bit faulty, had them cause me a problem sometimes.

 

[John R]

Does anyone have for sale a genuine stihl ignition module for a still MS 391? The part is no longer available from Stihl.

 

[junglebiker]

Really? I had no trouble getting one from a dealer in Australia.

 

[junglebiker]

Yeah. BTW, I learned a few things about MS391 ignition modules--if yours has the trapezoidal air filter and originally had a 1140-400-1301 coil, then the correct replacement is 1140-400-1303. If yours has the round, twist on air filter and originally had a 1140-400-1305 coil, (like the one I am working on) then the correct replacement is 1140-400-1307. Although they all look exactly the same, they are not interchangeable between the early saws (with the trapezoidal filters) and the later saws (with the round filters).

Got the new coil on and it seems to be exactly like the old one--no spark unless you are REALLY pulling fast. I reduced the coil/flywheel air gap down to 0.005" and that seemed to make no difference.

 

[ahjuma]

I've ju

I've got a MS391 in the shop, nice looking saw, has some time on it, but compression seems good. Owner says that it is incredibly hard to start, but once started works alright. I checked it out and found that it had no spark at all unless you really pulled the rope very fast--much faster than other saws--and then the resulting spark is thin and weak-looking. It's blue, (not yellow) and is just barely visible, to the point where I'm not sure if it really is intermittent or just so hard to see that I only see it once or twice for each pull of the cord. I assumed there was a problem with the coil, so got a new one. When it arrived, I checked both coils with a multimeter, which showed some difference, so I assumed the old one must be breaking down and installed the new one. Pulled the rope and found the new one behaves exactly like the old one. On other saws I've worked on recently, (like the MS720 I talk about in a different thread), I don't have to pull the rope very fast at all to see a fat, blue spark at the plug. What (if anything) is different about the MS391 ignition? Could it be that the flywheel magnets are weak? I'm sure I'm missing something.

I've got a MS391 in the shop, nice looking saw, has some time on it, but compression seems good. Owner says that it is incredibly hard to start, but once started works alright. I checked it out and found that it had no spark at all unless you really pulled the rope very fast--much faster than other saws--and then the resulting spark is thin and weak-looking. It's blue, (not yellow) and is just barely visible, to the point where I'm not sure if it really is intermittent or just so hard to see that I only see it once or twice for each pull of the cord. I assumed there was a problem with the coil, so got a new one. When it arrived, I checked both coils with a multimeter, which showed some difference, so I assumed the old one must be breaking down and installed the new one. Pulled the rope and found the new one behaves exactly like the old one. On other saws I've worked on recently, (like the MS720 I talk about in a different thread), I don't have to pull the rope very fast at all to see a fat, blue spark at the plug. What (if anything) is different about the MS391 ignition? Could it be that the flywheel magnets are weak? I'm sure I'm missing something.

I have just now returned my MS461 to service after dealing with the same symptoms you describe. This is the second Stihl saw that has had the same problem... the coil wire breaks off inside the spark plug boot. It's the second occasion in which I have cut the connector rubber with a razor knife to reach an inner steel hooked wire, pinched and soldered on a couple inches of braided copper wire pigtail, then electrical tape over the repair. Fired right up.
I prefer Stihl for its lighter weight, but that comes at a price... the recoil assembly is lame, I've replaced more cord and pawl sets than I care to admit. This repeated plug wire failure reveals the idiosyncracies I have to be aware of with this brand.