MS391 - not running, likely no spark - Help

[ppsathas]

Hello everyone,

New to the site. Homeowner from central MA with a decent size property, cutting my own firewood.
I'm facing some weird issues with my 391.
Was running poorly, not holding idle after warming up and a few 20" cuts; I took apart the manifold, carb etc., discovered some pinholes on the fuel feeder line from the top of the tank to the carb. Replaced it, serviced it with a new plug, air filter and tuned it nicely.
Was idling great, then started acting up again after a few cuts, dying of at idle. Then after a few attempts, would never start again. I suspected a spark issue, and indeed I was getting no spark. Bought a new (open box) genuine Stihl coil from ebay, still no spark. Tested it with both switch wires disconnected and have checked continuity between boot and iron core.
I have checked the resistance on both coils, both give me about 1.9Kohms. New BPR7A resistor spark plugs NGK give me about 7.9K and 8.3K ohms, bought two champion plugs, they both measure 160K ohms (very high resistance). I read the post about a potential flywheel issue, I took it out, checked the magnets and confirmed one is a N, the other a S magnet. I'm gapping the coil to about 10thou; have tried intentionally as high as 20thou to no avail.

I just can't figure out what else might be wrong. Could the new coil be faulty? Could it be the flywheel?
Wouldn't I be able to see spark at the electrode in a dark room?

Any thoughts would be greatly appreciated.
Petros

 

[Old2stroke]

You cannot get any useful information by checking a modern ignition module with an ohm meter because of the electronic components that do the primary switching. Only reliable method is to replace with a known good module. Does "open box" mean it may have been used? What is the "boot"? There has to be a ground source for the primary and on some modules it is done internally to the iron core, on others there is a ground lead brought out of the coil that must be placed under the head of a screw that mounts the coil and is then grounded through the iron core and the screw into the saw body. However the primary grounding is done, it's important that the path is clean and tight right from the coil, through the place it is mounted back to the cylinder where the spark plug is grounded. That you can verify with and ohm meter. If you are using a known good plug, you should have no trouble seeing the spark in a dark room. Are you sure the saw is being turned over fast enough? Some coils don't produce a very good spark at slow starter speeds.

 

[ppsathas]

You cannot get any useful information by checking a modern ignition module with an ohm meter because of the electronic components that do the primary switching. Only reliable method is to replace with a known good module. Does "open box" mean it may have been used? What is the "boot"? There has to be a ground source for the primary and on some modules it is done internally to the iron core, on others there is a ground lead brought out of the coil that must be placed under the head of a screw that mounts the coil and is then grounded through the iron core and the screw into the saw body. However the primary grounding is done, it's important that the path is clean and tight right from the coil, through the place it is mounted back to the cylinder where the spark plug is grounded. That you can verify with and ohm meter. If you are using a known good plug, you should have no trouble seeing the spark in a dark room. Are you sure the saw is being turned over fast enough? Some coils don't produce a very good spark at slow starter speeds.

Thanks for the advice. I'll check the above and report back

 

[ppsathas]

Checked all ground connections to the block, all checks out solid.
I'm leaning towards purchasing another coil. Anyone with a spare one that is known to be good?
The markings on the original coil are: 1140 1301 C, MC 1140F, MS 1140, MJ 171

 

[user 122190]

Wouldn't I be able to see spark at the electrode in a dark room?

Yes!

I can see a spark when I crank the engine. It helps to do this in a darkened garage. You can also grab the plug and feel the spark jolt.

We have a member here, who claims that he can tell the condition of the coil, by visual inspection of the spark while cranking the engine. I don't recall his name, but if you could search it, he can possible help.

 

[rupedoggy]

I claim you can get a reasonable idea by increasing the gap to about a 1/4 inch. If it jumps that you don't have coil problems usually. Mike

 

[Bob_w]

Hello everyone,

New to the site. Homeowner from central MA with a decent size property, cutting my own firewood.
I'm facing some weird issues with my 391.
Was running poorly, not holding idle after warming up and a few 20" cuts; I took apart the manifold, carb etc., discovered some pinholes on the fuel feeder line from the top of the tank to the carb. Replaced it, serviced it with a new plug, air filter and tuned it nicely.
Was idling great, then started acting up again after a few cuts, dying of at idle. Then after a few attempts, would never start again. I suspected a spark issue, and indeed I was getting no spark. Bought a new (open box) genuine Stihl coil from ebay, still no spark. Tested it with both switch wires disconnected and have checked continuity between boot and iron core.
I have checked the resistance on both coils, both give me about 1.9Kohms. New BPR7A resistor spark plugs NGK give me about 7.9K and 8.3K ohms, bought two champion plugs, they both measure 160K ohms (very high resistance). I read the post about a potential flywheel issue, I took it out, checked the magnets and confirmed one is a N, the other a S magnet. I'm gapping the coil to about 10thou; have tried intentionally as high as 20thou to no avail.

I just can't figure out what else might be wrong. Could the new coil be faulty? Could it be the flywheel?
Wouldn't I be able to see spark at the electrode in a dark room?

Any thoughts would be greatly appreciated.
Petros

 

[Bob_w]

Ok, Here is something new.I just bought a 391, with a 25 in bar. The owner had a hard time starting itm but eventually it screamed, so I got it. At night I started it again. The next day, nothing.

I already have a MS391which is on the bench apart for other reasons. but it would run for a while, and stop. So I started measuring resistance readings, etc. I transferred all the important parts, nothing.

I uplled the flywheel. This has in molded in woodcott key, which looked bad... I put it back together. VROOM, then the flywheel became loose. So I put it back together again. Vroom, same thing.

so I will be back running with a new flywheel!!

So then my small saw will be a MS391, and my big saw, Will be a MS391.

I hope that this helps.

 

[SteveSr]

Ok, Here is something new.I just bought a 391, with a 25 in bar. The owner had a hard time starting itm but eventually it screamed, so I got it. At night I started it again. The next day, nothing.

I already have a MS391which is on the bench apart for other reasons. but it would run for a while, and stop. So I started measuring resistance readings, etc. I transferred all the important parts, nothing.

I uplled the flywheel. This has in molded in woodcott key, which looked bad... I put it back together. VROOM, then the flywheel became loose. So I put it back together again. Vroom, same thing.

so I will be back running with a new flywheel!!

So then my small saw will be a MS391, and my big saw, Will be a MS391.

I hope that this helps.

You don't need a new flywheel... The key is only there to align the shaft and FW. If it is coming loose either the surfaces are contaminated or you are applying insufficient torque to the nut.

 

[Bob_w]

Thanks Steve, I will try that. The manual says 33Nm which appears to be 24.3 Ft-lb. The molded key is not that good, but if it will hold, that will be fine. What is the trick to holding the flywheel without breaking a fin or popping a piston?

 

[SteveSr]

Thanks Steve, I will try that. The manual says 33Nm which appears to be 24.3 Ft-lb. The molded key is not that good, but if it will hold, that will be fine. What is the trick to holding the flywheel without breaking a fin or popping a piston?

index the piston up past the exhaust port and stuff the cylinder with 1/4" rope or use a Stihl plastic piston stop.

 

[Bob_w]

Thanks Steve. I see from the manual, what the piston stop looks like.

It turns out that one of the cover mounting screw holes can also be used it you are careful! A 5 mm x 2'' screw that you needed for the puller can be inserted into the top to block the flywheel. If you place the flywheel so that the thicker part of the casting near the magnet hits the screw, you can apply the torque without damage.

It was interesting since the first time I put the flywheel back on .. the damaged key did not allow the wheel to set all the way down on the taper. LOL, the engine starts, and the flywheel comes loose? ... Shut down, A little precise filing made it work.

Now I have ran 2 tanks of gas through it! it cuts like a champ with the 25 in bar!

Timing was the problem!

Thanks for the help!