Rebuilt Stihl MS-391 not getting spark

[MorganE95]

So id like to explain the whole situation.
I inherited a Stihl MS-391 along with a couple other models, as my first chainsaws. ill call it Saw A .

And at some point or another and for some reason that I'm not exactly clear on. While i was running the saw, as i was first learning about chainsaws. And somehow managed to run it so hard that the threads on the cylinder where the spark plug screws into, had stripped almost completely because the spark plug had gotten blown out and took the threads with it. So when I noticed that. I made the plan to buy a new cylinder and rebuild the saw.

Some time went by and in that time i had gone out and purchased another MS-391 off of craigslist which started, and ran beautifully. I'll call that saw B.

So I finally managed to order the new cylinder and piston and ignition coil and sprocket for saw A. And then one of my workers proceeded to accidentally run saw B over with a trailer breaking the handlebar and bending the bar. And at this time i almost had Saw A put back together with my new parts. But upon putting it back together, i go to fire it up and no spark. So with a general basic knowledge of how chainsaws fire up. I tested to see if the spark plug was getting spark and nothing so i had came to the assumption of bad ignition coil even tho it was new. I said hey sometimes you just get unlucky and come across a faulty part. So to save me some time, i just took the ignition coil that i knew still worked great off of saw B. Along with the same spark plug and flywheel which i had just used and ran perfectly on Saw B. And i installed those parts onto Saw A, which completely the reconstruction of Saw A. But when I tested it. Still nothing.

I want to apologize for my long drawn out explanation but im stumped and haven't really gotten anywhere when asking the people i have so far. So I'm reaching out to the internet. So does anyone have any more tips cause im determined to get this thing running.

 

[angelo c]

Disconnect/remove the kill switch wiring and see if you still get "no spark". Should be the "little" wire coming from coil. Start there.

 

[MorganE95]

Disconnect/remove the kill switch wiring and see if you still get "no spark". Should be the "little" wire coming from coil. Start there.

Thanks i will try that but i tried that with the "new" ignition coil. But I dont think ive tried it with this ignition coil that I took from saw B.

 

[HarleyT]

These newer coils put out a very faint spark when testing. Do it in a dark room.

 

[cscltd]

Also, if new coil is aftermarket-then it could be DOA.
Recheck your coil gap, and/or put coils on other saw to test spark and as last resort, swap flywheels if getting spark on other only

 

[MorganE95]

So thats where im kinda puzzled because after doing a straight swap. Flywheels and coils. Still wont fire even tho it just was working on the other saw. It doesn't make any sense

 

[Old2stroke]

Did you swap spark plugs too?

 

[Joseph Acquisto]

So thats where im kinda puzzled because after doing a straight swap. Flywheels and coils. Still wont fire even tho it just was working on the other saw. It doesn't make any sense

There must be a wire that connects the coil to the kill switch. That could be crimped against the case somewhere and grounding out, just like the kill switch could do.

 

[Bob_w]

I did swap the plugs too, but nothing worked until I popped the flywheel and put it back in the right place. I may have been weak in the tightening, Since the flywheel came loose. I put it back, It ran, and loosened again The repair manual says 33Nm which is 24.3 Ft-lb. Tomorrow, I will be setting it there.

With the molded key, I've gotta have the proper torque.

I might switch flywheels if it doesn't hold with the torque.
My goal will be to switch all the outer parts back on the other machine, and see it still work, meaning the flywheel was to whole problem

 

[Bob_w]

There must be a wire that connects the coil to the kill switch. That could be crimped against the case somewhere and grounding out, just like the kill switch could do.

The kill switch terminal reads 2 megohms to gnd, so there is no short.

 

[stihltech]

Just to be clear, 2 mega ohms with the wire disconnected from the coil? Wire to ground should be should be infinite ( no reading). Otherwise there may be a short to ground.

 

[angelo c]

I did swap the plugs too, but nothing worked until I popped the flywheel and put it back in the right place. I may have been weak in the tightening, Since the flywheel came loose. I put it back, It ran, and loosened again The repair manual says 33Nm which is 24.3 Ft-lb. Tomorrow, I will be setting it there.

With the molded key, I've gotta have the proper torque.

I might switch flywheels if it doesn't hold with the torque.
My goal will be to switch all the outer parts back on the other machine, and see it still work, meaning the flywheel was to whole problem

If the flywheel jumped "out of time" it would still spark-just at the wrong time needed. It is rare but a flywheel has been known to "demagnatize" or poop the bed and not work.
Check to confirm the clip in the spark plug boot hasn't disconnected or broken. I'm betting on a short somewhere in the system.

 

[Old2stroke]

I did swap the plugs too, but nothing worked until I popped the flywheel and put it back in the right place. I may have been weak in the tightening, Since the flywheel came loose. I put it back, It ran, and loosened again The repair manual says 33Nm which is 24.3 Ft-lb. Tomorrow, I will be setting it there.

With the molded key, I've gotta have the proper torque.

I might switch flywheels if it doesn't hold with the torque.
My goal will be to switch all the outer parts back on the other machine, and see it still work, meaning the flywheel was to whole problem

As mentioned, it doesn't matter where the FW is located, if it goes past the coil fast enough it should make a spark. One thing that sometimes gets overlooked is the grounding of the coil. The coil is internally grounded to the laminated steel legs (electronic coils) and the legs are in turn grounded to the cylinder through the mounts and screws. On many saws the coil is mounted on the plastic body and there is a strap that must be there to finish the ground path back to the cylinder.

 

[MorganE95]

So id like to explain the whole situation.
I inherited a Stihl MS-391 along with a couple other models, as my first chainsaws. ill call it Saw A .

And at some point or another and for some reason that I'm not exactly clear on. While i was running the saw, as i was first learning about chainsaws. And somehow managed to run it so hard that the threads on the cylinder where the spark plug screws into, had stripped almost completely because the spark plug had gotten blown out and took the threads with it. So when I noticed that. I made the plan to buy a new cylinder and rebuild the saw.

Some time went by and in that time i had gone out and purchased another MS-391 off of craigslist which started, and ran beautifully. I'll call that saw B.

So I finally managed to order the new cylinder and piston and ignition coil and sprocket for saw A. And then one of my workers proceeded to accidentally run saw B over with a trailer breaking the handlebar and bending the bar. And at this time i almost had Saw A put back together with my new parts. But upon putting it back together, i go to fire it up and no spark. So with a general basic knowledge of how chainsaws fire up. I tested to see if the spark plug was getting spark and nothing so i had came to the assumption of bad ignition coil even tho it was new. I said hey sometimes you just get unlucky and come across a faulty part. So to save me some time, i just took the ignition coil that i knew still worked great off of saw B. Along with the same spark plug and flywheel which i had just used and ran perfectly on Saw B. And i installed those parts onto Saw A, which completely the reconstruction of Saw A. But when I tested it. Still nothing.

I want to apologize for my long drawn out explanation but im stumped and haven't really gotten anywhere when asking the people i have so far. So I'm reaching out to the internet. So does anyone have any more tips cause im determined to get this thing running.

UPDATE: so i hooked up a spark tester to it and I am getting a faint glow when i pull the rip cord. Any new suggestions. Still not starting

 

[Bubster]

Try taking the "Master Control Switch" completely off the saw and see if the spark gets hotter. The old 029s 039s etc were bad about the flat contact spring getting bent.

 

[Bubster]

Could also be a bad coil wire,and maybe even try putting the coil in your oven for a few.

 

[MorganE95]

Could also be a bad coil wire,and maybe even try putting the coil in your oven for a few.

So I had the thought of getting one of those spark testers that has the little light and looks just like a spark plug wire extender. Well after plugging that in. I guess I'm getting a very very small amount of spark but hardly any at all. It just glows like a little puny, super dim light when i try to fire it up. So my new guess would be a ****** aftermarket ignition coil. So I'll be buying a new stihl brand ignition coil and new spark plug. And will let you all know what the result of that is.

 

[Bob_w]

UPDATE: so i hooked up a spark tester to it and I am getting a faint glow when i pull the rip cord. Any new suggestions. Still not starting

My 391 reads about 2k to gnd from the coil wire. The frame of the coils should read zero ohms. The gap should be small. a business card is .012, from the magnets to the coil. Use something non magnetic

 

[irhunter]

1. Note that the flywheel probably needs to go on dry...no oil. Check the FSM to be sure.
2. How many coils do you currently own? I would try them all before buying another...and, I would try them with 1/2 teaspoon of mixed gas poured down the spark plug hole and a new plug installed.

Roy

 

[SteveSr]

1. Note that the flywheel probably MUST needs to go on dry...no oil. Check the FSM to be sure.
2. How many coils do you currently own? I would try them all before buying another...and, I would try them with 1/2 teaspoon of mixed gas poured down the spark plug hole and a new plug installed.

Roy

According to MANY Stihl Tech Bulletins.