391 spark issues?

[bearcat2]

I've got a 391 that is I think four years old and has about 60 cord of firewood under its belt. The other day, right in the middle of a cut it sputtered and died. Pulling the plug out and touching the threads to the jug while pulling it over I have no visible spark, even in a very dark room. Tried a new plug, same thing. Tried disconnecting the kill switch wire from the coil, same thing. So I assumed it was the coil and ordered a new coil. Got the new coil and installed it and same thing.
After a lot of head scratching I took a cordless drill and socket and spun the flywheel with that. With the drill spinning at full power it will create a decent spark... with either coil. But no spark unless you hold the trigger down completely on the drill so it is spinning at full speed. And when you start spinning it with the drill it takes a second or two of spinning before it starts to spark, it will continue to spark until you back off the trigger, but every time you pull the trigger it takes a second or two before it will start sparking.

Any ideas what could be causing this? There is no way to spin it fast enough or long enough with the recoil starter to create a spark.

 

[singinwoodwackr]

I've got a 391 that is I think four years old and has about 60 cord of firewood under its belt. The other day, right in the middle of a cut it sputtered and died. Pulling the plug out and touching the threads to the jug while pulling it over I have no visible spark, even in a very dark room. Tried a new plug, same thing. Tried disconnecting the kill switch wire from the coil, same thing. So I assumed it was the coil and ordered a new coil. Got the new coil and installed it and same thing.
After a lot of head scratching I took a cordless drill and socket and spun the flywheel with that. With the drill spinning at full power it will create a decent spark... with either coil. But no spark unless you hold the trigger down completely on the drill so it is spinning at full speed. And when you start spinning it with the drill it takes a second or two of spinning before it starts to spark, it will continue to spark until you back off the trigger, but every time you pull the trigger it takes a second or two before it will start sparking.

Any ideas what could be causing this? There is no way to spin it fast enough or long enough with the recoil starter to create a spark.

That’s how it works with the newer Stihl’s…need a drill on high speed to get spark. Coils rarely go bad.
trace all wires with a meter for continuity.
check spark screen, fuel lines and filter.
make sure the kill switch isn’t stuck.
check the contact spring…broken, off?
i recently replaced one on a 290…cheap fix.

 

[SteveSr]

Any ideas what could be causing this? There is no way to spin it fast enough or long enough with the recoil starter to create a spark.

Incorrect... It is instantaneous velocity as the FW magnets pass the coil pole pieces that matters. A drill motor is constant velocity but likely too slow to cause spark. Believe it or not you can create higher instantaneous velocity with the pull rope!

You may also want to pull the muffler to check for scoring on the piston. Post some photos.

 

[bearcat2]

If I can create higher instantaneous velocity with the pull rope, why can't I get any spark with the pull rope no matter how fast I pull it? That is how I have always checked for spark on any saws (I've mainly worked on older saws) and would have never thought of using a drill until I read of someone on here using one and I couldn't get spark with the pull rope.

Piston looked good, my phone camera sucks and you can't see anything in the pictures I tried to take.

 

[bearcat2]

Well I put a different pull start on it and have spark now with it. But it still won't fire. I've gave up for the night. Will ponder on it and try to figure something out tomorrow.

 

[cscltd]

Spark is very faint on new saws, need a dark room or put finger on plug and pull.
check the transfer port covers especially the flywheel side one as they come loose from their glue. That will cause saw just to die during cut. Squirt soapy water around them and pull rope if they seem still attached (common on some of the glue on ones from Stihl from a few years ago

 

[bearcat2]

Looks like the one transfer port cover is loose. Anybody actually have any luck with glueing them back on and actually having them work?

 

[cscltd]

1st I would try Local dealer
give them ser # and see if they will call Stihl to maybe do a goodwill warranty?

 

[Chevboy0167]

https://www.jbweld.com/product/high-heat-syringe

 

[SteveSr]

Looks like the one transfer port cover is loose. Anybody actually have any luck with glueing them back on and actually having them work?

This is the main way that these saws die. They overheat from running old gas, forcing the saw to cut with a dull chain, incorrect (too lean) tuning, not keeping the cylinder clean of sawdust, etc.

The piston is likely trashed with considerable transfer to the cylinder. Disassemble the engine and you'll see what I am talking about. You have more problems than just a unglued transfer cover.

Time to look for a new professional grade saw.