Stihl ms391 woes

[hacskaroly]

Although after adjusting spark plug gap to .020 like google says it should be instead of the .030

I generally gap my plugs at .025.

 

[LAWN BOY]

I generally gap my plugs at .025.

I tried that as well. Problem remained.

 

[LAWN BOY]

Try a different spark plug if you haven’t already. Either a new one or a compatible one from a running saw.

The one that's in it is basically new but I can try that anyways, thanks.

 

[Shane¹]

Have you determined it is the correct coil? I would also check the flywheel key to make sure it did not shear off

 

[LAWN BOY]

Have you determined it is the correct coil? I would also check the flywheel key to make sure it did not shear off

I’ll do that this wknd.

 

[hacskaroly]

I would also check the flywheel key to make sure it did not shear off

That was going to be my next thought, can't remember if it has a key or if the key is built into the flywheel. Without looking at it, I would guess that the flywheel is off some.

 

[LAWN BOY]

Ok, I got it running again today with a new plug, it ran several times but for no more than 8 seconds each. Maybe the dukes carb needs further adjusting and or cleaning. New NGK spark plug is in. Can a lousy pull on these saws with decomp used result in decomp coming out mid pull and making the jerking hand sensation we've discussed? There was some black fluid coming out the muffler, and the muffler region was producing a little smoke after dying. I see the new coil installed by the previous owner cracked, just the wire anyways, obviously still makes spark cuz I had it running. Here's a cool pic, does it look correct?

 

[hacskaroly]

Can a lousy pull on these saws with decomp used result in decomp coming out mid pull and making the jerking hand sensation we've discussed? There was some black fluid coming out the muffler, and the muffler region was producing a little smoke after dying.

Yes, pulling on the cord will create pressure that can pop out the decomp, even if it does not fire up. The black fluid coming out the muffler is fuel mixed with carbon (probably from the exhaust port and muffler). I have an 034 I am working on that does this too. The saw seems to flood easily. Sometimes when pulling on flooded saws, you will see the unburnt fuel coming out the muffler in good amounts. Smoke is present when there is not clean combustion. This can happen especially when unflooding a saw. Usually when a saw is running, you don't see any smoke it produces because it is evacuating it at a whole lot faster pace. When the saw moves slower like when yours dies, then you will see any smoke it produces because it isn't blowing it away as fast.

The coil will run with the break in the casing, that is until crud gets in there and starts to corrode the wiring. If you have access to a soldering iron, you can tin the wires with solder and then wrap it in electrical tape. With a break like that, there might be more damage to the wire that you don't see. You might have some strands broken under the sheathing and when the wire heats up from the ignition pulses, it might be causing issues with the spark at that point.

 

[LAWN BOY]

Yes, pulling on the cord will create pressure that can pop out the decomp, even if it does not fire up. The black fluid coming out the muffler is fuel mixed with carbon (probably from the exhaust port and muffler). I have an 034 I am working on that does this too. The saw seems to flood easily. Sometimes when pulling on flooded saws, you will see the unburnt fuel coming out the muffler in good amounts. Smoke is present when there is not clean combustion. This can happen especially when unflooding a saw. Usually when a saw is running, you don't see any smoke it produces because it is evacuating it at a whole lot faster pace. When the saw moves slower like when yours dies, then you will see any smoke it produces because it isn't blowing it away as fast.

The coil will run with the break in the casing, that is until crud gets in there and starts to corrode the wiring. If you have access to a soldering iron, you can tin the wires with solder and then wrap it in electrical tape. With a break like that, there might be more damage to the wire that you don't see. You might have some strands broken under the sheathing and when the wire heats up from the ignition pulses, it might be causing issues with the spark at that point.

Ok, thank you for all the info. I think next I’ll try cleaning and reinstalling the OEM carb. I would try adjusting the dukes ones further, but for some reason the adjustment screws don’t line up with the holes in the Stihl parts… If cleaning and reinstalling the OEM carb don’t work, I will clean and then adjust as logically as possible the dukes carb then reinstall that one. Then if problems remain I’ll check the flywheel key. Never did confirm coil gap as I don’t have many of my tools including my feeler gauge at this property, will be there in a week tho. Did get coil gap looking decent tho, looks good to my eyes. Could be causing spark issues like you said for sure if some of the strands are damaged.

 

[hacskaroly]

Never did confirm coil gap as I don’t have many of my tools including my feeler gauge at this property, will be there in a week tho. Did get coil gap looking decent tho, looks good to my eyes. Could be causing spark issues like you said for sure if some of the strands are damaged.

Instead of feeler gauges, you can use a business card or if your spark plugs came in a box, the cardboard box should be the same thickness and you can use that. Its hard to tell at this angle if yours is gapped correctly, but if you have something like I mentioned then you should be good to go. I have one of Mark Heimann's business cards I have been using for mine. I need to order something else from him to get a new card I have feeler gauges too, but seem to have better luck with business cards or spark plug boxes.

 

[hacskaroly]

Picture of spark plug and coil gapping tool...lol

 

[LAWN BOY]

Picture of spark plug and coil gapping tool...lol

View attachment 449918

I bought that same plug in the same box today for the saw

 

[FederalQ]

Frustrating. Verify that the choke buttery fly is fully open when it is off. The air filter or linkage could be keeping it partially closed. If that coil is AM the air filter may be as well. Fit and finish isn’t always the greatest.

 

[Shane¹]

Ok, I got it running again today with a new plug, it ran several times but for no more than 8 seconds each. Maybe the dukes carb needs further adjusting and or cleaning. New NGK spark plug is in. Can a lousy pull on these saws with decomp used result in decomp coming out mid pull and making the jerking hand sensation we've discussed? There was some black fluid coming out the muffler, and the muffler region was producing a little smoke after dying. I see the new coil installed by the previous owner cracked, just the wire anyways, obviously still makes spark cuz I had it running. Here's a cool pic, does it look correct? View attachment 449908View attachment 449909

I would definitely tape, heat shrink or replace that wire. The coil will fire to the spot with the least resistance so if that crack is anywhere near metal that could be where your spark is going especially when the plug gets under compression. Coil wire is pretty cheap should be able to just unthread that piece from the coil and replace it easy enough

 

[LAWN BOY]

Frustrating. Verify that the choke buttery fly is fully open when it is off. The air filter or linkage could be keeping it partially closed. If that coil is AM the air filter may be as well. Fit and finish isn’t always the greatest.

Air filter is tight/difficult to reinstall.

I would definitely tape, heat shrink or replace that wire. The coil will fire to the spot with the least resistance so if that crack is anywhere near metal that could be where your spark is going especially when the plug gets under compression. Coil wire is pretty cheap should be able to just unthread that piece from the coil and replace it easy enough

Ok, I can definitely try just taping it right away.

 

[Powerstroke Cowboy]

Sounds like the timing is off/advanced on the saw. That's what I believe your feeling when you yank the cord, not the decomp valve closing.

 

[Woodslasher]

I think the chinese coil is crap and is probably the cause of your issues.

 

[LAWN BOY]

I think I'll order a used OEM ignition coil and new OEM air filter when I get a chance.