Stihl MS391 chainsaw problems starting…Advice please

Newto2stroke

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Hi Experts. I‘m not new to chainsaws but I‘m new to fixing them. I need your help please!!

My story: I bought my Stihl MS391 2nd hand and within a few uses it started to behave badly (first mistake but we all learn) It wad sluggish, cut out and no chain oil. So I decided to learn about it, watched loads of advice on YouTube. I replaced the chain oil system, now works fine. Then chainsaw stopped completely. Fearing I had done something wrong, I did the basics. Replaced the carb, air filter, better fuel with 40:1 mix. All these parts needed replacing. It would still not start. Checks spark and magneto, it was fine but lots of play on the shaft. So striped down engine; Crank bearing shot to pieces as were both oil seals. Cleaned up piston, replaced rings, new bearings and seals and reconstructed engine with all the right gasket materials, red gasket glue. etc, bought all Stihl parts as I’d heard about Chinese copies!!! Good compression now (held by starter handle and does not drop) and no play in the crank shaft after reassembled the whole machine. But again it won’t fire up. Sparks fine…Compressions good….So thought it was a fuel problem. Im trying to start it just as on YoiTube…Choke until takes, But don‘t even get that far.

So I‘ve noticed, after putting on full choke to start with 5 pulls, the spark plug is very wet with fuel and seems to be alot of fuel in the carb lower paddle. it almost feels it’s flooding too easily. Or maybe thats OK, just don’t know what to expect?

Please offer your advice for me to try and check and I’ll work the problem with you. Maybe a bit of a time lag as i’m in London. I’ve worked hard on this so don’t want to give up!! Cheers
 
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Bob Hedgecutter

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You need a 391 in London? :D

Sounds like you have narrowed down your issue- fuel delivery, or rather too much fuel delivery.
Bit concerned about the red gasket glue use- was it RTV or Stihl shop Dirko? Neither are really required and may have gotten into system to cause a blockage if used along the intake tract.
Replaced the carb- with a correct fitment Stihl OEM one?
I would be tempted to go back to the old carb and see if it still has the same issues.
Main needle valve could be sticking, or the metering lever is in the incorrect position- allowing too much fuel into the system.

Have you tried to start it on WOT?
Tried draining the crankcase of excess fuel mix and any assembly oil used in the rebuild, dry the plug and then start as per usual starting instructions?
 

Newto2stroke

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Thanks Bob, well I’m just far enough outside London to a tree🤓. London is a big place. and I have a wood burner and a car😂.
This red stuff (Dirko HT), supplied by Stihl.
Squeezed it along the bottom half of the crank case, there is a groove, and when two halves go together it seals, as shown in the YouTube MS391 rebuild. I had to clean the old red stuff off with steel wool before I put the new stuff on. I’m pretty sure its only where it should be and a very long way from the intake. But point taken, so I’ll park that for the moment.
The Carb was a Stihl Walbro direct replacement one from Stihl supplier, as was the old one.

OK with the needle valve sticking or metering level. I need to read up on those bits, sound like sensitive adjustments, somewhere near/in carb. I did try putting the old carb back yesterday and still would not start.
What’s WOT mean? Is that neat fuel?
BTW, I thoight 2 stokes have a reed system for fuel but I can’t find it anywhere. They dogung up I heard?
I feels it’s a fuel issue so I’ll focus on that maybe and report back. I’ll check your suggestions. J
 

Bob Hedgecutter

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Dirko is good- RTV is not.
For the 391 being the biggest clamshell type engine in the Stihl line up- yes use sealant along the cylinder to cup joint.
Needle valve and its metering lever are internal within the carb and from a new supplied Stihl carb- should not need touched.

Reeds are old school and kind of disappeared from saws in the 1970s ish.
WOT means wide open throttle- NEVER use neat fuel in two stroke equipment.

If you have spark, have compression and fresh correct fuel mix- bout the only other obvious (because you are flooding fuel not starving for it) fault could be timing.
Coils and flywheel magnets do not like contact rubbing, but you are still getting spark- so we will assume they are still good enough after the bearing failure- however it is possible the key that locates the position of the flywheel relevant to the coil in rotation has shorn, become damaged, or went missing during the rebuild and the location of the magnets/coil relative to TDC (top dead centre) of the piston is incorrect and the plug is sparking too early or too late to ignite the compressed combustion gasses.
 

Newto2stroke

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Bob, thanks for more info. Yes spark ok (see later), compressions ok( holding pull string and machine does not drop), Dirko ok, very fresh 40:1 fuel mix.

So for timing…Due to a good spark, magneto gap is good and since I replaced bearings, the crankshaft spins concentric so ignition coil is getting a nice clean pass of the magnets, twice every rotation. So regarding the key on the shaft: The shaft has a gap and the magneto fan has a key that fits nicely in it. There is no key as such to loose in that. The two and mated well with no play tightened with the left hand thread.

I also looked at the timing alot. With this engine, the timing is almost set in stone. If the ignition coil gap is set right ( two screws allow perpendicular adjustment for gap width but no lateral movement), the coil can only sit in one place. So the whole mechanism is fixed but I was thinking how to confirm that the spark is firing at TDC. Its not that I can adjust the angle of firing relative to TDC, like on a car though. I am going to have a think if there is any possibility of something I have miss placed to cause a miss firing though! The firing issue does make sense as if the piston is not firing, I suppose the fuel will just keep pooling and flood everything.

OK about reeds, I get that, this is too new.

No neat fuel in this engine. Got that. Made that mistake a long time ago and seized an engine. Never again🤪. Allowed that once in a life time!

No fiddling with carb adjustments. What about the setting for Low. I had it at 1.5 turns form end stop. Would that make any difference or does that only come into affect when things are all working? I suppose the H is not touched until all is running, the setting the top revs?

So I have a few things to try later, after my trip to Tescos 🤪 to see how much my shopping has gone up this week.

At this stage should I try 50:1, is thats not the issue yet?

Jules
 

Bob Hedgecutter

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Whats wrong with ASDA? :ROFLMAO:
Fuel ratio does not need changed to "improve" things 40:1 versus 50:1 will make no difference to your issues.
1:45 am here- so will get back to you after I have slept on it.
 

Newto2stroke

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Bob, a little progress. Going back to basics, reading the on line operational manual, I tried to start it as detailed.
Full choke until it caught (7-8 pulls). Then to half choke and it fired up after 2-3 pulls. It ticked over in half choke then I move it to normal running and it ticked over. When operating the throttle, its sluggish with no real revs as such, feels like its running lean, not getting enough fuel to rev. Then after a minute, conks out. Have to go through that process again to get it started but conks out after a minute. I went through starting with the L and 1/4 turn from fully closed (anticlockwise) as says the book, increased it over various attempts to 1.5 turns at which point I got the ticking over going a bit better. The H is set at full anticlockwise, 3/4 turns.

So some progress. What does this prove so far: the spark is working and the timing must be close to OK. The compression works but as yet not fully proven as I’ve got no real power out of it yet. The mixture, well maybe that needs a tweak or maybe there is some fuel blockage somewhere which makes it conk out.

Interesting…Jules

PS. No ASDA near me. I did notice that my soya milk has suddenly gone up 30% this week, my same weekly bread up 20%.
 

Bob Hedgecutter

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I was going to suggest incorrect coil- the 391 had two choices and the shape of the air cleaner dictates which your saw should have.
But you got back to my first reply and the correct starting procedure got it going.
H in my opinion may be lean at 3/4 CCW (counter clock wise)- should be marked in black on the right hand side of the air box cover near the holes for the carb adjustment screws what the baseline settings are and never have I seen reference to a Stihl saw starting at just 1/4 turn out on the L circuit.
1/4 -3/4 turns out on either screw is going to make lean conditions and burn the saw up- basic standby is 1 full turn out on both as a starting point- fine tune from there.

Okay, you could be looking at several factors- piston not replaced- just ring, was the cylinder cleaned up of ALL transfer before reassembly?
Old enough for bearings to be shot and oil seals hard- probably leads to an age where all rubber components are getting to end of serviceable life- impulse line loose or cracked, fuel lines same, manifold cracked/holed/torn. AV mounts are probably due for replacement- loose AV's lead to more flex between tank and body that puts stresses on hoses and manifold joints. New fuel filter in new fuel line.

Continuing to run with the symptoms you describe is probably going to tear the top end apart again.
New oil seals could also have been damaged while fitting and causing air leaks, which make a saw impossible to tune.
Ideally a pressure and vacuum test will confirm or deny any air leaks and lead us back to carb settings and fuel delivery.
 

Oakie

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Bob, a little progress. Going back to basics, reading the on line operational manual, I tried to start it as detailed.
Full choke until it caught (7-8 pulls). Then to half choke and it fired up after 2-3 pulls. It ticked over in half choke then I move it to normal running and it ticked over. When operating the throttle, its sluggish with no real revs as such, feels like its running lean, not getting enough fuel to rev. Then after a minute, conks out. Have to go through that process again to get it started but conks out after a minute. I went through starting with the L and 1/4 turn from fully closed (anticlockwise) as says the book, increased it over various attempts to 1.5 turns at which point I got the ticking over going a bit better. The H is set at full anticlockwise, 3/4 turns.

So some progress. What does this prove so far: the spark is working and the timing must be close to OK. The compression works but as yet not fully proven as I’ve got no real power out of it yet. The mixture, well maybe that needs a tweak or maybe there is some fuel blockage somewhere which makes it conk out.

Interesting…Jules

PS. No ASDA near me. I did notice that my soya milk has suddenly gone up 30% this week, my same weekly bread up 20%.
There is known problems with these saws.
You need to take off enough parts to see the carb butterfly, and the smaller butterfly
above it, the small butterfly opens along with the main butterfly in the carb when you pull the throttle,
what happens is, when you let go the throttle, the small / top butterfly sticks open a little or a lot, usually
depends on how fast you pull the throttle and let it go, the butterfly in the carb can also stick, this all happens
because of using fuel with ethanol in it, it basically gums up the pins that rotate with the butterfly / flap on them,
Use some penetrating oil to wash and free up the flap / butterfly, especially the top one, it only gets nudged to the open
position when the throttle is pulled, and it can not come back to its closed position again because of the gum like
corn oil they sell these days instead of fuel, and no, using trufuel will not solve the issue, it takes some light oil and
a bit of wiggling the flap / butterfly open and closed until your sure it will close of its own accord.

Not saying this is the issue, but it is a know issue that causes running problems with these saws.
The main issues is not coming back to idle, this can happen after a tank or two of perfect running,
another symptom is the saw will run richer than it should, it won't reach full speed,
and it will cause it to be difficult to start, it presents as an intermittent problem, but does get worse.
Watch to the end,
This guy explains about strato causing saw to run lean, which he says burns out the ignition coil,
he says he put 4 coils on, worth a look as a coil issue could be your problem.
 

Bob Hedgecutter

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There is known problems with these saws.
You need to take off enough parts to see the carb butterfly, and the smaller butterfly
above it, the small butterfly opens along with the main butterfly in the carb when you pull the throttle,
what happens is, when you let go the throttle, the small / top butterfly sticks open a little or a lot, usually
depends on how fast you pull the throttle and let it go, the butterfly in the carb can also stick, this all happens
because of using fuel with ethanol in it, it basically gums up the pins that rotate with the butterfly / flap on them,
Use some penetrating oil to wash and free up the flap / butterfly, especially the top one, it only gets nudged to the open
position when the throttle is pulled, and it can not come back to its closed position again because of the gum like
corn oil they sell these days instead of fuel, and no, using trufuel will not solve the issue, it takes some light oil and
a bit of wiggling the flap / butterfly open and closed until your sure it will close of its own accord.

Not saying this is the issue, but it is a know issue that causes running problems with these saws.
The main issues is not coming back to idle, this can happen after a tank or two of perfect running,
another symptom is the saw will run richer than it should, it won't reach full speed,
and it will cause it to be difficult to start, it presents as an intermittent problem, but does get worse.
Watch to the end,
This guy explains about strato causing saw to run lean, which he says burns out the ignition coil,
he says he put 4 coils on, worth a look as a coil issue could be your problem.

Nice- but are they running ethanol fuel in the UK yet?
I can't say I have heard of the gumming up of pins over here, but we do not have ethanol in our pump fuel yet.
 

Newto2stroke

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Well Bob, came down this morning, having left it last night, went through the start up sequence and she started like a dream. So everything I had done was correct. loads of power and everything works. After 20 mins, the flywheel pin sheered🤪. but at least I know I can replace that flywheel and all will come good. Such is life. Thanks so far. J B1C68C96-8C34-4D29-83B9-374FFE6E1892.jpeg
 

Newto2stroke

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Bob, there are flywheels new £100+ from suppliers or EBAY 2nd hand £30.

I found one for an MS362C which looks the same. see photos. What do you reckon? my fly first first 2 photos ( broken pin and one fin missing) and then EBAY two photos, to compare. They look identical. The only difference is my one has number 1140 400 1204B while the EBAY has 1140 400 1205B? Is that enough difference ti mean anything, otherwise 9C52B5F3-AF2E-4E5F-B284-FC76C143E7AF.jpeg 8AD089B1-C8F0-4CB4-98E0-A362ECEE9039.png E6E8733A-DE46-464D-8152-B1992CF04039.jpeg 4FD798F3-1F7B-48E4-A8DC-578205BBC934.png
 

Oakie

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Gas with ethanol is in all the pumps here / Ireland.
I use Aspen with a little more oil added to break in new saws.
But there is anything up to 20% ethanol in the fuels here, most suspect more.
My first issue with it was when my son half filled his quad, and left it for 6 months.
Went to run it, took tank off, looked like candle wax only rock hard like varnish,
it rotted through the steel pick up pipe in the tank, and gummed up the carb.

His sisters quad on the other hand was perfect, she used the fuel I told them both to,
its called miles fuel, I found out about it when a local chainsaw man alerted me,
the guards and ambulance service use it too in their vehicles, so it seems they
ordered it specially to avoid any possible trouble with corn gas, it never
gums up, works a treat.

But yes, corn oil does many things to an engine, even to the moving parts of a carb.
The carb in the top video even looks like the flap / butterfly is chemically discolored.
You can see the waxy residue inside the carb above when he opens it.

Flywheel key sheard!, it happens, at least with a Stihl you can get one, the key on my
Efco GS520 broke, its built into the flywheel, so had to source a new flywheel for a new saw
and it was not easy, there was one in New Zealand, none anywhere else I looked at the time,
a new one was a ridiculous price, Efco would not help at all even though the saw was new
and under warranty, they ignored my emails after refusing to refund me because the saw never
ran correctly from new, I even tried to return it never having fuel or oil in it. Never buy an EFCO
product.

Hope the OP gets sorted after all the work he has done.
 
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Oakie

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I was going to suggest incorrect coil- the 391 had two choices and the shape of the air cleaner dictates which your saw should have.
But you got back to my first reply and the correct starting procedure got it going.
H in my opinion may be lean at 3/4 CCW (counter clock wise)- should be marked in black on the right hand side of the air box cover near the holes for the carb adjustment screws what the baseline settings are and never have I seen reference to a Stihl saw starting at just 1/4 turn out on the L circuit.
1/4 -3/4 turns out on either screw is going to make lean conditions and burn the saw up- basic standby is 1 full turn out on both as a starting point- fine tune from there.

Okay, you could be looking at several factors- piston not replaced- just ring, was the cylinder cleaned up of ALL transfer before reassembly?
Old enough for bearings to be shot and oil seals hard- probably leads to an age where all rubber components are getting to end of serviceable life- impulse line loose or cracked, fuel lines same, manifold cracked/holed/torn. AV mounts are probably due for replacement- loose AV's lead to more flex between tank and body that puts stresses on hoses and manifold joints. New fuel filter in new fuel line.

Continuing to run with the symptoms you describe is probably going to tear the top end apart again.
New oil seals could also have been damaged while fitting and causing air leaks, which make a saw impossible to tune.
Ideally a pressure and vacuum test will confirm or deny any air leaks and lead us back to carb settings and fuel delivery.
The OP never mentioned cleaning the Cylinder, he said he cleaned the Piston, so yes, could be still suffering
from a cylinder with aluminium smeared onto it, that could also be responsible for the good compression he
got when he says saw can hang from pull cord without turning over, I hope am wrong here as taking it all apart
will be double work.
 

Newto2stroke

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i have got the machine running really well now. It feels powerful and responsive, all my hard work paid off until the flywheen key sheered. Oh dear, never mind, an ebay flywheel in on route to me. all my hard work had paid off.
 

Oakie

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i have got the machine running really well now. It feels powerful and responsive, all my hard work paid off until the flywheen key sheered. Oh dear, never mind, an ebay flywheel in on route to me. all my hard work had paid off.
Good to hear, clean the end of the crank where the flywheel sits on very well,
Nail Polish remover / Tesco, don't think Aldi keep it, am in Ireland.
Clean the inside of the flywheel bore very well, and find the correct torque setting
for the flywheel, I add a dot of locktiite around the key, just a dot on each side of it,
and torque up right away, leave overnight to give it a chance to harden well, do not
put too much of it on, it woks best near the key as any excess can squish into the slot
the key fits into, I would not put any on the tapers, up to you though, I never used to put
any loctite on them, until I got a saw with a loose flywheel and broken key.
 
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Bob Hedgecutter

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Gas with ethanol is in all the pumps here / Ireland.
I use Aspen with a little more oil added to break in new saws.
But there is anything up to 20% ethanol in the fuels here, most suspect more.
My first issue with it was when my son half filled his quad, and left it for 6 months.
Went to run it, took tank off, looked like candle wax only rock hard like varnish,
it rotted through the steel pick up pipe in the tank, and gummed up the carb.

His sisters quad on the other hand was perfect, she used the fuel I told them both to,
its called miles fuel, I found out about it when a local chainsaw man alerted me,
the guards and ambulance service use it too in their vehicles, so it seems they
ordered it specially to avoid any possible trouble with corn gas, it never
gums up, works a treat.

But yes, corn oil does many things to an engine, even to the moving parts of a carb.
The carb in the top video even looks like the flap / butterfly is chemically discolored.
You can see the waxy residue inside the carb above when he opens it.

Flywheel key sheard!, it happens, at least with a Stihl you can get one, the key on my
Efco GS520 broke, its built into the flywheel, so had to source a new flywheel for a new saw
and it was not easy, there was one in New Zealand, none anywhere else I looked at the time,
a new one was a ridiculous price, Efco would not help at all even though the saw was new
and under warranty, they ignored my emails after refusing to refund me because the saw never
ran correctly from new, I even tried to return it never having fuel or oil in it. Never buy an EFCO
product.

Hope the OP gets sorted after all the work he has done.

If you have Gardai, you are South of the border- is the petrol the same as UK petrol?
Not that this matters much as it seems a loose fitted flywheel might have been the main issue- not fuel.
 

Bob Hedgecutter

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Bob, there are flywheels new £100+ from suppliers or EBAY 2nd hand £30.

I found one for an MS362C which looks the same. see photos. What do you reckon? my fly first first 2 photos ( broken pin and one fin missing) and then EBAY two photos, to compare. They look identical. The only difference is my one has number 1140 400 1204B while the EBAY has 1140 400 1205B? Is that enough difference ti mean anything, otherwise View attachment 1406 View attachment 1408 View attachment 1407 View attachment 1409

Off of the top of my head, I cannot quote 391 flywheel numbers- so do not know, but usually flywheels are mated to certain coils- so it is best to mate same with same. There were two different coils/flywheel combinations for the 391 and the shape of the air filter was the identification as to which your saw had.

However I do agree a key is not NEEDED, but handy to have. All the key does is locate the flywheel in relationship to the crank to set the correct timing- the taper and torque of the nut keep it in place. A sheared key means fit was loose and timing could shift.
On no key saws I usually lap the fit of wheel on crank, degrease, fit the wheel with the aid of dot punch marks or vivid ink marks to align key position to crank keyway and do the crank nut up until it is tight enough to hold.
If you wish to be "safe" and buy a new genuine flywheel that has good key, buy the same part number- but sit back and think about the cost thus far invested in genuine parts- on what is basically a large consumer grade saw, not a pro grade saw, versus the cost of a good condition 044/440?
 

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There looks to be enough of the stub left on your key to locate correctly on the crank- if you drift out the remains stuck in the crank and clean up any burring before fitting.
The missing fin- up to you, I would be tempted to find the fin directly opposite and shape it the same to keep the flywheel balanced. I get many a saw in that has bits of fin missing and they run okay- but when something is asked to do say 12,000 unloaded RPM's balance becomes more important. I am willing to bet because you thought the nut was tight enough, the missing fin and subsequent unbalanced rotation caused the not tight enough nut to back off and open the taper enough to throw the flywheel and shear the key- had the key sheared with the wheel in tight taper fit- there would be no stub left on the shear.
Eventually unbalanced flywheels accelerate wear to main crank bearings and oil seals- the more fins missing, the faster this may happen. Many a saw is running with one or part missing fins.
 

Newto2stroke

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There looks to be enough of the stub left on your key to locate correctly on the crank- if you drift out the remains stuck in the crank and clean up any burring before fitting.
The missing fin- up to you, I would be tempted to find the fin directly opposite and shape it the same to keep the flywheel balanced. I get many a saw in that has bits of fin missing and they run okay- but when something is asked to do say 12,000 unloaded RPM's balance becomes more important. I am willing to bet because you thought the nut was tight enough, the missing fin and subsequent unbalanced rotation caused the not tight enough nut to back off and open the taper enough to throw the flywheel and shear the key- had the key sheared with the wheel in tight taper fit- there would be no stub left on the shear.
Eventually unbalanced flywheels accelerate wear to main crank bearings and oil seals- the more fins missing, the faster this may happen. Many a saw is running with one or part missing fins.
Great advice Bob. I will do as you say and shape the concentric fins to balance the wheel. I‘m sure there will not be a serious reduction in air flow as a consequence. It’s a cost benefit I suppose; balanced wheel versus air flow. Unbalanced wheels spinning at 12,000 rmp takes the cake on that one as the forces on the wheel are BIG. I do like your hypothesis that the unbalanced broken finned flywheel added to the unscrewing of the nut and consequential sheering of the key. I could do the maths and work out the forces.

OK about the different flywheels and ignition coils to be a matched pair. So my take on that is ( as a physicist) the magnets embedded in the flywheels could be made of the same material (which will have a specific permeability to magnetic fields; their want to become magnetic and to what extent when magnetised by an electro magnet in the factory). So if the physical sizes of the magnets on the two types are the same, it is unlikely for the mangetic field to be vastly different. So what I am saying is, if the physical size of the magnets are the same, and with the same orientation of N & S( it has to be as the crank turns the same way on all MS391’s) I would bet my bottom dollar the flywheels are the same. The fact that the flywheels have two different number types for two different ignition coil types concerns me slightly as all my research online suggests the MS391 and MS362 have the same flywheels, but two different potential coil types. Of course I may be completely wrong, stihl may have spent some serious money on development and the two flywheels may have two different magnet field strengths, in which case I will find out when I spin my fly wheel with the electic drill and see no spark, or reduced spark from my EBAY £34 replacement! We theorists always think too much!!! But magnets spinning at 12000 rpm would have to have a seriously different field strength to make a difference.

BTW, my ignition coil is the wrong part number for this air filter type but it works fine!!!

So my present options are: the the new EBAY flywheel or balance and loctite the old flywheel. I do enjoy a challenge.

I’ll report back if Parcel Force have overcome the Russian Hack and can deliver my flywheel !!!
 
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