MS391 wont start

[treesap]

was using my dads 391 to mill (go ahead, hate on it), I noticed it starting be down on power, let off the throttle to let it cool off for a minute, and it just died instantly, pulled on it some, didnt start, pulled the plug, got spark, put a little gas down the intake, still wont start

seems like it isnt getting compression, decomp valve open or closed feels the same, was a bear to start this morning, now it wont start at all

any ideas?
would hate to have to tell my dad I blew his saw up lol

 

[Chaplain242]

Got an inspection camera? Look in bore and see if you see big scores from a broken ring...

 

[treesap]

Got an inspection camera? Look in bore and see if you see big scores from a broken ring...

ive got one but the usb plug is pretty well trashed

any other ideas?

 

[treesap]

I forgot to mention, the spark plug seemed dry, although I can in fact say there is 50-1 fuel mix in the saw right now, tried the plug from my leaf blower (Not dry) still didnt work

first time ive ever seen a dry plug on a two stroke, usually they are nasty from all the oil

 

[Jonny]

You can see a little of the P&C with a flashlight, take off the muffler and have a peek.

Milling is a tough task on big pro grade saws. The farm and ranch saws will have their days numbered if you try.

Get the compression tested if the P&C look ok.

What were you milling? Small stuff? 12” wood?

 

[islandarb]

That saw is toast. Ripping is a different ballgame. @Jonny is right. Done the alaskan mill thing..... prosaws or die. Using an 880. Just happened to get 391 from a bud who did not want it. Nice saw but not strong enough for ripping. If yours was running lean it probably blew.

 

[treesap]

Small stuff?

nah, will have to measure but im pretty sure around 20" or so poplar

was stopping every foot or so and letting it cool down, didnt have any issues till literally a foot and a half from the end of my last slab for the day, got a little weak, had to back off the pressure allot then it just died when I let go of the throttle, kinda like low fuel/sucking air

 

[islandarb]

Something in a fuel line or in a carb jet?

 

[treesap]

That saw is toast

hope not lol, havent milled much with it at all, saw is almost brand new (Maybe 20-25 tanks thru it)

was rocking it in the cut (To take the corners off instead of plowing thru a massive cut in one go) being fairly gentle, home made ripping chain

considering it was $700 and its almost new, my dad would be pissed if it blew, gonna take a look tomorrow, dont want to dig too far since its not my saw

 

[treesap]

Something in a fuel line or in a carb jet?

I will check tomorrow, I was thinking the same kind of thing, poured a little gas down the intake and it still wouldnt fire, had good spark, im guessing carb is clogged (There was a little junk in the fuel, nothing terrible, and the filter should have defiantly caught it, air filter was nasty, maybe something got in behind the air filter and into the carb?

 

[Jehinten]

Your description sounds like my 661 when I scored the piston milling some oak. Take the muffler off and look in there with a flashlight, and slowly pull the rope, you'll be able to see any scoring on the muffler side of the piston.

I've got a 391 tore down in my shop right now, it's the first decent saw I ever bought for myself over 10 years ago. Originally just for firewood, then I used it regularly once I got into this business. A little heavy but pulls a 20" very well. I have ran a 28" on it occasionally, before I had a bigger saw, and it ran it well as long as it's sharp.

Good news is that rebuilding a top end is pretty easy. You'll need a T-27 torx for most of the disassembly

And I'd agree with the others, apart from noodling planks the length of the bar, I wouldn't recommend continuing to mill with that saw.

 

[Jehinten]

air filter was nasty, maybe something got in behind the air filter and into the carb?

Certainly possible, milling creates a finer sawdust and lots of it. I clean out my air filter probably 2-3 times more often when milling than normal

home made ripping chain

I made my own ripping chain as well with a bench grinder, then struggled to get the angles right while hand filing. Muscle memory did not want to file at that angle... anyways that's what I believe caused my 661 to get too hot was a improperly sharpened mill chain.

gonna take a look tomorrow, dont want to dig too far since its not my saw

Already mentioned, but taking the muffler off is minimal and is the easiest way to see inside the cylinder without tearing it down.

 

[treesap]

Certainly possible, milling creates a finer sawdust and lots of it. I clean out my air filter probably 2-3 times more often when milling than normal




I made my own ripping chain as well with a bench grinder, then struggled to get the angles right while hand filing. Muscle memory did not want to file at that angle... anyways that's what I believe caused my 661 to get too hot was a improperly sharpened mill chain.



Already mentioned, but taking the muffler off is minimal and is the easiest way to see inside the cylinder without tearing it down.

thanks man, I did notice the decomp valve seems like it may have a piece of sawdust in it, when I push it in I can rotate the know smoothly, when its out its real chunky when I rotate it, id imagine that could be where my low compression is coming from

 

[Jehinten]

thanks man, I did notice the decomp valve seems like it may have a piece of sawdust in it, when I push it in I can rotate the know smoothly, when its out its real chunky when I rotate it, id imagine that could be where my low compression is coming from

I've not tried that on my saw, so I'm not sure if it's normal, but when the decomp valve is out is when it's sealed. So it makes sense that it would have a little more resistance

 

[treesap]

I've not tried that on my saw, so I'm not sure if it's normal, but when the decomp valve is out is when it's sealed. So it makes sense that it would have a little more resistance

more resistance sounds right, but this almost feels like it has notches in it when I rotate it, I feel the fact it has super low compression might is why it was so down on power (Went from pushing probably 35-40 pounds on the mill to get the rpm's down any to like 10 pounds)

gonna pull the muffler and check for scoring, then to a compression test
still wondering why the spark plug was dry
I know its not seized since it spins SUPER free when the spark plug is out

 

[Jehinten]

more resistance sounds right, but this almost feels like it has notches in it when I rotate it, I feel the fact it has super low compression might is why it was so down on power (Went from pushing probably 35-40 pounds on the mill to get the rpm's down any to like 10 pounds)

gonna pull the muffler and check for scoring, then to a compression test
still wondering why the spark plug was dry
I know its not seized since it spins SUPER free when the spark plug is out

I'm out in the shop now, and the decomp valve feels just like you've described. Should be normal operation for it, as mine wasn't giving me any problems with low compression.


However if you feel like you want to inspect it further, once the plastic is off the decomp valve can be unscrewed from the top of the cylinder. Personally I'd leave it alone until other test come back as normal. Just to ensure that you don't have any problems getting it out or back in causing future compression problems.

 

[treesap]

sounds good, im about to go out and take a look at this cylinder and piston

 

[treesap]

pulled the muffler off, has scoring at the top half of the piston ring seems ok, compression test came back to about 85-90 psi (Seems low), has strong spark and air, only other option is carb, gonna pull it apart later I guess, hoping its just a piece of sawdust in a jet, or possibly a stuck choke valve, anyone know how much of a pain itll be to pull this carb apart? gonna need a seal kit?

 

[Jehinten]

pulled the muffler off, has scoring at the top half of the piston ring seems ok, compression test came back to about 85-90 psi (Seems low), has strong spark and air, only other option is carb, gonna pull it apart later I guess, hoping its just a piece of sawdust in a jet, or possibly a stuck choke valve, anyone know how much of a pain itll be to pull this carb apart? gonna need a seal kit?

Maybe I'm missing something, what are you still looking for after finding the scored piston? That will stop your saw from running correctly. As for compression (don't take this for fact) I think it's supposed to be around 140-150psi. The scored piston can be the cause of the low compression and the scoring is likely a result of getting too hot during the milling.

 

[rico]

It seems you fucked this saw up by asking what is essentially a home owner saw to do something it wasn't built to do...The results. She got crispy hot and you scored the piston. OOPS! Lesson learned?