MS 391 Not Starting

[Sam R]

Hey folks,

I've got a 391 in the shop right now that came in with complaint of won't start.
Been through the diagnosis; checked it from top to bottom. Good spark, tank vent works, tank holds pressure, fuel lines good no cracks or leaks. Good spark, even put a new plug. Torched the spark arrester even though it looked pretty clear. P/v test was good, .5 bar for at least 20 sec.
Air filter looks good, fuel filter in it was too big. I didn't have the exact one so I put a 0000 350 3502 which is similar.
I even swapped the carb & comp valve for new. Also reset magneto air gap to spec. Made sure the chain brake was disengaged.

It just won't fire up. Got it to start for a minute when I started fiddling with it before I did anything it ran rough and smoked a lot. Would not restart after.
And once in my diagnosis it fired up for a couple seconds when I had the ZAT4 on it. Died. Sounded like from running out of gas.
Also put fresh Motomix in it.
I need some ideas. At this point I'm stumped.

One other thing, I took the spark plug out, muffler confirms that it's getting some gas. There was a fine mist in combustion chamber and muffler was wet inside.
Also compression is good as verified by drop test w/ deco valve engaged.
I'm thinking maybe I flooded it and I'll try again after lunch here. But I've never had a saw be this hard to start that didn't have a bad carb or serious air leak.

Thanks folks

 

[Sam R]

Also also, the p&c look good. Not scored. Rings free.

 

[merc_man]

Sounds flooded to me.
Pull plug and blow compressed air to dry

Sent from my SM-J320W8 using Tapatalk

 

[Chainsaw Jim]

You may have flooded it when you tested the fuel tank for holding pressure. It'll only hold so much before the popoff releases to let fuel flow into the crankcase.
Also if the carbs popoff (metering lever spring) is too weak it will flood very easy when starting.

 

[chipper1]

Both the above.
Also while you have the plug out pull the motor over to clear the bottom end.
Next would be to inspect the flywheel at the crank, did it turn, ie ignition timing.

 

[Sam R]

Ah Chipper, that's the kind of response I was looking for!
Didn't get to get back to it today I'll look at it again on Monday. I'll keep you all posted.
I had an FS-55 suffering a similar issue today, that turned out that I jammed the shaft in too far (nickel for every time that's happened) and the crankshaft just couldn't really get it done. Pulled it out, corrected the seat and it ran just fine. Unfortunately barring the chain brake there's really no drag like that on a chainsaw.

 

[dmb2613]

Flooded and most likely carb problems causing it to flood

 

[chipper1]

I even swapped the carb & comp valve for new.

Flooded and most likely carb problems causing it to flood

I agree that it could be flooded, but hopefully the carb he put on it as stated above wasn't the problem, although it wouldn't be the first time something like that happened.

 

[dmb2613]

also try cranking it with the decompression valve out not in

 

[Sam R]

I kind of think it's fuel delivery related although the carb I swapped in was brand new, Stihl carb not a knockoff. Not unheard of of course but I doubt it. Funny thing when I pulled the new carb off it seemed dry. I wondered if maybe the impulse line was somehow getting compressed and not functioning. But like I'd said it had a mist of fuel in the chamber when I pulled the plug.
And dmb I tried that with the decomp valve - tried it with a brand new one too in case that was an issue. In fact with the decomp pulled out it has a ton of compression, drop test held it up with the 20" bar and chain and tank of gas.

Any ideas on how the carb could be causing it to flood? It's got limiter caps and weirdly the L screw was only turned out 1/4 when it came in. I played around with it but I don't think the owner tried anything like that and so I'm forced to assume it last ran at those carb settings.
I'll try hitting it again on Monday, any flooding should have evaporated by then. Like I said it ran for a minute or two when I first started fiddling, although it smoked pretty bad and sounded rough. Had a good acceleration though. Just wouldn't restart.

 

[dmb2613]

I have had new oem carbs to have a piece of casting wondering around it them , but a Stihl will flood EZ sometimes , I flood about half of the ones I rebuild lol on the first start up , stay with it you will fix it. Had one once that had week spark under compression, took me a week off and on to fix it

 

[Sam R]

Update on the 391, pulled it off the shelf today, just thought I'd see what it'd do. It's sat for a week. Fired up on like the 2nd pull. Switched to half choke and it fired right up. Ran flat out like hell, good acceleration - still kind of smoky - might just be left over crud from the flood. Turned it off, went to restart it - with decomp pulled out and could hear it pop loud but not start. Half choked it again and got it idling. Put the tach on it it was idling at like 2000! I don't *know* if the owner fiddled with it - sounds kind of carby to me. Anyway I tuned it up to spec- can't remember exactly off hand I have a print out of them all - 3100 or something I think. Still good acceleration, leaned the H just a tad because it was topping out a little below where it should be.
Switched it off and now I'll check it again tomorrow once it's cold again. I think maybe it was flooded before and provided it starts up again tomorrow the coil & timing must be okay. Ran and idled fine after I turned up the idle... I'm thinking maybe the carb is on its way to needing a good cleaning but as far as I can see, topping up the tank with fresh gas, changing the plug & fuel filter and torching the spark arrester has basically got it tip top again.

 

[user 122190]

topping up the tank with fresh gas, changing the plug & fuel filter and torching the spark arrester has basically got it tip top again.

Good yearly maintenance items, plus the air filter. I don't do mine yearly, but once the problems occur, I do them all. Keep them on hand and your down time is near zero.

 

[Sam R]

You got it, and I try and stress that to customers - some listen better than others. Had a Husqvarna trimmer in that was so messed up somehow the guy's been using it without a trigger or kill switch... Yeah, it's got problems

 

[JustinSL]

If you ever run into a saw you think is flooded if you turn the choke all the way off and in the run position and hold it wide open and crank it after a few cranks it will clear itself and start. Make sure you keep it running until it clears itself up. I’ve done this to saws that were flooded to the point fuel was coming out the exhaust and it works.

 

[chipper1]

You got it, and I try and stress that to customers - some listen better than others.

You might not like me as a customer .

 

[Sam R]

Oh I like you fine, your kind keeps me employed!

 

[Sam R]

If you ever run into a saw you think is flooded if you turn the choke all the way off and in the run position and hold it wide open and crank it after a few cranks it will clear itself and start. Make sure you keep it running until it clears itself up. I’ve done this to saws that were flooded to the point fuel was coming out the exhaust and it works.

I'll keep that in my back pocket. Usually what I've taken to doing is blow it out with the compressed air through the spark plug port. Thanks.

 

[JustinSL]

I'll keep that in my back pocket. Usually what I've taken to doing is blow it out with the compressed air through the spark plug port. Thanks.

That works too , the way I mentioned is great in the field if you don’t have access to tools. Usually that’s where stuff goes wrong for me is when I’m out in the woods cutting. Your way is probably a little safer. Here is where I learned it from.

 

[chipper1]

Oh I like you fine, your kind keeps me employed!

Not my kind, I've never had a piece of OPEquipment serviced at a shop, not once, but I have had a saw ported lol.
I've also let others do some work on some for me.
I'm not much for bringing anything to a shop, or for listening to them.
Not saying all shops are bad, but many don't know squat.
If you run the equipment on a normal basis as in every other month at minimum and then it gets a good workout now and then most of the problems people see will not happen, that's my maintenance program.
With regards to saws, sharp chains, clean air filters, good fuel, and good mix.