MS391 slow getting back to idle

I noticed last evening while cutting up some firewood that my 391 doesn't always drop right back to idle after a cut or throttle blip. I was busy trying to get that all finished up so I didn't pay much attention to it at the time, but later I got to thinking about it. It used to stall sometimes when the throttle closed and I had adjusted the carb and got rid of that issue last year. I haven't played with the carb yet to see if it's just a tweek needed.

I'll try to get it out later and make some cuts and pay more attention to it's behavior. It's date night at our house this evening so I may not get to it tonight...

I'm really not looking forward to having to tear it apart, but will if I have to. I don't have any of the specialty tools for checking leak down and vacumn checks. I'm fairly handy with most tools and ready to jump in if I need to. I have rebuilt engines and heads in the past, even ported my old Blaster, so I'm not totally ignorant of mechanics.

I'm not sure what year this saw is, I had gotten it used from my brother seven years or so ago. I did a MM on it a few years ago and it runs great for all the bad press this model gets. The MM and tune woke it up quite well.

Are there any tips on what to look and listen for while operating it to clue in on if it has a bad seal? It starts cold and hot normally, just doesn't return to idle like it used to. I will check it out some more if you guys can point me in the right direction. I've always been able to tear stuff apart and put it back together and do modifications, but I'm not always very good at troubleshooting a problem to get straight to the issue causing the problem.

Possible tuning issue? Air leak?

What should I be checking and listening for?

Treat me like a total nube... LOL I'm much better at being an operator than a fixer... LOL

 

Carb tuning should be for tuning. If the Saw is failing, stalling or acting squirrelly, tuning it so it will run is masking another issue. In light of it running fine before having to make an adjustment.
Ever change the fuel filter? That would be a reasonable first step.

 

Thx, I didn't get the chance to get out to cut with it this evening, but I'll definitly do a carb tune after warming it up next time I'm out. Hoping it's just adjustments vs. teardown. I admit to being lazy with the tweeker, once I get em tuned up and cutting great, the tweeker tends to find it's way to the bottom of the toolbox.

And no I haven't changed the fuel filter. I will pick one up next time I get by the shop. Thx M2theB

 

My first guess would be an air-leak.
Possible crank seal , maybe a carb boot ?

My MS 250 had a similar issue. Seemed to act OK when it was cold but once it got some heat in it the idle increased quite a bit. Turned out to be a crank seal.
This saw sat for a couple years with no use and that's when it started acting up ; after just sitting.

By the time you disassemble and do a vacuum/pressure test you might just as well put a couple seals in it.

Not a hard job but just one of those that is a pain in the a$$

 

Start really simple and see if the throttle fully closes upon release. Then, inspect the fuel system (dump the old fuel) and look at the filter as mentioned above. Replace anything split or soft (fuel, impulse line... visually inspect the boot if you can with that model).

Clean everything up, replace the spark plug, and see what ya got. The LA screw could need a tweak as could the L screw as Scotty said. If you really go out of spec (or against the limiter, counter clockwise) and the issue persists, proceed to a carb rebuild. Fuel, compression, spark= running saw.

I pull the muffler on saws with running problems but you can wait on that...

 

Thanks guys, you're giving some good stuff to look into. That's the kick start I need. I wasn't able to get out and play today cuz work was pretty busy (work from home) and by quittin' time it was raining here. I'm sure it isn't old fuel, I've run about 6 gallons thru this and the 250 over the last couple months and the other saw is running great.

Hoping for a decent day out and I'll start with the suggested basics. I've never changed the spark plug in it either, it's always run great. As I mentioned before, I'm a user and if it ain't broke, I don't think to fix it.

Thanks again.

 

Just don't use any more of those Coronofire plugs!

 

I saw a great deal on SARsBlast plugs listed earlier... I'll have to see if I can cough up enough cash for one of those...

 

While it is warm, turn it on each side and see if the idle changes. If it does one of the crank seals is leaking.

 

Hard to find USA made plugs so I've switched to NGK for my Stihl's. Yours takes the BPMR 7 A. Pull all the covers and give it a good cleaning with the air hose. New OEM air filter and fuel filter.

 

Awesome tip... Never heard that before. Thx

I keep it pretty clean in there with the compressor. I always ran NGK in the snowmobiles, but to be honest, I've never changed a plug in any of my chainsaws before. I will be picking up some parts over the weekend if we get out. I'm sure it's due for some TLC.

 

Got the w/s block off changed for above zero.? Temps affect RAD and mix can be affected by frosted carb too cold, and too hot and can be lean . KISS before a carb clean or powerhead da

 

Good tip. I keep up on mine but forgot to mention it.

 

It's in the summer position. I typically only switch it over when I go out to cut on a really cold day and then put it back to summer when done cuz I'm usually not out when it's that cold. Default position for me is summer cuz I'm pretty keen on making sure not to over heat stuff. And I always listen for the burble when I pull the throttle and hear the change when it gets into good wood. I will double check that and all the other suggestions that have been noted and I'll clean the fuel tank out real good when I change the fuel filter.

Thanks guys for all the tips. GREAT info.

 

I just got in from cutting some HL logs up. Before leaving the garage, I did some checking.
I cleaned out the fuel tank and looked over the lines.
Fuel filter still looked pretty much like a new one. (I'm a stickler for cleaning up before gassing up)
Cleaned the air filter and looked over the intake boot. (What I could see of it without tearing it all down) I usually blow out the filter every other tank of fuel mix.
Pulled the spark plug and it looks a tad on the rich side. OK by me.
Cold start was normal.
I let it warm a moment and then limbed some small stuff to get er warmed up.
I cut a few 16" HL rounds then opened the Lo jet a tad under an 1/8 turn and bumped up the idle stop.
I cut a bunch more and it never acted up at all. Hi jet setting cleans right up in the cut and pulls hard like usual.
Warm starts were the normal one pull.
It revs quick right up off idle, no hesitation at all.

Thanks for talking me down off the ledge...

However. It will sit and idle all day, but if I set it over on it's side like mentioned above, the idle does come up just barely a noticable bit, not very much. I don't have a tach to be exact with it but is wasn't much. It did it on both sides and setting it back up didn't really bring it back down till I blip the throttle. Other than that it ran great and never did the slow to idle at all this evening. I only got to run about a quarter tank through it so it never got "hot" like when running a couple tanks through it.

So I'm OK with it for now, but I am curious about the setting it on its' side idle thing. Does this mean the seals are on their way out? Is there a little leak or bad leak thing, or if they're leaking at all is it a "fix it right away" situation?
Can they run this way for a while or are new seals imminent in the very near future? I'd rather not work on it till fall if it can be put off, but I don't want to damage it by running it if it shouldn't be run.

Thanks again for all the great info. I'm always looking to learn more. And sometimes I already knew it but just forgot since I don't work on em very much. I'll keep a close eye on it and note any changes.

I'm still planning on picking up filters and a plug over the weekend if we get out.

 

If the seals are leaking then I would not want to run it further...you risk going lean and seizing it up...but from what you said I think I would have to do further diagnostics to decide if it needs seals or not...but if it does, or you just want to CYA, they are cheap and not that hard to install (unless there is something different about this specific model)

 

OK, I watched some videos last night and I'll be picking up a pair of crank seals as soon as I can too. I'm learning new stuff all the time. This site is great.

Thanks all again for the tips.

Thanks to the admin and staff here at this site that make this all happen. It's great that the experienced folks have a place to share knowledge and newbies get the chance to absorb it and become experienced while saving some coin and having some fun as well.

 

If the idle changes when tipped on it's side (usually a gain in RPM's) then one or both seals are leaking.

 

If it like the 039, and I think it is, the seals are very easy to change out. But it’s a full tear down to get to them. They compress in a groove when the cylinder gets sucked back down. I’d love know if some one has a way to change them another way.