Stihl MS390 problem

crocboy25

I have a stihl ms 390. It has been a good saw and have used it for about a year. A few weeks ago I was cutting with it and had it start shutting off right after I cut through some logs. It would restart right away and I would cut another log. as soon as it cut through the log, it would shut off. It kept doing it. I stopped using it and took it to a Green South dealer and had them service. I thought maybe the fuel filter or something was clogged. I brought it home and tried it again. Same thing but this time I noticed it doing it after about 5 minutes.... It felt like it was over heating. I took it back to GS. They adjusted the idle on it and said that was the problem. I got it home today and tried it again. Same thing. It runs fine for about 5 minutes but then starts to feel really hot and starts shutting down after I cut through a log.... I am really frusterated with it and dont feel like throwing away more money at Green South. Any ideas guys????

Thanks

crocboy25

Also, there doesnt appear to be an oil leak and the oiler for the bar appears to be working ok....

KevinE

Is the dealership located at a different altitude to where you live mate? If so, I'd get it tuned to your altitude. It sounds a lot like a "L" screw & idle screw issue to me?

If you can, warm it up in a few cuts, then rich the "L" screw up till it idles OK. Be safe of course, but don't worry too much if the chain starts to spin at this stage.

Once again, if you can, drop the idle down using the idle screw till the chain stops spinning. Depending on how worn the clutch springs are, you may not be able to stop the chain spinning when the saw is idling correctly. You may have to go back & forth between the "L" screw & the idle screw a few times to get it right.

Then go buy some new clutch springs & prepare to start cursing lol!

NevC

My small Husky did the same thing recently. The problem was it was running rich at revs, the dealer sorted it out on the spot and it is now running well.

crocboy25

Was it overheating too?

crocboy25

The dealership is at the same altitude that I am.
In your statement below you state, "If you can", why wouldnt I be able to? I have never worked on the "L" screw of idle screw before.

NevC

Got a bit warm, so I stopped using it until it was sorted. It did not sound as good as normal, like restricted in revs.

crocboy25

Mine has that sound too. Even as it was cutting into some logs it sounds a bit restricted.... so you said yours was running too rich during revs?

Tony Knight

If the saw was running rich I doubt wether a excessive overheating problem would exist. Try lean, ie leaning out. A plug chop while under load may give a indication?

If indeed the saw is overheating for what ever reason (hard to tell from hear) I would check why the excess thermal and then examine the cdi as these fail under extreme heat.

I would also try and bring pressure to bare on the dealer that has taken your hard earned sheckles without any problem resolution.

Regards

Tony

crocboy25

What is a plug chop while under load?

Tony Knight

Plug chopping is usually used to check the fuel/air mixtures after tuning or modifications that effect power and tuning. Used to add further security that the motor is not running to lean from the mid to high range band. The process I follow.

Warm the saw up making sure it starts easily and idles at all angles, chain brake off crack or goose the saw a couple of times making sure that it returns to idle (chain stops rolling) straight away (if its running down to idle slowly then theres a tuning issue or air leak somewhere). If it passes my simple pre work checks then find a good lump of a log and start cutting a vertical down, I may take half a dozen rings plus in quick succession off to get the saw good and warm. Then on the last cut halfway through the log under power and load (ie cutting) I kill the saw (shut it off). Then I take it to the bench and remove the spark plug and examine the color and burnt and unburnt fuel residue. Key indicators of poor fuel mixture and or tuning at the medium to high will be display in the form of carbon deposits, ie general overview rich-black and oily, lean - off white to light grey hard and crusty. A reasonable burn to me would be dry and bronze colored. All I have indicated to you is a guide only, others may do it differently and without the saw its impossible to advise on any settings.

Food for thought, the issue you describe (cutting out after 5 minutes) could be a mired of reasons caused by over heating, the overheating also could have many causes and if your not mechanically minded it may serve you if its your bread and butter to have it professionally looked at (again).

Regards


Tony

crocboy25

Thanks for the information Tony. I took the saw back to green south AGAIN and tried to demo it to them. Wouldnt you know it... the D*** thing worked like a charm on some softwood they had and didnt choke out at all. I ran it for like 5 minutes too. Really iritated me, I came home later in the day and JUST tried it out again. I could tell by the way it sounded when it started that it was going to mess up. Sure enough, on the first cut through it choked out after the first log got cut. I was livid. I recranked it(started on first pull) and tried another log. I revved the heck out of it.... sounded good. Cut into another log.... Same thing. Choked out after it cut through. I had an idea though.... I started it again and just let it sit there and idle for like 2 minutes. I picked it up and tried to rev it and it choked out when I touched the throttle.... I am about ready to get rid of this thing. Seriously. I guess I am going back to Green South again tomorrow for the 5th time about this issue and try to demonstrate the problem. I am going to take some wood with me this time though. In any event, tomorrow they are going to see this SOB do what I have been telling them it does. Dont think they believe me....

KevinE

If the clutch springs are worn, the chain may run on when the saw is idling correctly.

If you have never tuned a saw before, disregard what I said above & either get someone who knows how to show you, or find a reliable dealer to fix it.

Personally, I'd have thrown the thing in the bin by now lol!

crocboy25

i am so far beyong pissed off with this thing but it seems like it should be an easy fix if I can ever get a pro to see what I am talking about it. Everytime I have taken to the shop and I leave, they crank it a few times and make a cute with it and say "yep it runs" and leave it at that. If i dont get some satisfaction from green south tomorrow, I am going to take it to a new place and maybe even sell the friggin thing. I paid around 600 for it and dont wanna walk away ya know? alot of money to toss aside.

Steve Tipton

Check that the fuel tank breather is not blocked. This may be a small hole in the filler cap or a tube from the fuel tank.
Also check that the muffler or exhaust port is not clogged with carbon.

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