Constant revving and reluctance to turn off

[Coreytroy]

Item: Stihl MS 380

I run a mix of 42:1 in my Stihl, and after a first use to cut timber in ready for the colder months, I had trouble with my Stihl not wanting to slow to idle at times, and once it even continued to rev flat out, even though I flicked the switch to off, and pumped the trigger in an attempt to stop it.
It eventually stopped but I think only after the fuel dried up when tilting the saw.

When I returned from the cutting, I gave it a clean down, and the only thing I noticed was a little dust on the air filter, but not a mass amount.

What would cause this revving, lack of air?

[AKDoug]

More likely too much air. Sounds like you have some sort of air leak.

[Andyshine77]

Air leak, that caused overheating. The saw needs to be vacuum and pressure tested, and all the lines need to be inspected.

[Coreytroy]

Is that something I can check/solve, or a service centre job?

When the rain stops for long enough, I will fuel it up and give it a start to see if I get the same result.

If it seems ok, can I assume my clean out fixed the air leak, or it may reoccur once heating up again?

[Andyshine77]

Is that something I can check/solve, or a service centre job?

When the rain stops for long enough, I will fuel it up and give it a start to see if I get the same result.

If it seems ok, can I assume my clean out fixed the air leak, or it may reoccur once heating up again?

Don't run the saw until you know what's wrong. If you keep running it, you'll cause more and more damage. No!! cleaning the saw will not fix any air leak. To be honest if it was running like you described, it likely has some damage already. Take the exhaust off and have a look at the piston and cylinder through the exhaust port and report back, pics always help.

If you don't have the tools, It's best you take the saw in for service, but be prepared for the bill to be more than the saws value.

[Coreytroy]

Is that something I can check/solve, or a service centre job?

When the rain stops for long enough, I will fuel it up and give it a start to see if I get the same result.

If it seems ok, can I assume my clean out fixed the air leak, or it may reoccur once heating up again?

Take the exhaust off and have a look at the piston and cylinder through the exhaust port and report back, pics always help.

Thanks for the reply.
I will have a look tomorrow after work, and upload some pics if I can remove the exhaust.

[bandmiller2]

You may want to check the carb. throttle disc and its screws, if loose they can wedge open. Be sure theirs nothing hanging up with the throttle linkage. Check idle speed screw first. Frank C.

[thecfarm]

Coreytroy,welcome to the forum.I had kinda high idle issue with my 372 husky last year. I got that fixed like the next day.One time it was not bad in price to fix. Maybe $60. Than about 6 months later more trouble. And $250 that time.They had to pull the jug. It might sound like it's running out of gas??? Not a good thing. I don't even let my high rev when it's starts to run out of gas. I  don't know much about a chainsaw. I have to pay the price to have it fixed.

[joe_indi]

Air leak, that caused overheating. The saw needs to be vacuum and pressure tested, and all the lines need to be inspected.

Air leak for sure. The 380 has a non metallic cylinder gasket and M5X18 cylinder screws that tend to come loose. If the screw(s) become loose, the first thing to blow is the cylinder gasket.

[JohnG28]

An air leak wouldn't cause it to rev with the kill switch turned off though.  ???

[Coreytroy]

You may want to check the carb. throttle disc and its screws, if loose they can wedge open. Be sure theirs nothing hanging up with the throttle linkage. Check idle speed screw first. Frank C.

Thanks Frank. I will check that out before looking at dismantling anything else.

I do feel that if I refuelled it and started it would run fine, but may change once warmed up again.

[Al_Smith]

If the kill switch isn't getting it something is wobbling plus it has a loose grounding bond .

Sounds to be if possibley the shock mounts went bad causing the engine to rock out stressing the intake boot .If you haven't seen it before you cannot believe how fast a saw will run with an air leak ahead of the carb on just idle .I'll bet an easy 7-8000 rpm .I have seen it,twice as matter of fact on the same saw .Both times torn shock mount  and torn boot .

[Andyshine77]

An air leak wouldn't cause it to rev with the kill switch turned off though.  ???

With enough heat, air and fuel, an engine can run without any spark.

[Coreytroy]

 

 


  

View of piston inside cylinder through exhaust.


 
View of air filter.
What do you think? Is the lines on the piston expected?
Pulling the starter cord slowly moves the piston smoothly without issue.
Should a new piston and ring set be done now before any damage, or is it ok for now?

I have never changed a piston out, but would assume I need:
1 x piston incl. rings
1 x new cylinder gasket
Is that all if I replaced it? Or do I not need to?

All bolts were tight, except for the pin through the throttle trigger was coming out. Pushed it back in.
Started MS380, seems good.

[Andyshine77]

That's not good. What you're seeing is scuffling caused by lack of fuel, oil and excessive heat, likely do to an air leak. The saw will at least need a new piston. The cylinder will need to be cleaned up, or replace. I'm actually surprized it runs at all. Nevertheless don't run the saw any more, you'll just cause more damage to the cylinder.

[celliott]

I have never changed a piston out, but would assume I need:
1 x piston incl. rings
1 x new cylinder gasket
Is that all if I replaced it? Or do I not need to?

Yup, you'll need all of that.
Your picture shows the piston is scored, like andyshine said. Excessive heat(caused by running lean due to an air leak) caused aluminum from the piston to transfer onto the hard cylinder walls. You will also need to find a way to clean this aluminum transfer off, and make the cylinder walls smooth again. If it has scored through the hard plating on the cylinder, you will need a new cylinder as well. But try and clean the aluminum off, sometimes that can make it seem like the cylinder is scored when it's just stubborn aluminum transfer. Try muriatic acid and a 3M pad, or very light grit sandpaper, others have different methods that work as well.
Something else you will need to do when changing the piston, is, find out why it scored in the first place, or it will just do it again. Like others have said, you probably have an air leak somewhere. Could be the crank seals, intake boot, impulse line, or other areas. The saw really needs to be pressure\vac tested to find the leak so you can fix it.
Good luck 

[celliott]

And the reluctance to turn off- I have a husky 357xp, and it did the same no-shut off thing. The kill switch wire was frayed right near the end where it connected to the coil. The fraying was causing it to ground out on the case. Holding the kill switch down would sometimes shut it off after a moment, not right away, sometimes it wouldn't even shut down, had to use the choke.
Take the starter recoil off, and inspect all the wires for cracks\breaks, and check the grounds.

[JohnG28]

Yeah, that looks pretty rough. It is probably a good idea to do a pressure and vacuum test before you tear the p/c off. This will find the air leak before you open it up, potentially hiding the cause. I don't believe the OP said it has run since it shut off the last time, so it may well have shut off because it was fried, not because it ran out of fuel.  :( If it was running out of fuel that probably made things worse. In the future, if a saw won't turn off choke it to kill. It's a little harder with the Stihl master switch but still do able.

[AKDoug]

You cylinder is more than likely severely damaged also. It may be salvageable.  You are going to need to locate the cause of the airleak or the next rebuild is going do the same thing. 

[Coreytroy]

Thanks for the reply guys, much appreciated.

I have a family member who would have a cylinder pressure tester, it that what is required to do a pressure test?
Plug it into the plug hole and pressurise to see if any leaks occur?
But if the rings/piston is scored, wont the air pressure leak from around this now though?

I also found a new non oem cylinder and piston set that I can buy, but will wait for a reply to confirm to see if there is an air leak first.

Not 100% sure about the air leak being the problem, as after thinking about things, and having several types of fuel mix ratios for several 2 stroke tools in the shed, I may have accidently use fuel with much less oil mix than 42:1.
If this was the case, and I use fuel with only about 90:1 mix ratio, would that have caused the piston scoring as pictured if I did?

[Ianab]

Pressure testing is different to compression testing. You need to ensure that the crankcase doesn't leak air. From worn crank seals, failed gasket etc.  There are other reasons for a saw running lean (hole in fuel line, blocked jets in carb or wrong adjustments etc.

A compression tester is a different animal. It will tell you the piston and rings are stuffed, but we already know that.  :-\

But what the guys are saying is to get that crankcase tested before or during any rebuild. If the crank seals are gone the new piston will burn up in minutes. Same as any other lean fault, so it's important to work out what actually caused this.

ian

[Andyshine77]

90:1 could cause the damage. To vac/pressure test the saw you'll need to seal off the intake and exhaust ports. Vac and pressure testing is a must when when repairing a saw in that condition, even if it turns out not to be the cause of the failure. You're best off trying to save the cylinder you have, aftermarket cylinders are hit and miss when it comes to quality.

[Coreytroy]

Could you advise what should I do from here, in what order?
Do I have the Vac/Pressure test done before replacing parts, or do I replace the parts then have a Vac/Pressure test done?

I assume if I need to do a Vac/Pressure test, I could drop it into my local Stihl service centre and they could do that for me, or does any local car service station do this testing also?
I want to try to replace the parts myself to avoid the added cost of a service centre.

[Al_Smith]

Well the facts first ,the 380 is a later number given to the 038 magnum .Long regarded as the best hot weather saw Stihl ever built .I personally think it's the best period .

To get auto ignition on a two cycle would be very hard to do if not impossible .To run with acceleration it would require a spark ignition or else it would be so sporadic it would barely run at all .

If you had an air leak in a crankcase seal that would bypass the carb and thus not provide the fuel it would require to accelerate .It might speed up but soon die off due to lack of fuel

[Al_Smith]

Next .If it had a leak as I previously said in the intake tract  ahead of the carb the rapid inrush of air would create a huge pressure drop accross the carb and be able to suck in huge amounts of fuel through just the low speed jet .

One weak link on the 038/380 is the top most right hand annular buffer .That point takes the most stress on the cut and as I also previously said can tear you will miss it .Pretty soon torn boot .

You're going to need to tear it down anyway so just see if I'm right .

Also it looks like most of the flocking is gone on the air filter which won't make it run lean but it doesn't help the saw any .Throw away that danged Amsoil too while your at it .