Anyone ever seen this?

[Full Chisel]

Had an MS391 come in the other day...very low-hour, basically new saw. Customer described problem as erradic idle. Saw starts and revs but does not return to idle and at times will not shut off. Suspected air leak or ignition problem. Our mechanic ran the saw and while running pulled the plug lead...it kept running! Saw turns about 8k with no spark, very steady RPM for as long as you let it run. Throttle blip kills it. This one has us baffled...the only explanation we have come up with is that a massive air leak is causing the cylinder to heat up and cause spontaneous combustion under compression...think diesel engine. There is spark, when the lead is pulled with pliers a visible blue arc is produced. But this thing runs just fine (well not fine, but runs) without it!

 

[alexcagle]

Let me guess. Bosch plug?
Blowers do that.

 

[Full Chisel]

Can't say for sure but most Stihls seem to come with Bosch plugs stock. What is the difference and why would the plug type have an effect with the lead removed?

 

[Thommo]

Tip of the plug must be red hot so it acts like a glow plug.

 

[LegDeLimber]

Along with the spark plug glowing, I'd also look at carbon buildup or a lean condition causing some flecks of aluminum to be a hot spot.
Pictures of the plug , Be very careful whne removing it to not bump off any buildup or wiskers etc.
Also a view into the muffler (where the exhaust gas enters)
might be interesting to see.

 

[Conquistador3]

First try using an NGK spark plug (cheap and widely available) to rule out the Bosch spark plug. Those things aren't as bad as Champion's but there's a good reason even the local Stihl dealership does not use them.

If that doesn't sort the problem, then you should run a leakdown test (ro rule out an air leak somewhere) and if that comes clean, take off the cylinder and look for carbon buildups.

That's all I can think off.

 

[alexcagle]

They showed us a blower with a bosch spark plug in a BR600 that stayed running after he pulled the plug wire off at gold school.
The said to remember one simple rhyme: "bosch is bad for blowers".
I would suggest putting a higher octane fuel in it, and the NGK plug as noted above by @Conquistador3 .
Low octane fuel explodes easier. Try mid-grade fuel at least, as Stihl recommends.
I ran into a customers cutoff saw that had 50-1 mix, plus had a small amount of diesel in the fuel mix by accident that was left in the fuel can when they filled it up at the gas station. It scored the piston on a TS800 that was about two weeks old in about 30 minutes. When I pulled it apart, I could smell the diesel right away.
I asked him if he smelled the diesel when he was filling it up. He said when you work on heavy construction job sites that most of the equipment runs on diesel and gets topped off every day; that everything smells of diesel. Including the truck he drives every day.
I must admit though, I've never seen a chainsaw stay running from that condition after the switch was shut off. Except when there was a problem with the shut off switch of course.

 

[CR888]

Ghost saw.

 

[Full Chisel]

We tried a different plug and it still ran without the lead attatched. And there should be very minimal carbon buildup. When I say "low hour saw" I mean like 10 minutes tops. It was basically brand new...I don't even think it had been put in wood.

 

[HarleyT]

How far away is the lead when you pull it off the plug?

 

[pioneerguy600]

It needs an ignition source so its either the sparkplug or something else inside the cylinder igniting the incoming fuel charge. If there were no ignition source then the engine would die. If a plug change does not stop the problem the cylinder has to come off to find the source of heat causing ignition.

 

[bikemike]

I bet it was some rc nitro fuel. I tried some in a crapsman and it ran but didn't deliver enough fuel but still ran and cut ok only way to shut it down was to choke it after it was warmed up. Drained the tank put fresh mix in it and took quite a long time for all of the nitro fuel to exit the system before it ran normal again. After that the saw ran better than it ever has

 

[pioneerguy600]

I bet it was some rc nitro fuel. I tried some in a crapsman and it ran but didn't deliver enough fuel but still ran and cut ok only way to shut it down was to choke it after it was warmed up. Drained the tank put fresh mix in it and took quite a long time for all of the nitro fuel to exit the system before it ran normal again. After that the saw ran better than it ever has

If the fuel was other than normal pump gas and oil mix then yes I could see that being the case .

 

[bikemike]

I forgot to ask if the exhaust fumes smell like a normal 2 cycle or tart and eye burning

 

[bikemike]

If the fuel was other than normal pump gas and oil mix then yes I could see that being the case .

Yeah rc car, plane, heli nitro fuel is like a very potent diesel fuel. Alcohol nitro methane and castor oil and sometimes synthetic oil mix. In the sunlight you can light it on fire in the driveway and you won't hardly se it

 

[Full Chisel]

How far away is the lead when you pull it off the plug?

As far away as we could get it...several inches.

It needs an ignition source so its either the sparkplug or something else inside the cylinder igniting the incoming fuel charge. If there were no ignition source then the engine would die. If a plug change does not stop the problem the cylinder has to come off to find the source of heat causing ignition.

We are going to dig into it more but we're waiting on a response from a higher up Stihl Tech.

I bet it was some rc nitro fuel. I tried some in a crapsman and it ran but didn't deliver enough fuel but still ran and cut ok only way to shut it down was to choke it after it was warmed up. Drained the tank put fresh mix in it and took quite a long time for all of the nitro fuel to exit the system before it ran normal again. After that the saw ran better than it ever has

Saw was fueled when it came in...didn't notice any odd fumes or anything off with the fuel.

 

[bikemike]

As far away as we could get it...several inches.



We are going to dig into it more but we're waiting on a response from a higher up Stihl Tech.



Saw was fueled when it came in...didn't notice any odd fumes or anything off with the fuel.

The fuel unburnt or raw won't have much scent to it and with pump gas you won't se to. It just take a little bit to do a whole with this stuff. They say it's a mixed fuel for a 2 cycle engine they have some for 4 cycle too.

 

[bikemike]

nitro fuel burning vid. Just did this a few minutes ago

 

[wde_1978]

How about a video of that saw running when it shouldn't (maybe someone already asked though)?

 

[Full Chisel]

I'll see about getting it uploaded...we did get video proof! Might not be until Monday or Tuesday though.