Anyone ever seen this?

[ChoppyChoppy]

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.

"bad" as Champions? Pretty much all that's used here, haven't had a problem in the nearly 40 years the shop has been fixing small engines.

 

[Conquistador3]

"bad" as Champions? Pretty much all that's used here, haven't had a problem in the nearly 40 years the shop has been fixing small engines.

It's the only brand I've personally seen melt an electrode and having the ceramic insulation break off.

 

[Full Chisel]

It's the only brand I've personally seen melt an electrode and having the ceramic insulation break off.

And that is almost surely beyond the fault of the plug...that type of failure should have never occurred under normal operating conditions!

 

[Cj genever]

It's dieseling. Combination of compression, heat, and octane. Higher compression increases potential as does higher temps and lower octane. I would think there is something very wrong with the saw to diesel with pump grade. How does the p/c and ring look? Are there any obvious spots that are overheating?

 

[Full Chisel]

That is the conclusion we came up with, but we have not yet dug into the saw to see what is going on internally. It's very possible Stihl will want the saw back, so we are waiting to hear from them.

 

[Rx7man]

I'd be curious what the compression is on it too.. while you have it.
Perhaps it's just cursed like my old XR500 that detonated like heck when it warmed up. replaced the ENTIRE ignition system and it still does it... (chevron 94 octane fuel in it too)

 

[lefturnfreek]

We need video cause that would be saw ****....

 

[a. palmer jr.]

I had a Honda CR250R that did that, I actually had to remove the carb to get the engine to stop. It turned out to be a cracked intake manifold.

 

[bikemike]

I'd be curious what the compression is on it too.. while you have it.
Perhaps it's just cursed like my old XR500 that detonated like heck when it warmed up. replaced the ENTIRE ignition system and it still does it... (chevron 94 octane fuel in it too)

On ur xr I wouldn't be surprised if the comp release was sticking they had that issue.

 

[HarleyT]

On ur xr I wouldn't be surprised if the comp release was sticking they had that issue.

ur???

 

[bikemike]

ur???

Your xr500. I'm lazy. "Ur"

 

[HarleyT]

Oh, sorry.

 

[bikemike]

Oh, sorry.

No prob. I have many talents, reading and writing is not some of them

 

[Big Block]

 

[JonCraig]

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

It's Monday, and we're dying to see it!

 

[Little Al]

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.

Had a similar problem a while back,the problem turned out to be part of the last couple of threads in the spark plug hole had partly broken away & were poking into the cylinder about 2mm below& just to the side of the earth electrode with the lead in place the the spark was jumping to the swarf rather than the plug electrode so judging by the colour it was getting red hot, disconnect the lead & the heat was firing the mix in the cylinder in a hit & miss way but enough to sort of keep it running Broke it off with pliers cleaned up the thread & was as it should be.

 

[Full Chisel]

That is the thing that gets me, when it runs with the lead removed the RPM is very stable and does not change or cut out until throttle is blipped. I would think if it was something causing spontaneous combustion it would run rough with varying speed.

Our mechanic is in today, I will see if we can get the video up later this afternoon...

 

[bikemike]

My new toy

 

[Rio95]

I had this exact thing happen to me once using one of those cheap chinese 62cc chainsaws. It would rev like crazy with no throttle, and even when i killed the spark, it continued to run. As described, a throttle blip killed it instantly. It got incredibly hot. I pulled it apart and found a massive leak in the intake manifold between the carburetor and the cylinder, and after patching the plastic manifold with filler, and sanding it back, it has run fine even since.

 

[Full Chisel]

Well I wasn't able to get video uploaded today but we did find out what was going on with this saw. Something got caught in between the flywheel and case/cylinder and ate a small hole through the transfer tunnel causing an air leak. Apparently this created a hot spot and allowed spontaneous combustion to occur.

Still trying to get video uploaded...thought I had it downloaded to my phone but it didn't work.