Saw running with plug wire off!?

[Full Chisel]

We had a MS391 do that once, it would run at a steady RPM with the plug boot pulled. Found a pinhole leak in the transfer tunnel where a piece of debris got behind the flywheel and damaged it. Apparently that created enough heat to where it was dieseling. It definitely gets your attention!

 

[heyduke]

Oh yeah I was a little in awe, gas engines aren't supposed to act this way normally. I'm trying to upload the video, but I keep getting a compatibility error

vids are too big. the software for this site reject them. you have to upload the vid to youtube, then copy and paste the url into your post.

 

[HarleyT]

I remember way back, when we would have an engine that was running poorly, usually dirty points, and if you yanked off the plug wire to shut the engine down, and if the wire was still close enough to the plug tip, it would keep running.
You could see it arc, and the engine would actually run better!

 

[IyaMan]

Dieseling is another way of describing pre-ignition, and almost certainly happening if running hot enough to melt the plug wire. Though it could also be detonation (also from excessive heat). You better start taking it a bit easier with the milling, let the saw cool more often. And make sure your oiler isn't clogged (happened to me once on a small saw, such that when there was little/no oil on the chain, caused so much friction that when powered off it kept running). Though it could also be from running low octane fuel. What kind of saw is it, and is it stock?

 

[Boomer 87]

Like @astnmacgto said to it wouldnt hurt a milling saw to run alittle fatter fuel wise that would also keep temps down

 

[Boomer 87]

I remember way back, when we would have an engine that was running poorly, usually dirty points, and if you yanked off the plug wire to shut the engine down, and if the wire was still close enough to the plug tip, it would keep running.
You could see it arc, and the engine would actually run better!

Probably ran better bc spark has to get hotter to jump a long gap.

 

[Odog]

Dieseling is another way of describing pre-ignition, and almost certainly happening if running hot enough to melt the plug wire. Though it could also be detonation (also from excessive heat). You better start taking it a bit easier with the milling, let the saw cool more often. And make sure your oiler isn't clogged (happened to me once on a small saw, such that when there was little/no oil on the chain, caused so much friction that when powered off it kept running). Though it could also be from running low octane fuel. What kind of saw is it, and is it stock?

I'll check the oiler and make sure its not plugged up. The saw is husqvarna 2101, and it's pretty stock, only thing I've done it add a velocity stack and changed to a different air filter .

 

[Franny K]

I have two stroke run away but not in a chainsaw. One theory that seems possible is when running too hot a piece of carbon gets stuck in the plug and glows enough to in this case keep it running.

 

[a. palmer jr.]

Yep, it's probably a hot spot in the combustion chamber that glows and continues the spark. I had a Honda CR250 once that I had to pull the carburetor off to shut it down. Turned out it had an intake leak that produced a lean burn that lit up the hot spot, probably carbon.

 

[Odog]

OK hopefully this works. This is the saw running with plug wire off

 

[Ozhoo]

Worked Way cool!

 

[Efisher26]

Freakin spectacular


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[Boomer 87]

Wow musta had a hell of a hot spot!

 

[IyaMan]

I'll check the oiler and make sure its not plugged up.

Also, check the plug and gap, and also clean out the fan/engine of any sawdust. Then try running under normal load or light milling. After about 10 minutes of running, see if it turns off correctly.

And how did the plug wire come off? Did you remove it or did it just vibrate off? And was the engine switched off when the plug wire was off as it kept running? As mentioned by someone else, it could be the coil still giving a jumping ignition if the power switch was still on. Pre-ignition/detonation that keeps it running occurs if the power was switched off.

 

[Odog]

Also, check the plug and gap, and also clean out the fan/engine of any sawdust. Then try running under normal load or light milling. After about 10 minutes of running, see if it turns off correctly.

And how did the plug wire come off? Did you remove it or did it just vibrate off? And was the engine switched off when the plug wire was off as it kept running? As mentioned by someone else, it could be the coil still giving a jumping ignition if the power switch was still on. Pre-ignition/detonation that keeps it running occurs if the power was switched off.

I raneed the saw at idle and it oiled ok, I pushed the hand oiler and it worked good as well. I switched chains and blocked some wood at about half throttle, no issues so I ran her up to wide open and it ran like a champ.

Yesterday the plug wire vibrated off, and the kill switch was in the run position. I shut the saw off with the kill switch, then put the plug wire on.

 

[n240sxguy]

If the plug wire was off, and the kill switch grounds the spark that wasn't getting there anyway, how did it stop the engine?


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[Odog]

If the plug wire was off, and the kill switch grounds the spark that wasn't getting there anyway, how did it stop the engine?


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I have no idea, but that's how I shut it off.

 

[astnmacgto]

That's unbelievable, if the kill switch did in fact kill it then I wonder where it was getting spark from. Because even if it was arcing to the case or handle some where the electrode should be insulated from all of it

 

[n240sxguy]

I have no idea, but that's how I shut it off.

Good enough. I've heard horror stories about old diesels that wouldn't shut off. My brother in law has an 80's Mercedes diesel. It has a manual fuel shutoff lever on the top of the engine in case it keeps running.


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[Odog]

That's unbelievable, if the kill switch did in fact kill it then I wonder where it was getting spark from. Because even if it was arcing to the case or handle some where the electrode should be insulated from all of it

If it was arcing to the handle or somewhere else shouldn't I have felt a shock from it?