I'd never believe it

jayp1017 · 12-29-2018, 04:58 PM

I'd never believe it had I not seen it with my own two eyes. A friend and I were working on 96 polaris snowmobile. It was running fine besides rough idol then all of a sudden it rev up to like 6500 rpm for no reason at all. Tried shutting it down with the key and safety kill but nothing. So removed BOTH spark plug wires and it was still running wide open! We couldn't believe it...totally defies anything I've ever known about engines. Only shut down after we put hands over the carb and choked it out! So we've named the sled Christine after the movie with the possessed car. Thought I'd share.

drift240se · 12-29-2018, 05:06 PM

Thats pretty crazy. have seen this once. stuck valves and a lot of gas was in the motor. took about 20 seconds to burn out the extra fuel

dundeebarnbuggy · 12-29-2018, 05:31 PM

Sounds like you have something red hot causing ignition in the cylinder[s]. Spark plug protruding too far allowing exposed threads to get hot? Or, could a huge buildup of carbon causing enough compression to diesel. Keep us posted on what you find?

CP241 · 12-29-2018, 06:43 PM

I'm guessing it is a 2stroke? It's called a runaway engine.

That's common if they run lean. The cylinder gets stupid hot, hot enough to ignite the fuel without the spark plug. It will go sky high RPM and the best way to shut it off is to choke it out and starve it for air. You can pull the spark plug if it's easy, sometimes that's easier to get to than the intake to choke it out.

If you dont kill it, it only gets hotter until the engine goes pop.
Happens to diesels too, but I've never seen a diesel snowmobile. Any diesel tech keeps a big steel plate handy for just such occasions.

Need to Find out why it is running lean or it will happen again

jayp1017 · 12-29-2018, 10:16 PM

It is running lean. It's the PTO crank seal...replacing it tomorrow. It's not a snowmobile I really care about honestly but it only has 2000 miles on it. Not going to split the case unless I see crank run out or play in bearing...hoping for the best with trying to slide a new seal in.

jakesnake27 · 12-29-2018, 10:25 PM

I've never heard of anything like this, thanks for sharing!

Gazoo · 12-29-2018, 10:52 PM

Pre-ignition. Happened to my snowmobile too years back. Freaky phenomenon....

EzgoTXT2002 · 12-31-2018, 03:12 PM

Scary RPM for sure !

I've seen 2 strokes run backwards, but not a runaway on a two stroke.
Plenty of diesel runaways on video but never in person.

I keep a thick plywood shut off plate in the engine compartment of my
Lariat diesel truck for just such an emergency.

Mooncarter · 12-31-2018, 03:39 PM

Had this happen on an old Cadillac one time. Not WOT but running after pulling wires off. Ran hot.

CP241 · 12-31-2018, 04:20 PM

I had a Polaris 4wheeler that was a 2stroke that had a carb problem run away on me once. Very scary indeed

12-29-2018, 04:58 PM	#1
jayp1017 Getting Wild Join Date: Nov 2016 Location: Kalamazoo Michigan Posts: 132	I'd never believe it I'd never believe it had I not seen it with my own two eyes. A friend and I were working on 96 polaris snowmobile. It was running fine besides rough idol then all of a sudden it rev up to like 6500 rpm for no reason at all. Tried shutting it down with the key and safety kill but nothing. So removed BOTH spark plug wires and it was still running wide open! We couldn't believe it...totally defies anything I've ever known about engines. Only shut down after we put hands over the carb and choked it out! So we've named the sled Christine after the movie with the possessed car. Thought I'd share.

12-29-2018, 05:06 PM	#2
drift240se Gone Wild Join Date: Oct 2013 Location: Martinsburg WV Posts: 281	Re: I'd never believe it Thats pretty crazy. have seen this once. stuck valves and a lot of gas was in the motor. took about 20 seconds to burn out the extra fuel

12-29-2018, 05:31 PM	#3
dundeebarnbuggy Gone Wild Join Date: Oct 2016 Location: Dundee quebec Posts: 3,190	Re: I'd never believe it Sounds like you have something red hot causing ignition in the cylinder[s]. Spark plug protruding too far allowing exposed threads to get hot? Or, could a huge buildup of carbon causing enough compression to diesel. Keep us posted on what you find?

12-29-2018, 06:43 PM	#4
CP241 Nincompoop village idiot Join Date: Apr 2018 Location: Michigan Posts: 9,673	Re: I'd never believe it I'm guessing it is a 2stroke? It's called a runaway engine. That's common if they run lean. The cylinder gets stupid hot, hot enough to ignite the fuel without the spark plug. It will go sky high RPM and the best way to shut it off is to choke it out and starve it for air. You can pull the spark plug if it's easy, sometimes that's easier to get to than the intake to choke it out. If you dont kill it, it only gets hotter until the engine goes pop. Happens to diesels too, but I've never seen a diesel snowmobile. Any diesel tech keeps a big steel plate handy for just such occasions. Need to Find out why it is running lean or it will happen again

12-29-2018, 10:16 PM	#5
jayp1017 Getting Wild Join Date: Nov 2016 Location: Kalamazoo Michigan Posts: 132	Re: I'd never believe it It is running lean. It's the PTO crank seal...replacing it tomorrow. It's not a snowmobile I really care about honestly but it only has 2000 miles on it. Not going to split the case unless I see crank run out or play in bearing...hoping for the best with trying to slide a new seal in.

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12-29-2018, 10:25 PM	#6
jakesnake27 Gone Wild Join Date: Aug 2009 Posts: 14,245	Re: I'd never believe it I've never heard of anything like this, thanks for sharing!
jakesnake27 Gone Wild Join Date: Aug 2009 Posts: 14,245

12-29-2018, 10:52 PM	#7
Gazoo Gone Wild Join Date: Dec 2017 Location: Montreal, Quebec, Canada Posts: 1,130	Re: I'd never believe it Pre-ignition. Happened to my snowmobile too years back. Freaky phenomenon....

12-31-2018, 03:12 PM	#8
EzgoTXT2002 Getting Wild Join Date: Nov 2018 Location: Ringgold, Georgia Posts: 149	Re: I'd never believe it Scary RPM for sure ! I've seen 2 strokes run backwards, but not a runaway on a two stroke. Plenty of diesel runaways on video but never in person. I keep a thick plywood shut off plate in the engine compartment of my Lariat diesel truck for just such an emergency.

12-31-2018, 03:39 PM	#9
Mooncarter Gone Insane Join Date: Oct 2016 Location: Richmond, VA Posts: 14,214	Re: I'd never believe it Had this happen on an old Cadillac one time. Not WOT but running after pulling wires off. Ran hot.

12-31-2018, 04:20 PM	#10
CP241 Nincompoop village idiot Join Date: Apr 2018 Location: Michigan Posts: 9,673	Re: I'd never believe it I had a Polaris 4wheeler that was a 2stroke that had a carb problem run away on me once. Very scary indeed