Motor won't quit!

[BufoBolt]

Hi all, earlier today started my 73 250WR on a stand. It was idling way way too high. On trying to stop the motor with the kill switch no joy. Turned off the fuel tap and it was still screaming at 5000 revs 3 minutes later. No problem I pulled off the plug cap but the motor just kept running. Wound out the air slide screw and still screaming. Stuffing a heavy rag up the tail pipe had no effect. I finally killed the demonic beast by pulling off the fuel pipe and waiting for the carb to run dry.

OK I figured out the following - exhaust leak between manifold stopped me from suffocating motor, leaking fuel tap didn't stop fuel flow to the carb, lack of slack in throttle cable kept slide open.

But how did it run for at least 5 minutes without the plug lead attached? The motor was not particularly hot. Have I been buying and installing spark plugs needlessly all these years?! And what can I rig up to be sure that when I want the motor to quit it really will quit. I don't have much confidence in electrical kill switches anymore. Thanks, Kris

 

[Gord]

I have had the same thing happen on an H1 500, when the slides stuck open. I think it might only happens with 2 strokes, and I think it's called dieseling. Someone else will be able to explain it better than me. I haven't had it happen with my 73 Husky yet. Pretty scary though.
And it has nothing to do with your kill switch. I turned off the kill switch and the key to my H1, as well as all three plug wires and it had no affect.

 

[Bigbill]

The carbon in the head turns red hot like a glow plug.

My buddy rebuild a Harley chopper and my other buddy went for a ride they doubled up on it. It had no kill switch. They got on the m parkway in Bridgeport, the carb stuck they were past Norwalk when the gas ran out. This was 12am at night.

I'm always very careful on the first rides. I use the cable plumbing system and the carb slide is oiled too. Make sure the cable is free.

 

[fran...k.]

At least it didn't rev high enough to self destruct. Probably the best thing is to just to put it into gear and stall it out. Quite possibly the base gasket leaks, Possibly a piece of carbon came loose and lodged in the spark plug. Have a look. Of course it could in part involve the slide wasn't able to go down all the way because of what the cable dictated.

 

[Bigbill]

Always make sure the throttle cable has some free play in the housing. Same thing on the clutch cable.

 

[justintendo]

always check to make sure your kill switch actually kills spark, and throttle has play in it, fuel tap really works. dangerous to not do so, and quite honestly cardinal safety mistakes.
not a good idea to let the bike just run out of fuel, fuel lubes a 2 stroke as they have no motor oil like a 4 stroke.
the bike just sat screamin for minutes? oh boy, thats scary. pull the filter and seal off intake air instead of letting it sit there.
the others are probably right about carbon getting hot and keep the engine dieseling. you likely have a air leak somewhere but you have already identified several thing to fix like a new petcock, diagnose kill switch, and investigate the perhaps root issue of why its idling so high to begin with. is it just the lack of cable slack?

 

[Gord]

I could be wrong but a new kill switch has nothing to do with what happened. He could have had 3 kill switches and it wouldn't have stalled the motor once it revs and diesels like he said. He also pulled the plug wire off. When the head is glowing, like Bill said, it doesn't need spark to keep running.

 

[justintendo]

could be true, hard to tell what caused the bike to get to the dieseling condition. its possible a working kill would have shut the motor off. after the kill didnt work (which we dont know if it ever worked) he simply turned the fuel tap off while it sat at 5 grand for minutes. its likely it got to the diesel state during this time. a motor doesnt get combustion temps hot enough to run without spark as soon as you start it, it takes awhile. he states as soon as he started it it was idling high, which we dont know if that was because of the no-slack throttle or a lean air leak or what. sounds to me like several things wrong here.

 

[Long_Stroke]

Grab something (anything) and hold it over the exhaust to stop the motor. Rag, piece of wood, gloved hand, etc.

 

[dkwkid]

Seadoo 2strokes do this under no-load conditions(running the motor on the trailer after a day of running to blow the water out of the exhaust) I pull the choke-it is a butterfy valve. Some guys open the throttle wide-cool fuel stops the dieseling as there is no spark to keep it going

 

[BufoBolt]

Many thanks for all your replies.

I have uploaded a photo of the runaway beast. In fact this was its first engine run since a total bike resto including professional rebuild of the engine bottom end - new bearings and seals, newly rebuilt and balanced crank and nos rod. The tank shown in the photo is a spare which I used just for the engine run. It was old and tired - and so was the fuel petcock! For sure I was not focused on the detail and ran into a perfect storm of small build errors which when it came to it prevented me from shutting down the run away motor. For anyone interested I have also posted a before photo of the bike.

My attention had been on trying to start the bike. Any thoughts on safety and making sure I could stop it just didn't figure in the equation

From your comments the most plausible cause was dieseling although I have never come across this condition before and don't know much about it. I was using a 20:1 mixture. Would this have helped cause it?

Another factor I should mention here is that although it is a WR I couldn't find an original 250WR cylinder when rebuilding it and went with a used cylinder from a 1972 250 Cr. Wanting a more flexible power range of the WR I thought it would interesting to try the modification described in the April 74 Cycle News article reproduced in Husky Club Newsletter #23. The cylinder had already had its ports modified when I acquired it and I had a Husky 250 DH reed block lying around. I just had to get the piston machined with additional ports and the bottom skirt trimmed.

I don't think these mods would change the compression ratio from that of a stock 250Cr but I am thinking maybe having the reed set up made it easier for the dieseling condition to occur.

I am drawing out this postmortem a little because I want to be sure it doesn't happen again. I vividly remember the wave of incomprehension followed by disbelief followed by panic that hit me after I pulled off the plug lead, and that screaming two stroke crescendo of noise and blue haze just kept on coming! I wouldn't want that to happen again. Kris

 

[justintendo]

Grab something (anything) and hold it over the exhaust to stop the motor. Rag, piece of wood, gloved hand, etc.

wont work if theres a leak in the exhaust, as the poster found out. my 87 250 had a leak between the chamber and silencer, and often i would forget to remove the exhaust wash plug. bike would still start 1st/2nd kick..

 

[dartyppyt]

I had a suzuki do that when I was racing. I had to actually jump off the bike. The crank seal went bad and I even pulled the spark plug cap.
The motor gets so hot that it ignites the fuel and keeps it running.
The only way I got mine to stop is shutting the fuel off.

 

[fran...k.]

Many thanks for all your replies.

.... In fact this was its first engine run since a total bike resto including professional rebuild of the engine bottom end - new bearings and seals, newly rebuilt and balanced crank and nos rod. ....Kris

How about the top end was it de coked as the term is in my British motorcycle literature?

Just because something is professionaly rebuilt doesn't mean the main guy did the work.

 

[2premo]

like fran...k. said put it in gear, had this happen on a Yamaha in 74, but knew that piece of info ahead of time
the 2 stroke version is called glow plug, when the timing is advanced enough it will heat the plug tip quickly and leaning the fuel as in shutting off the petcock makes it worse

http://en.wikipedia.org/wiki/Glow_plug_(model_engine)

 

[Kartwheel68]

You have an air leak. The easiest way to shut it off is flip the choke on, if you have a Bing with no choke, hold the tickler down until it stops.

 

[grouty]

You have an air leak. The easiest way to shut it off is flip the choke on, if you have a Bing with no choke, hold the tickler down until it stops.

If you have a Bing you can flip the float bowl off. Handy little wire clip on the bottom !. I have seen that save many a run-away Maico

 

[Auto5guy]

An air leaking, dieseling, runaway two stroke is spinning at a fantastic rpm but is not really generating much for power. Put it in gear, foot on the brake, dump the clutch. Problem solved. The lean burn can't overcome an actual load and the bike won't get away from you.

 

[Gord]

The easiest way to shut it off is flip the choke on.

Above, I told of my H1 500 doing the same thing. It happened instantly. As soon as I started the bike, I knew something was wrong and could not shut it off. The choke, like Kartwheel68 said, is the only thing that stopped it.

 

[dartyppyt]

Take engine out next time. Put a handle on it and 48 in bar!

Turn that baby into a hot saw!