Bike stays running without spark?

mwoj · Post by **mwoj** » 08:27 am Nov 29 2016

So I picked up another 1985 kdx200. A whole bunch of parts. Runs greats, tons of power. Anyway, Ive only had it for a few days. Yesterday I finished riding i was holding in clutch, backing the bike up. All of a sudden the throttle revs up, and stays at about 5k rpm (if i had to guess). I tried twisting the throttle tube forward, and nothing. I try the kill switch and nothing. I then go ahead and shut off gas. After a few more seconds it stays running. So Im basically like screw it, and reach to disconnect the spark plug wire. I unplug it completely, and the bike STAYS RUNNING at like 5k rpm. Side note, the bike didnt have an air filter in it at the time. HOW DO I FIX THIS?

I ended up putting it into like 3rd or fourth gear, holding front brake, and dumping the clutch so the engine stalled. But whats the real fix

Post by **Julien D** » 08:29 am Nov 29 2016

Sounds like lean runaway. Firstly, don't run it without an air filter. Secondly, don't let it run out of fuel.

mwoj · Post by **mwoj** » 09:13 am Nov 29 2016

Oh yeah I know. i have a new air filter on now. The old one got destroyed by a mouse or whatever (previous owner cleaned intake/ carb). Do you think it could have just been caused be the lack of an air filter?

Jim B · Post by **Jim B** » 12:14 pm Nov 29 2016

Julien D wrote:Sounds like lean runaway. Firstly, don't run it without an air filter. Secondly, don't let it run out of fuel.

This. I had an old '84 CR250 that used to do that. Just like mwoj, I had to occasionally kill the engine by putting it in gear, holding the brake and releasing the clutch. It turned out the base gasket for the cylinder was installed incorrectly (not by me

) and it was causing an air leak. I'd be surprised if an air leak (may or may not be from the base gasket) was not the cause of the OP's problem.

Jim B · Post by **Jim B** » 12:17 pm Nov 29 2016

mwoj wrote:Do you think it could have just been caused be the lack of an air filter?

I don't think it was caused by lack of an air filter.

two-stroke-brit · Post by **two-stroke-brit** » 12:46 pm Nov 29 2016

a leak down test will rule in or out an air leak or bad seals.

Post by **Julien D** » 09:10 pm Nov 29 2016

Fuel starvation, probably. Massive air leak will contribute. No air filter would compound the problem.

bob_jones · Post by **bob_jones** » 02:18 am Nov 30 2016

Air leak. Absolutely crucial to fix. Lean condition runaway can / will cause major damage.

Dekon · Post by **Dekon** » 08:44 pm Nov 30 2016

I agree with the others, air leak. But, I would also check the plug. On a YZ125 I had, a piece of steel somehow got through the fuel filter and "welded" itself onto the spark plug. It acted as a glow plug causing the engine to keep on running even as I was holding the kill switch down and after I pulled the spark plug cap. The only difference was the bike didn't rev out like yours is doing.

pumpguy · Post by **pumpguy** » 10:31 am Dec 01 2016

Stays running without spark???

Putting aside the air leak issue described above, there has to be a source of ignition. Frequently running on is caused by a glowing carbon ember in the combustion chamber. Think model airplane engine glow plug.

I would look for carbon deposits as a possible cause.

bob_jones · Post by **bob_jones** » 10:53 am Dec 01 2016

Respectfully sir, It's heat. That's why lean runaway is such a dire situation. It essentially turns into a diesel engine. The heat in the combustion chamber itself is the ignition source. It's so lean that yes it does in fact create that much heat. Then the more it revs, the hotter it is. Further, it doesnt matter if the throttle is closed becasue air is coming in from the leak unchecked rather than the carb. The only way to stop it is to eliminate fuel, air, or compression. If you can't do that fast enough, then it will only be stopped by kinetic forces via something letting go in a catastrophic failure.

Yes you are correct that a burning ember could be providing an ignition source. But no it is not correct that there "has to" be. Further, if it were simply undesired combustion taking place without undesired air intake (leak) then the engine would continue to idle but not Rev because there is no uncontrolled air intake. Like when our cars use to continue to "diesel" after turning them of. Ingnition continued but, absent additional air or fuel, they wouldnt Rev on their own like his bike is.

EDIT: Yes we are not there with the bike and yes it could be something other than lean condition. However, lean condition could become dire if the runaway finally happens with him on the bike. Even if it never runs away, lean condition can do plenty of damage anyhow.

pumpguy · Post by **pumpguy** » 02:11 pm Dec 01 2016

Thanks for your explanation Bob, I learned something.

Friedom · Post by **Friedom** » 05:52 pm Dec 01 2016

How does your plug look?

"Fun Wheel Drive"

mwoj · Post by **mwoj** » 07:20 pm Dec 01 2016

Idk if im bugging or something. But like, the bikes fine. I tried to run it today (first time after the issue/new air filter) and I couldnt get it to stay running. So I check the gas, and its almost empty. When the runaway happened I was facing up hill. I dont know if its even possible that this caused it? But I ran it today, runs great now. To make sure, I tossed some flower near the top end gaskets and it didnt get blown out/sucked in. And, I know, prolly not the best way to check for an air leak.
Could the runaway have been caused by air getting sucked into the fuel line?>

Friedom · Post by **Friedom** » 07:52 pm Dec 01 2016

Oh yeah, when you're running out of gas it can surge from leaning out.

"Fun Wheel Drive"

bob_jones · Post by **bob_jones** » 11:14 pm Dec 01 2016

Yes mine does that on the last drops of fuel too. But I'd be real leary that yours kept revving without electricity. A bike of that vintage really should get an leakdown test. It's worth keeping an eye on.

mwoj · Post by **mwoj** » 06:14 am Dec 02 2016

Yeah I'll probably get a leak down tester to make sure nothings wrong. Thank you guys for all your help!

antonyp · Post by **antonyp** » 06:54 am Dec 02 2016

mwoj wrote:Yeah I'll probably get a leak down tester to make sure nothings wrong. Thank you guys for all your help!

If you have the mechanical knowledge and access to parts for said bike (e.g. cylinder and head gaskets) I would do a teardown of the top end to clean and inspect. I would have done that anyway on a recently "recommissioned" vintage bike even if I did not have any problems with it - but that's just me and my humble opinion [emoji12]

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Post by **KDXGarage** » 07:15 am Dec 02 2016

You said you rode it with no filter. Why did you think this was OK??

Friedom · Post by **Friedom** » 11:03 am Dec 02 2016

Because he was excited, it was running, and the filter hadn't arrived yet. And he avoided the dust when riding..

Just a guess.

Not based on my own decisions or experience.

...much...

"Fun Wheel Drive"

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