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Discussion Starter #1
Some of you may have seen my JS550 thread over in the Kawasaki section. Basically I've been having trouble getting the ski to idle and it will randomly 'runaway' like a deisel. Killing the spark does nothing. The only way to stop it is to choke it out or prime it and flood it.

Hopefully this thread will help someone down the road and save a few engines. The first time it happened to me I was in a panic trying to stop the engine before it blew.

SO - I'd like to hear some of your thoughts/opinions on the subject.

Engine Runaway

Engine runaway is when the motor revs up freely on it's own. Preignition is taking place in which no spark is needed. A red spark plug or peice of glowing carbon preignites the fuel mixture and causes engine to continue picking up speed until failure.

Causes

In order for this to happen, there has to be a lean condition present. Any kind of air leak could potentially lead to destruction. Intake gaskets, carb gaskets and crank seals seem to be common culprits.

I'm guessing a pourly tuned carb could also be a problem? I'm starting to think my crank seals are fine but my high mixture screw is too lean. The ski has higher compression than stock but I think we adjusted the carb back to stock for a starting point.

How to stop it

I was absolutely shocked when my ski started to runaway. I immediately grabbed the spark wires and it continued to pick up speed. My second reaction was to put my hand over the intake but my flame arrester plate was still on, so I wasn't able to choke it out with my hand. Finally I grabbed my primer and pumped it a few times to flood it. Shew!
 

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Your explaination is pretty much what I've always heard as a cause for a run-away engine = air leak somewhere in the system, which results in a lean condition, which in turn causes the cylinders to get so hot that it will actually keep running without spark.

Best way to stop it, is to do what you did by hitting the primer, holding the throttle wide open and try to flood it. I've always been told never to pull the spark plug wires on a running engine, but I don't know the reason.

It usually only happens on the trailer when there is no load on the impeller, but I know one guy that claims it happened to him while riding the ski for a very short burst. Now the problem is locating the source of the problem.........it can be tough to track down sometimes.
 

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Discussion Starter #3
It makes sense that it would have a hard time revving out like that while under load. It's a lot easier for the engine to turn out of water.

On second glance, I don't think a pourly adjusted carb would cause a ski to runaway. If your throttle plate isn't stuck open...air has to be coming in from somewhere.
 

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On second glance, I don't think a pourly adjusted carb would cause a ski to runaway. If your throttle plate isn't stuck open...air has to be coming in from somewhere.
I think you are right. Someone else posted a photo of a hole burnt through a piston, that I bet was from a lean condition caused by the carb. on that one cylinder. If your engine only had one carb, and it was not properly adjusted, it seems that it could lead to the run-away situation. Probably wouldn't cause a run-away on a multiple carbed machine, since the other cylinders would help keep it in check, but you still get that one cylinder that gets cooked.
 

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Discussion Starter #5
^ Yep...looks like you may be right. My JS has a single carb so it's looking like a pour adjustment may have caused it.

After pulling the engine - I was able to take a look at the front and rear seals. Both appear to be in fine condition with no signs of any leakage. I'm pretty much puzzled at this point.
 

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I know this is an old thread, but I have just had the following experience on my Seadoo GSX Ltd. I did fix the issue by re-pumping the throttle, so does this sound like overun still?

After a great day on the ski with family yesterday, i took the ski out of the water and started my usual post ski clean up.. this includes running the ski for circa 10 mins on clean water to flush her out, toward the end of this i gave her a few extra revs only to find that it didn't return to idle, anyway I pressed the stop button to turn the engine off and this did nothing, so then i pulled the lanyard/fob and still nothing, so then i pulled the battery terminals (click release) off - this still did nothing!!! I started to pull the spark plug leads off when i thought i will just give the throttle another squirt, this cleared it as it had locked on, but obviously a bit concerned that the start/stop didn't work and really concerned that the Fob removal (lanyard kill switch) didn't do anything either!! - does anyone have any experience of this?
 

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I was trying to tune my cabs and had the same issue where it rev'd loud and high and nothing stopped it. I ended up shutting the fuel shutoff off in order for it to finaLLY die out. I think that this is due to the carbs needing to be adjusted still. I found out i was running really lean and opened up the low speed screw to allow more fuel, I have not had it in the water since but will try it before i put away for the season. It seems to make sense that its a carb adjustment.
 

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Has anyone mentioned to you what your high rev issue was out of water?> I had this happen to me as well but it was on my trailer in the driveway and since i opened up the low end screws a half turn to almost 1.5 turns out, it has been fine. sounds like for me it was not getting enough fuel. This also caused me not to get on plane when on the water too. i could not get it up when i gave it throttle all the way. Never died but would not plane up in water.
 
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