Lean runaway scenario on a motor

**rocketman1** · 24th January 2008, 18:24

Has anyone heard on a lean runaway situation on motor where a normal motor will rev out of control till it blows itself to bits.
Apparently happens now & then on motorcycle and aircraft engines, & deisels
Any one know a reason why this should happen?

**crazyxr250rider** · 24th January 2008, 22:12

Only heard of this in diesel engines where theres been a LPG leak or similar and the engine started reving on its own way past the the normal rev limit causing it to (predictably) blow up. I canot imagine this happening to an engine with direct control of air flow ie. throttle slide,butterfly ,ect.Unless there was an air leak of some desription then that would be a whole different story....

**Disco Dan** · 24th January 2008, 22:33

That must be on YouTube somewhere!

**Kickaha** · 25th January 2008, 05:41

Originally Posted by rocketman1

Has anyone heard on a lean runaway situation on motor where a normal motor will rev out of control till it blows itself to bits. ie no throttle
Apparently happens now & then on motorcycle and aircraft engines, & deisels
Any one know a reason why this should happen?

I've had it happen on my IT200 when the barrel came loose so it was very lean and it revved it's nuts off, I did mange to shut it down though

**B0000M** · 25th January 2008, 06:51

had it happen on a jetski,(2 stroke 650 twin) it reved and revved, took the leads off, which had no effect managed to stop it by removing a spark plug- which went flying across the shed never to be seen again,

luckliy it didnt get as far as blowing itself to bits

**skidMark** · 25th January 2008, 07:41

Originally Posted by B0000M

had it happen on a jetski,(2 stroke 650 twin) it reved and revved, took the leads off, which had no effect managed to stop it by removing a spark plug- which went flying across the shed never to be seen again,

luckliy it didnt get as far as blowing itself to bits

jesus you were keen.....i wouldve just clamped the fuel line/ ripped it off.

**Ktmboy** · 25th January 2008, 07:43

Have heard of this before and my old RD350 used to do a similar thing. Sometimes when you throttled off it would keep revving at 12 grand and not button off. The only way to stop was to hit the kill switch and the brakes. Fuckin dangerous in the wet at 80mph. Turned out to be leaky inlets.

I also had a funny occurance once on a YZ125 (back in the seventies). After we had given it an over haul. When we started it up it was sweet. But when we dropped the clutch the freakin went backwards and through the rider over the bars.

Funniest thing.

Something to do with the timing being 180 deg out I think.

**takitimu** · 25th January 2008, 10:06

Originally Posted by skidMark

jesus you were keen.....i wouldve just clamped the fuel line/ ripped it off.

That does sound the safer if less quick instant option, that spark plug must have left the head at some serious speed.

**bungbung** · 25th January 2008, 11:39

You will still have to wait until the carb bowl(s) empties out

**B0000M** · 25th January 2008, 16:39

spark plug spanner was at hand, screwdriver for hose clamps wasnt. fuel tap was broken... not many alternatives. and yes it did leave at some speed. didnt really think about it going flying as i was undoing it, i gues i was lucky i wasnt in its path

**pete376403** · 25th January 2008, 16:58

Originally Posted by Kickaha

I've had it happen on my IT200 when the barrel came loose so it was very lean and it revved it's nuts off, I did mange to shut it down though

My IT400 did that too - on an adventure ride, going up a hill in a nice orderly queue and suddenly the thing flat took off. I dropped it (IT "brakes" being what they were and turned the fuel off, but it seemed to take a very long time to stop. Only had a short 12mm open end spanner in the tool kit so had to retighten the nuts every now and then. Bought a cheap socket set at a garage and pulled the head off, then tightened the barrel properly. (two strokes are great like that) Worked ok after that but never as fast as it was while the barrel was loose

**dammad1** · 25th January 2008, 17:58

Originally Posted by Ktmboy

I also had a funny occurance once on a YZ125 (back in the seventies). After we had given it an over haul. When we started it up it was sweet. But when we dropped the clutch the freakin went backwards and through the rider over the bars.

Funniest thing.

Something to do with the timing being 180 deg out I think.

I saw the same thing on my brothers old IT200 took him a while to figure out what was going on.

**Sir_hoppalot** · 28th January 2008, 17:47

This can happen if the throttle lead has a kink in it or gets pinched.

**Titanium** · 28th January 2008, 18:18

It is simply an air leak after the carby that causes a lean runaway.

**cheese** · 28th January 2008, 20:01

Originally Posted by Ktmboy

I also had a funny occurance once on a YZ125 (back in the seventies). After we had given it an over haul. When we started it up it was sweet. But when we dropped the clutch the freakin went backwards and through the rider over the bars.

Funniest thing.

Something to do with the timing being 180 deg out I think.

LOL that is some funny shit!