lean runaway scenario

[rocketman1]

Has anyone heard of a lean runaway scenario on a motor , where the motor revs out of control, till it blows up.
Any one have a reason why this would happen?

[Ixion]

I've seen it happen on two strokes. Never on a four stroke. None of them actually blew up, but they were hitting the red line and beyond for a while .

One wouldn't stop even with the ignition off! Had to turn the fuel off and wait for it to run out.

[Motu]

It happens often on diesels - they run with an open intake,so they take a full charge of air whatever the speed or load....engine speed is controlled by fuel quantity.....the fuel is controlled by the govenor.If fuel is supplied by an external source,then the engine will run with it too however much is supplied....if it is over supplied with fuel it will rev beyond it's physical limits.The govenor can't cut back the fuel to lower the revs....so it runs uncontrolled.I've had it happen when I filled an engine with oil too fast,and then started it without letting it drain to the sump (nearly knock off time) The oil still in the rocker cover was pumped through the breather into the intake.Very scary!!!!!:Oops: Two stroke diesels can do it too as they can run backwards - the centrifugal govenor can't work with reverse rotation.....so they can just rev to whatever the limits are...or are not.So a 2 stroke diesel will often run a dephaser in the pump drive so the timing goes out in reverse.

I've had it happen on an industrial motor too - it lost it's throttle plate,so that gave it WOT,and of course the govenor couldn't stop that either.Luckily a side valve engine has some rev limits built in.