runnaway motor

[Part-time]

My neighbours '84 evinrude 60 hp twin was just started for the first time this year.
Last year it had no isues.
It started fine but has an eratic miss... then it starts!
the miss goes away and the RPM's creep up (no tack to know exact numbers)
but higher than normal idle RPM.
then slows back down and has miss again.
During this time if you turn the key off engine will shut off.
After a little while the engine will rev up and down going a bit higher in rpm's
and then the RPM's will go wide open.
I mean this engine was just screaming!!!
Turn the key off, nothing, it just keeps reving out of control.
This is in neutral, linkage is in the position for idle, never moved, and the carbs are in the idle position as well.
What the heck is going on???

 

[HighTrim]

Re: runnaway motor

Was the owner revving the motor before it ran away on you? Im also assuming you are doing this on the muffs?

It is a scary thing isnt it!? Best way to kill a motor going into thermal runaway is to cover the carb intake with a rag. As you found out, turning off key wont help.

BTW, love the French River, grew up there practically as a kid at my Uncles Camp.

 

[Part-time]

Re: runnaway motor

Was the owner revving the motor before it ran away on you? Im also assuming you are doing this on the muffs?

It is a scary thing isnt it!? Best way to kill a motor going into thermal runaway is to cover the carb intake with a rag. As you found out, turning off key wont help.

BTW, love the French River, grew up there practically as a kid at my Uncles Camp.

No to the first and Yes to the rest... but what will cause this, and how to fix it?
Thanks, PT.

 

[glenn property of pam]

Re: runnaway motor

intresting the key kills the magneto so that should cut out the spark and stop a run away. check that all the carb butterflys are closed all the way things like the cold start lever could be holding the carbs slightly open. also check that on the non linkage side of the carbs the roll pin is not caught on the carb bolt holding one carb open. then next check would be trying to pump the fuel bulb as a carb thats running empty can cause a problem.

 

[nwcove]

Re: runnaway motor

sounds like the motor could use an aggressive decarb. jmo

 

[Part-time]

Re: runnaway motor

Could the primer cause this??
A short in the ignition switch holding the primer open and at the same time not shutting the engine off when turned off?
But if the key is held pushed in on a running motor would the primer give the engine enough gas to run away like this?

 

[nwcove]

Re: runnaway motor

intresting the key kills the magneto so that should cut out the spark and stop a run away. check that all the carb butterflys are closed all the way things like the cold start lever could be holding the carbs slightly open. also check that on the non linkage side of the carbs the roll pin is not caught on the carb bolt holding one carb open. then next check would be trying to pump the fuel bulb as a carb thats running empty can cause a problem.

killing the spark wont stop a run-a-way engine. you need to either stop the fuel or stop the air. ( fuel is hard to stop as there is plenty in the float bowl(s) ).

 

[AlTn]

Re: runnaway motor

do a google search on runaway conditions in 2 stroke engines...scary stuff as autoignition is scary...seems one constant is a lean condition combined with a no load situation...as nw states stop the lean condition by shutting off the air, or oddly enough, cause a too rich condition by applying the choke or flood it with the primer...bottom line seems to be to correct the lean condition at idle

 

[HighTrim]

Re: runnaway motor

Yeah Im guessing there is either an air leak in the fuel line, low speed needle set too lean, crankcase leak, intake manifold leak, carb gasket leak, etc....

Stopping the air is the best method as mentioned. Pulling the fuel line will still give the motor enough time to detonate, potentially severely injuring you, as the fuel in the bowl will keep it running.

Turning off the key does not kill it since it no longer needs spark. Preignition is taking place. This can be caused by a red spark plug, or even a chunk of carbon in the cylinder that is glowing. This preignites the fuel mixture and causes it to continue picking up RPMS until destruction. This is also due to the lack of backpressure on the exhaust, which is why it happens on the muffs.

Give her a decarb, and try to source out the reason for the lean condition.

 

[Part-time]

Re: runnaway motor

OK thanks guys.