Johnson goes into WOT on it's own

[Fistful]

Model: 1987 Johnson 48 SPL

Issue: Yesterday as I was running the motor with muffs for the first start up of the year (modulated idles for about 10 min), I was at a bout 1/3 throttle for about 30 sec when the motor suddenly sped up to WOT on it's own. I pulled it back to neutral, turned the key off, and pulled the fuel line from the engine itself and still it continued, wide open until it finished off the remaining fuel (about 30-45 sec.).

Quick history: This also happened last fall when I was winterizing it in the same manner. At that time the muffs had unbenounced to me slipped off allowing the motor to heat up. I figured that was the cause then (I proceeded to restart and finish winterizing it an hour later -to cool down - with no issues).

I hope some of you have some ideas. Thanks in advance for your assitance with a 1st time poster.

Tom

 

[JB]

Re: Johnson goes into WOT on it's own

Normal behavior, Fistful. It is called thermal runaway and is caused by revving the engine without a load on it.

Never rev the engine over about 1200rpm without a load.

 

[Fistful]

Re: Johnson goes into WOT on it's own

Thanks JB. I think you nailed it. I guess I got away lucky - no damage other than a blow head gasket (probably due to have failed soon anyway). I will be much more careful in the future. Funny thing is many winterizing instructions speak of running at "1/2 throttle" while fogging out the engine.

My runaway occured at 1/3 throttle (I don't have a tacometer).

Thanks again for your advice.

 

[noelm]

Re: Johnson goes into WOT on it's own

runaway can occur for various reasons, one stated here, and another from a very lean burn, just like when you run out of fuel, except you are still getting fuel and it keeps going, even with key off, only way to stop it is to choke off the air supply! pretty exciting isn't it?

 

[F_R]

Re: Johnson goes into WOT on it's own

Whoever wrote those instructions had a case of stupid. Never run a motor at half throttle in neutral. Never run it above fast idle in neutral.

 

[kenmyfam]

Re: Johnson goes into WOT on it's own

Whoever wrote those instructions had a case of stupid. Never run a motor at half throttle in neutral. Never run it above fast idle in neutral.

Totally agree. Stupidity !!!!