RUNAWAY JOHNSON OUTBOARDS (why do they do that)

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fisherman67

Cadet
Joined
Jan 27, 2011
Messages
11
Why do the johnson 2 cylinder outboards have a tendacy to runaway to high rpm's when you have them hooked to a flushing device? It has happened to me twice while I was flushing the motor, turned off the key, the fuel line was already unhooked and I even pulled the spark plug wires off and it still would not shut down, only after about a minute of running away at very high rpm's did it finally shut down. This is a 48hp 1986 johnson. It also happened to my brother's 55hp when he was flushing it out. Happened from an idle without throttling it up. Help, why does it do this?
 
nwcove

nwcove

Admiral
Joined
May 16, 2011
Messages
6,293
Re: RUNAWAY JOHNSON OUTBOARDS (why do they do that)

not just omc's that do that!!! a good decarb will lessen the chance of it happening. jmo
 
HighTrim

HighTrim

Supreme Mariner
Joined
Jun 21, 2007
Messages
10,486
Re: RUNAWAY JOHNSON OUTBOARDS (why do they do that)

Carbon is igniting in the cylinder. As stated, a decarb may help prevent it.

A google search of thermal runaway will provide some good reading.
 
8

89renken175

Cadet
Joined
Feb 1, 2012
Messages
13
Re: RUNAWAY JOHNSON OUTBOARDS (why do they do that)

On a flush kit there is no back compression to slow her down. You don't ever want rev any higher than 1500 rpm out of the water on muffs only. I've heard of powerheads running away with everything disconnected and coming apart when reved to high.
 
B

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: RUNAWAY JOHNSON OUTBOARDS (why do they do that)

I agree with the carbon causing the prob. If it ever happens again, grab a shop towel or some rags and stuff over the air silencer intakes.
 
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