Johnson 1964 28hp over rev

[Kotoski]

Hi folks, First time on forum and I have a problem.
I just rebuilt my carb on my 28 and she starts first pull. My problem is that when I crank the throttle in neutral and then throttle back she will run away (over rev) and not come down until I hit the kill switch to reduce rpm's then she will idle perfectly. Anybody have a clue to this issue?

 

[JB]

Re: Johnson 1964 28hp over rev

Hi, Kotoski. :welcome:

That is called thermal runaway and it is the reason we caution never to rev outboards in Neutral. It usually requires removal of the fuel source.

 

[HighTrim]

Re: Johnson 1964 28hp over rev

Try a thorough decarb with Seafoam. There is a thread in the FAQS to help with this.

These Big Twins are notorious over revvers. Killing the ignition usually solves it, so not technically thermal per se. More likely high vacuum coupled with the forces of the already spinning flywheel. Just guesses there.

Carbon build up I feel is a large contributing factor to true thermal runaway due to the glowing embers igniting the fuel/air mixture under compression.

If in a true thermal runaway mode, the spark plugs are redundant so killing the ignition is pointless. The best way to stop a true runaway is the pull the fuel line and cover the intake with a rag to choke it out.

 

[Kotoski]

Re: Johnson 1964 28hp over rev

Thanks for the info, when I hit the stop button she revved down so by your info this was not thermal runaway.
My biggest question was answered that this is normal behavior which I had not experienced before.

Thanks again, Kotoski