Runaway Motor

[C-Beans]

After dipping my carburetor I put it all back together and, thankfully, tested out the motor (Tohatsu m40d) before launching the following morning.
Was cleaning and testing because I could go slow and go fast but the motor acted funny in the middle.
Anyway, started it up and it ran rough for a minute but warmed up. While I was tweeking the throttle to see if I fixed that rough spot between speeds the engine just, on it's own, goes full throttle!

I put it in neutral and the propeller stopped but the engine continued to run as hard as it could and really smooth, no shaking or skipping, but I couldn't stop it.

I turned off the key, nothing.
I pulled the emergency lanyard, nothing.
Shut off the battery, still nothing.
Pulled the spark plug wires, nada.
Engine was still screaming at full throttle!
Disconnected the fuel line and she finally ran out of gas.

Any ideas?
I've heard of deiseling but not at such high RMPs.

This may not mean much but I'll mention I added Sea Foam to the gas and also got a few shocks from the spark plug cables while disconnecting them though as I mentioned the battery was shut off.

Thanks. Any thoughts would be appreciated.

 

[GA_Boater]

You did the no-no. Ran the motor at high speed on muffs and it ran away.

This is one of the things we warn about over and over again.

This is also in the wrong section. Moving it to the Tohatsu section.

 

[C-Beans]

Ah! Thank you for moving my post. Led me to answers. Im curious if thermal runaway occurs with the motor in a bag or trashcan full of water.
Hope I didn't burn it up.

 

[Alumarine]

I've seen that happen on a friends 2 stroke Carabela motocross bike.
Started it up and it went full throttle.
Hit kill switch nothing.
Put it in second gear and popped the clutch and the chain broke and flew 50 feet behind bike.
Pull plug wire, nothing.
Laid it on it side and it ran for another 30 seconds.
Turned out head was cracked. Not sure if that's why it ran away but after getting the head welded it ran fine.

 

[ahicks]

Runaway like this is typical for any 2 stroke motor. High rpms with no load is a no-no for sure. Now you see why. It's not about how you are cooling it. It's ALL about the high rpms.

Stuffing a rag in the intake, or if so equipped, closing the choke(s) is about the quickest way to shut them down.