MERC 40 Runaway Motor

[jodee95]

Hello All Need some help. I have a 1980 Merc 40hp motor running well no issues until this. The engine accelerated out of control I was lucky I was running off of a garden house engine sped up to very high rpm's. I shut key switch off and motor continued to run. I pulled of both spark plugs leads and motor continued to run for at least 5 seconds more. Any ideas what this could be? It only happen one time key switch works fine now starts and shuts off. Has anyone seen anything like this before.I think I pushed down the throttle some while I started it be not to an extreme. I have no clue never seen this before Thanks for your help.

 

[roncoop75]

Re: MERC 40 Runaway Motor

So THAT'S why they say to never rev a 2 stroke too high.

 

[Blevins]

Re: MERC 40 Runaway Motor

Black/yellow wire maybe? I don't know though the wire only controls the key kill control.

 

[SeaKaye12]

Re: MERC 40 Runaway Motor

Your title line is accurate; that's described as a runaway engine

In place of a spark from the plugs; fuel is ignited by glowing carbon
The glowing carbon acts like a spark plug, and the carb was sucking fuel as it needed it, to stay running. It ran-away like a diesel with no governor on it.

There are a ton of posts on here relating to that. People have had their engines blow up from out of control over-revving. Not just outboards either. Sounds like a scary experience.

Blocking the air intake will stop it. But; how are you gonna access the carbs when the engine is revving wildly and you're expecting it to blow up any minute?

Chuck

 

[achris]

Re: MERC 40 Runaway Motor

You don't need to access the carb, just leave the key on and hit the choke button.... The engine will flood out...

Chris.......

 

[j_martin]

Re: MERC 40 Runaway Motor

To answer the original question. It's time for a serious de-carbon session.

http://forums.iboats.com/showthread.php?t=158076

hope it helps
John