1999 2cyl. 40 hp 2 stroke- "takes off" and runs high RPM

[Mark Gall]

I am working on a 1999, 2 cyl. 40 hp 2 stroke Johnson outboard. Most of the time it runs pretty good, but on occasion the guy has a very difficult time getting it to idle properly. Went through all of the Sync and Link procedures and adjustments several times. Decided to take the carbs off and check them out. Tore down, cleaned, and put kits in both carbs. After putting both carbs back on, I was making "final" adjustments, when the motor sped up to almost 3000 rpms, and would not slow down or shut off at the switch. Even after pulling both plug wires, it continued to run until I choked off air to both carbs. Never seen this happen in over 30 years of messing with boat motors. An ideas?

 

[Boobie]

That's normal for these motors if you over rev them on a flusher.

 

[bill_w]

It sounds like the motor has a lot of carbon in it.. the carbon got hot enough to fire the incoming fuel-air mix by itself, that's why pulling the plug wires didn't shut it off

 

[Docksidemarineservices]

Bill_w is correct. Using cheap 2cyc oil will carbon up your engine and cause this problem. Decarb your engine and use only XD50 oil.