89 Johnson 70HP runs WOT for ~5min but then acts up...

[Lord Tywin]

If I run this around 70% throttle it will run all day. If I push it to full throttle it runs fine for about 5 minutes or so. Then it acts like it drops a cylinder. It will "fast idle" back home or if I turn it off for ~5 minutes it goes back to normal. One thought was some trash in a carb but I rebuilt all 3 and not only were they clean already, it still does this. I have an actual service manual and can do almost anything needed, I just don't know even where to start diagnosing it. Some kind of ignition problem?

Any ideas welcomed!

 

[TunaFish389]

Look up S.L.O.W mode for your engine. Lots of info. Has to do with overheat and head sensor.

 

[Lord Tywin]

Looking into that and it sometimes (on youtube) looks/sounds similar. Will investigate the overheat sensor. It pees cold water and the engine isnt hot to the touch.

 

[Lord Tywin]

It was in fact the overheat sensor. ran it WOT til it produced the result, disconnected the sensor and fired it up. Ran great. Plugged it back in and problem returned. Thanks for pointing me in the right direction!

 

[TunaFish389]

Yeah I don't know too much about it personally but have heard about it. That's why I didn't want to go into detail but glad you found out what is causing issue.

 

[Lord Tywin]

So now I need to find out how to determine if it's a bad sensor or if there is a cooling problem.

 

[TunaFish389]

I am sure there is a way to test the sensor. Maybe in a pan of hot water. I am just guessing here but might be a change in resistance or even a open/close trigger.

 

[Lord Tywin]

I believe there's an open/close that can be tested with a meter...a pan of water would be the fit. Put a thermometer in the water and check for continuance etc...good idea!