Johnson 90 2 speed

[clayton8673]

Hi. I have an issue with my 1996 Johnson 90 hp that has been going on for a while. It is a 90 degree J90TLEDA with about 550 hours that I got last summer and is on a 16 ft Larson with a 135 rating.

The problem is that when you go to full throttle the boat responds smoothly, gets on plane well, and runs up to 37-39 mph (depends on boat load). This would all seem great except that at some point after reaching this top end speed the engine will suddenly increase power noticeably and raise top speed to a steady 41-44 mph (again based on load).

This power increase can come anywhere from about 10 seconds to 20 minutes after going to full throttle and the amount of time taken is inconsistent from run to run but most often is about 30 seconds to 2 minutes. It happens in all temp and weather conditions.

I rebuilt the carbs with no effect.
I tried taking fuel pressure reading with boat running wot and things seemed ok but it was hard to get accurate readings with the guage needle bouncing around. While I was out there I also took air box cover off and tried a little manual richening with a spray bottle at wot also with no positive effect. Obviously my initial thoughts led me to suspect fuel issues but I'm doubtful of that now.

I have quite a bit of auto and motorcycle experience but obviously don't know much about outboards. Anybody got any ideas for what to check next?

Thanks,
Ken

 

[Silvertip]

Re: Johnson 90 2 speed

Sounds like you may have a cylinder that's either loading up at lower speeds and comes back on line after a longer high speed run. Make a run where you have not yet experienced the power gain. Kill the engine and read the plugs. Bet you find one loaded up. Could be a carb issue or weak spark. Put the plugs back in and run it again until it gains the power you mention. Run it that way for a good distance. Again -- kill the engine and read the plugs again. If you had one loaded up before, it probably is not loaded up then.

 

[clayton8673]

Re: Johnson 90 2 speed

Wow thanks for the quick reply.
Can a v-4 outboard like that seem that smooth and torquey on just 3 cyl?
Everything feels and sounds normal even when it isn't pulling fast but I guess the motor is pretty well isolated.

Ken

 

[WillyBWright]

Re: Johnson 90 2 speed

You can check for spark while running with a timing light. Connect one to each plug wire on successive runs. If it sparks, it flashes. Tape the trigger on and set it someplace convenient to see while driving. I think you'll find that you have one cutting in and out. It can have smooth acceleration running on three. On four I'll bet it'd jump out of the water.