1990 Johnson 15HP Fuel?Air, Idle problems

[btkees]

So, I graciously inherited this motor. It has been sitting for years as far as I know. I got it to start! I am now having issues trying to get it to idle smoothly. I have to constantly play with with the choke to find just the right fuel/air ratio. Once there it will idle fine but when out on the water the vibration will of course move the choke and stall out. Or if I can maintain a good ratio the motor will run high then low with out any adjustments. I have rebuilt the carb and fuel pump, replaced the fuel line. Tweaked with the idle set screw. Kinda lost now. Any recommendation are greatly appreciated. Thanks in advance.

 

[jasper60103]

Re: 1990 Johnson 15HP Fuel?Air, Idle problems

btkees,
welcome aboard.
If its been sitting for years, there's a few maintenance items you need to take care of first.
Start with the Top Secret Files sticky. Lots of good info, or take the motor to your local outboard mechanic
if wrenching is not your thing.
Good luck.

 

[the machinist]

Re: 1990 Johnson 15HP Fuel?Air, Idle problems

Welcome to iboats forum. As jasper said, do some research on reserecting motors that have been setting a while. You are on the right track & there is not much in those newer carbs that can go wrong other than debris. But with those plastic topped carb idle jet shafts, the threads are FINE, so if you have read 1 1/2 turns out is a starting point, not so on these, double it as a start on these carbs.

You may also consider doing a compression check just for jollies. I suspect they may be OK with that new a motor unless something is internally wrong, but good to be sure anyway.

My guess is that you may have it running only on one cylinder. If so with it running, pull one spark plug wire, replace it & pull the other. If it dies when you pull one wire that is the good cylinder & it is dead on the other. Could be only a bad spark plug or corroded wire anywhere from the coil ground to anywhere in between, even inside the spark plug boot.

Could also be a air leak anywhere in the fuel system or the plastic carb top is warped or cracked at the idle screw boss.