1987 40 hp Johnson problems

[mike carson]

I have a 1987 40 hp Johnson outboard, while running full throttle RPMs up and down without touching the throttle. Removed old gas replaced with fresh gas and oil. Replaced gas filter, gas line with new connectors and prime bulb, still same problem when motor is working properly wide throttle is around 5300 RPMs now you start motor up throttle sometimes rpms increase sometimes no change rpms between 1100 1300 without touching throttle all of a sudden it takes off wide open then back down around 1100 still have not moved throttle. With motor running using test light with ground when pushed through rubber cap on spark pug, top pug this did not change the motot but when did the same on bottom pug it killed the engine. Replaced spark pugs reset gap, still doing same thing. Could this be my coil or power pack what is the difference between the 2, what would your next move be. HELP PLZ I'm at a loss

 

[Daviet]

Re: 1987 40 hp Johnson problems

Couple of thoughts, try pumping the primer bulb when it acts up and see if it helps, maybe you have a fuel pump going out.
Check the spark with an air gap tester and see if your spark will jump a 7/16" air gap and is bright blue. You might try and squirt some fuel mixture into the upper carb and see if it picks up, if so you have a carb problem.

 

[mike carson]

Re: 1987 40 hp Johnson problems

Couple of thoughts, try pumping the primer bulb when it acts up and see if it helps, maybe you have a fuel pump going out.
Check the spark with an air gap tester and see if your spark will jump a 7/16" air gap and is bright blue. You might try and squirt some fuel mixture into the upper carb and see if it picks up, if so you have a carb problem.

Forget to say I replaced fuel pump had both carburetors rebuilt also.

 

[mike carson]

Re: 1987 40 hp Johnson problems

Forget to say I replaced fuel pump had both carburetors rebuilt also.

Also forget to say yes primer bulb will make it better but stll run rough

 

[Daviet]

Re: 1987 40 hp Johnson problems

Well, you either have a fuel or ignition problem. Use a timing light when it is acting up and see if both cylinders have a constant flash.
If so, squirt some fuel into each carb, if you have a fuel problem in a carb it should pick up when you squirt fuel into it.

 

[mike carson]

Re: 1987 40 hp Johnson problems

Well, you either have a fuel or ignition problem. Use a timing light when it is acting up and see if both cylinders have a constant flash.
If so, squirt some fuel into each carb, if you have a fuel problem in a carb it should pick up when you squirt fuel into it.

I seem to think may be the coil, i have over 100 pis on compression test, with test light with ground wire with engine running stick through rubber cap on spark pug when testing top pug it made no change in engine but when did the same on bottom pug it shut down engine that is why i was thinking coil am i wrong i have put $600 in this motor still having problems also wanted to ask were you in the military? Old Coastie here son serving now on his 9 year he is in Louisiana getting ready for the storm, thanks for your help buddy I'm sure i will continue to bug with questions.

 

[Daviet]

Re: 1987 40 hp Johnson problems

You could have a needle/seat problem in the carb starving it for fuel, it would act the same as a bad coil.
That's why I asked if you checked the spark with an air gap tester. If you have spark, then it is probably a carb problem.

 

[mike carson]

Re: 1987 40 hp Johnson problems

Even just had carburetors rebuilt