90 hp johnson stutter/misfire above 4000rpm

[logger93]

Took my 83 j90tlcte out for a test drive, post seafoam. Sure starts up easier and idles a lot smother. However it did not get rid of the misfire/ stutter above 4000rpm's. In fact after 5 years of ownership and running 4500 rpms, then 4300,(low end of spectrum I know) now down to 4100 at WOT.It always runs strong initially and then somewhere between 1hr and mid trip it starts to mi****. There is no stutter or stammer ever below 4000rpm's.The tach dosen't drop during misfire and briefly it will surge as if it is running properly. Lost a couple hundred after seafoam, although still have seafoam in tank.
I have replaced or had replaced p/p's, stator, timer base, secondary ignition coils, plugs , wires. I have pulled fuel tank and replaced lines to engine. Mechanics have been unable to replicate stammer in /at shop
All four plugs burn a light golden brown color. Plug housing's are zip tied to rubber housing to prevent grounding with hood latch.
I am going to rebuild fuel pump because I know I can do it, and then look to rebuild carbs. I read from clymers( i know, I know) that there were two different production runs on carbs. Any way to id whether 82 or 83 model easily so I can order rebuild kits. Any tips specific to these carbs for rebuild. I have read FAQ's on subject.
All plugs look same . Don't think I am getting h2o in cylinders. I am limited in electrical testing equipment.All I have is a multimeter.
Any and all insight into what is causing this mi**** and the lowish RPMS at WOT would be appreciated. Joe

 

[emdsapmgr]

Re: 90 hp johnson stutter/misfire above 4000rpm

I'd be interested in what is going on with the ignition system at max throttle. Have you ever put an inductive timing light on each plug wire when the engine starts to sputter at 4200? The light will show the flashes from each plug wire and may show that one is either misfiring or has a weak fire. The carb kit(s) you need is 439076, which will cover either of the carbs you may have. It's a universal kit.

 

[countvlad]

Re: 90 hp johnson stutter/misfire above 4000rpm

Took my 83 j90tlcte out for a test drive, post seafoam. Sure starts up easier and idles a lot smother. However it did not get rid of the misfire/ stutter above 4000rpm's. In fact after 5 years of ownership and running 4500 rpms, then 4300,(low end of spectrum I know) now down to 4100 at WOT.It always runs strong initially and then somewhere between 1hr and mid trip it starts to mi****. There is no stutter or stammer ever below 4000rpm's.The tach dosen't drop during misfire and briefly it will surge as if it is running properly. Lost a couple hundred after seafoam, although still have seafoam in tank.
I have replaced or had replaced p/p's, stator, timer base, secondary ignition coils, plugs , wires. I have pulled fuel tank and replaced lines to engine. Mechanics have been unable to replicate stammer in /at shop
All four plugs burn a light golden brown color. Plug housing's are zip tied to rubber housing to prevent grounding with hood latch.
I am going to rebuild fuel pump because I know I can do it, and then look to rebuild carbs. I read from clymers( i know, I know) that there were two different production runs on carbs. Any way to id whether 82 or 83 model easily so I can order rebuild kits. Any tips specific to these carbs for rebuild. I have read FAQ's on subject.
All plugs look same . Don't think I am getting h2o in cylinders. I am limited in electrical testing equipment.All I have is a multimeter.
Any and all insight into what is causing this mi**** and the lowish RPMS at WOT would be appreciated. Joe

what is the manufct recomeneded WOT range for that motor? low 4000 sounds low.....

seafoam might've helped but if the carbs are really plugged even seafoam cant help.... when was last time carbs were rebuild??? i would rebuild the carbs and go from there...

also find out the zise of the prop.... you might be putting too much stress on the motor with the prop that you curently have.....

if it runs strong for a while then misfires possible theres a gasket leak somewhere, ignition problem....

 

[logger93]

Re: 90 hp johnson stutter/misfire above 4000rpm

silly question but how do i use timing light @40mph on the water?

 

[emdsapmgr]

Re: 90 hp johnson stutter/misfire above 4000rpm

Have buddy drive the boat. With the cowling off, put the timing probe on each wire and check for the flashes from the light. You will see misfires or no fires that way.