1998 Johnson 25hp dropping a cylinder when warm

Joined
Jul 14, 2012
Messages
8
first post guys. been reading this forum for a long time. here is my current problem.

25hp tiller - went from running fine 2 months ago - to having a weird "pulse" losing RPMs at full throttle after running 20 minutes or so, then clearing up. i thought maybe junk in carb. cleaned carb. same problem but not a crippling problem. still ran "okay".

then it wouldn't idle to save my life. does a kicking sputter when at idle that will kill motor if i didn't have it revved. started to worry.

second to last time i took it out - after warming up (running about 15 minutes) all the sudden violent misfiring and engine kicking. lost plane and would only run at just above idle. if i tried to gun it it would drop a cylinder and violently shake. i shut motor down and drifted for a few minutes, then started up and ran for 45 minutes straight no problem. still doesn't like to idle more than a minute or so.

last time i took it out. after putting in water and heading to dock, idled waiting for fiance to park the truck, maybe 5-10 minutes. she gets in, push off from dock, get 200 yards away on a plane and BAM! dropping a cylinder, motor violently shaking. i shut it down. waiting 5 minutes drifting. started up and got on a plane. immediately after reaching max rpm it started again misfiring. idled back to dock with my head hung low. i tried to keep it cranked while idling so i could pull a plug in gear trying to check what cylinder is dropping but felt like i was gonna fall out the boat. (14ft carolina skiff).

so what do you guys think? bad coil(s), powerpack maybe? i have read here that electronics tend to show they are bad when the engine warms up. how do i test?

btw, brand new water pump. water felt hot when engine was warm so i changed impeller. water still hot in tell-tale and exhaust of course. could be that our water temp is 86 now down here in Savannah.

let me know what you guys think?
 

hidef

Lieutenant
Joined
Aug 22, 2009
Messages
1,465
Re: 1998 Johnson 25hp dropping a cylinder when warm

What is your model number? OMC made the 25 in twins and triples back in 98. Yours sounds like a twin. You changed the water pump what about the thermostat? Has that been changed out? The electronics used by the twins is very reliable and very rarely causes any issues. It is possible that you have a power pack going bad but sounds like your engine is going into SLOW Mode when it starts to over heat.
 
Joined
Jul 14, 2012
Messages
8
Re: 1998 Johnson 25hp dropping a cylinder when warm

Model Number J25REEB
2 cylinder

dont know if the thermostat has ever been replaced. i have had this motor for 6 years and it has run great. powerpack is 5 years old. coils have never been replaced as far as i know.

so do you think the thermostat is bad or that the motor is still running hot?
 

hidef

Lieutenant
Joined
Aug 22, 2009
Messages
1,465
Re: 1998 Johnson 25hp dropping a cylinder when warm

I would start with the thermostat first if it is bad it will cause the motor to overheat.
 

Haffiman

Commander
Joined
Dec 17, 2009
Messages
2,454
Re: 1998 Johnson 25hp dropping a cylinder when warm

Have you tried to disconnect the temp switch and restart the engine when it happens?
 
Joined
Jul 14, 2012
Messages
8
Re: 1998 Johnson 25hp dropping a cylinder when warm

where do i find the thermostat switch? also, where can i find parts online for this motor? seems i have the dumb when it comes to interweb searching today.
 
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