1998 Johnson 50 intermittent misfire

[TMW123]

Well, I though I fixed all the issues on this engine. Not so anymore. It started having an intermittent issue where it begins to misfire at higher rpms. Carbs were cleaned prior to this and it runs great sometimes. It will idle fine but as soon as revs go up it misfires. It does not always do this but if I run it for more then a few minutes it will happen.

I already replaced the impeller and thermostat and no alarms go off so i doubt it is entering SLOW mode. I removed the coil and power pack. Connections on power pack were very corroded so I ordered a used one which is coming soon as I need it regardless. My question is, assuming the power pack does not fix it, will a spark tester show that there is a problem if it only occurs at high RPM?

 

[oldboat1]

If you are saying it started acting up after you cleaned the carbs, that would appear to be a place to start.

Use a timing light if you want to check spark at high rpm. If the light drops out, so too with the spark.

 

[TMW123]

I cleaned the carbs and it ran great after. This is something that just suddenly came up. Now that I think of it, maybe it is SLOW mode. Is there a way to temporarily bypass it for troubleshooting? Maybe unplugging the brown wire from the power pack? What would cause slow to kick in besides an overheat condition? Temp sensor malfunction?

 

[flyingscott]

Slow mode stops the motor from revving over 2800 rpm. It will also run extremely rough.

 

[racerone]

Is this the 3 cylinder version ?

 

[TMW123]

No, this is the 2 cylinder. Definitely runs rough when the problem occurs.

 

[oldboat1]

Your post wants to preclude overheating, but have you tested? 160F is overheating. If it's too hot to touch on the top of the head, it's too hot to run.

 

[TMW123]

No, I haven't. If the power pack doesn't resolve I will definitely be testing that. Only reason I do not think it is overheat related is that my alarm works when grounded so I should be getting that along with slow since it is the same circuit? Or am I mistaken? I can hold my fingers on the area near the spark plugs for a few seconds before it starts to burn. Water drops do not sizzle or boil. When you mean the top of the cylinder head, do you mean by the thermostat cover or where? Thanks.

 

[TMW123]

As always thanks to everyone who helped out. The power pack resolved my issue.