Spark Plug ?

[goconnor]

I have a 72 Johnson 6hp. I have j6c spark plugs in the motor now. I use the motor to push my 14ft jon boat. I went out today and the motor ran good for about 20 mins then it started sputtering. It never shut off but it ran rough. We were running mostly at low rpm when it was running rough but when we bumped up the throttle it sounded better. Is it possible that the spark plugs need to be switch out to the J4 or do you think there might be something else wrong.

 

[Vic.S]

Re: Spark Plug ?

J6C are the listed plugs. (J4C are cooler) (gap should be 0.030")

Check the plugs to start with. Put your spare set in.
Or check to see if both are firing, if not swap them over to see if its a dodgy plug

Check for sparks at each plug.

If you have no spark at one or other plug it could be a defective coil, condenser, breaker or even lead.

Playing the choke make any improvement? If so suspect dirt in carb