one plug black, the other still new looking

[mikeyzx2]

Hi all, fishing weather is fast arriving so I need to get motors shipshape ASAP. <br /><br />Back in the summer, I replaced the spark plugs on my '85 Johnson eight horse, model J8RCOB. I took boat for a spin, I guess about an hour or so off and on, wide open and midrange. At first it was a booger to get started. I finally got it running and it was fine. Well after I got home I pulled the plugs for inspection. I don't remember now which cylinder, but one plugs was already getting black, whereas the other was still new looking except maybe a little brown tint. The motor seemed fine to me, it wasn't missing or acting like it was dropping cylinder. <br /><br />Before I changed the plugs, the other two were both black. Don't think leaking head gasket, compression was even 120/120. I discussed this with OBJ one day and he hinted to weak or random spark on cylinder. Reckon I'll be taking two motors to test tank soon to find out if that's the case.<br /><br />By the way, I went through checklist: fuel/comp/cooling/spark is in progress when time allows.<br /><br />Mike

 

[mikeyzx2]

Re: one plug black, the other still new looking

I just rechecked the compression, the numbers I mentioned were from before this dilemma. I actually have 150/150, so I must've done it incorrectly the previous time. That's pretty high (good?) for a 20 year old motor.

 

[Solittle]

Re: one plug black, the other still new looking

Did you rebuild the carb? If not - - -

 

[mikeyzx2]

Re: one plug black, the other still new looking

Yeah SoLittle, I didn't clarify the part about fuel. It has rebuilt fuel pump, and I cleaned and rebuilt the carb, and adjusted float. <br /><br />I'm just curious about the one plug turning black so quick. The other one looked like it should after a short time running. I cleaned them both up, but the one still has a blackened insulator. I'm using L77JC4's. It has new thermostat and impeller is good. <br /><br />Hope that helps a little........

 

[Solittle]

Re: one plug black, the other still new looking

Thje difference in color tells you that the conditions inside of the cylinders are different. That could be the fuel/air mix (carbs or reeds) or spark. At this point I would suggest that you trouble shoot the ignition side of the equation. Using a spark tester see if you are getting a bright blue spark that will jump a 1/4-3/8" gap. If they don't look the same swap the plug, ignition wire and coil between cylinders to see if what you observe swaps cylinders.

 

[mikeyzx2]

Re: one plug black, the other still new looking

thanks SoLittle.....those are my intentions since OBJ mentioned it months back. <br /><br />I need to change something around: I rebuilt the carb when I got the motor. Okay, I had to replace the manifold on it, it had a stripped stud so the carb was loose. No sweat got a donor part. My nimrod butt didn't replace the gasket on it at the time. With that said and out in the open, could a slightly lean condition be possible? And finally, I did the float adjustment but haven't run it since then. My bad.....<br /><br />I think OBJ mentioned that a weaker firing cylinder causes that blackening. Both plugs were black before all this. <br /><br />I'll do some work and get back with new info.<br /><br />Mike

 

[mikeyzx2]

Re: one plug black, the other still new looking

UPDATE:<br /><br />I visited my mechanic yesterday and told him the deal about the plugs. I got some ideas from him, like weak spark, possible crankshaft seal leak, etc. So I pulled the flywheel last night to look for evidence of any leak there, and found the charge coil to be leaking potting compound from where the core goes through it. No doubt about that, it leaked a little onto the plate. Got one ordered, good thing I don't have charging on this motor, about $30 difference between coils. Gonna take it to his shop Friday and replace the coil and set the air gap on it, and throw it in the tank. He told me to throw it in the river and buy a new one, the booger bear.......