Carb Problem? 2 out of 4 Spark plugs Have carbon buildup?

[michaud500]

Hey all
I have a 1969 Mercury 500 with Thunderbolt ignition, Serial # 2929264. I rebuilt the carbs and fuel pump around 8 years ago. Last time I used the boat I noticed the engine running ruff and not starting after a long run. So I pulled the plugs and noticed that the plugs for the 1&2 cylinders had normal carbon build up but the plugs for the 3 & 4th cylinders had some fuel on them and a little bit of what looks like black oil around the edge of the plug, not on the electrode and clean.

Is this a carb issue or an ignition issue?

Thanks in advance

 

[jimmbo]

Could be either, so some troubleshooting is in order. First thing is a compression test. then can the spark jump a 3/8 gap? You might have a high fuel level on the bottom carb(inlet valve not sealing, or a rich idle mixture. Another possibility is the Fuel Pump Diaphragm, if it using 3 and 4 for is pulses

 

[michaud500]

Thanks for the information. I will try and do the trouble shooting asap and will post what happens.

 

[michaud500]

So I have done the compression check on 3 of the 4 cylinders, couldn't get the gauge on the bottom cylinder. They were all at 114 to 115, the #3 which is the main issue was 115.

Spark test on all 4 was strong. I did also squeeze the plug wire ends that attach to the plugs with a pliers to make sure that there was a solid connection to the plug from the wire, seemed a little loose.

Should I go back out with the boat or should I take off the carbs and clean them out first?
Thanks for the help....