99' Johnson 225 Ocean Pro Rough Idle, Clean Spark Plug on one cylinder.

[Bteach]

Hi everyone. 2 stroke 99 Johnson 225 here. Its used year round and was taken out this weekend. Had a rough idle but ran fine at WOT. Decided to check/change the spark plugs when I got home and eventually got into it last night. The top, port side cylinder spark plug looked incredibly clean in comparison to the other 5. The middle port side cylinder looked like it had alot more carbon buildup than the others as well. I shined a light into the cylinder and the head looked clean as well, especially compared to the middle port cylinder head. I plan to check the compression when i get off work today but i am curious about the condition of the plugs. I read faulty powerpacks or ignition coils could could cause these issues. The boat has seemed to run fine recently and nothing suspect has come to light other than the rough idle i experienced when out on it this past weekend.

 

[racerone]

Clean sparkplug is caused by -----No spark , no fuel , too much fuel , no compression , no crankcase compression , faulty sparkplug.------Take your pick on how / where to start your trouble shooting.-----These motors run very well on 5 of 6 cylinders.---Do you have a working tachometer ?

 

[Bteach]

I do not but it sounds as though i should invest in one. Would the dirty spark plug be that cylinder trying to keep up. The clean spark plug was the top port plug, the dirty one was directly underneath it.

 

[Bteach]

The tachometer i have does not function correctly but the alarms on it work so I havent replaced it.

 

[racerone]

Not sure where the idea of -----" that cylinder trying to keep up "---- would come from.-----Your motor has 6 carburetors / 6 seperate crankcases./ 6 ignition coils.-----Start some orderly trouble shooting.----Post your compression test values here.-----With a working tachometer you would notice a loss of RPM and take action immediately.-----Remember a restricted carburetor would starve ONE cylinder of fuel and of course OIL.------That could lead to expensive repairs.

 

[oldboat1]

I would measure spark first to see if a strong pulse is available at that plug. Set the open air tester to about a half inch. If no spark, check continuity of plug wire, coil boot to plug boot. If no continuity, repair or replace. If continuity, test coil (resistance test from plug boot to coil ground, multimeter set at 1K ohms resistance. See if the finding is the same on one or two of the other coils.) Test/measure spark with all plugs removed, same as with compression testing.

It's possible, if unlikely, the top plug is water washed.

A shortcut to checking the top plug is to attach a timing gun to the wire, and turn over the motor. No flash means no spark.