1999 Johnson 70 hp outboard. I take very good care of this motor. Recently
it started idling rough and was misfiring and running on only 2 cylinders.
I would try to accelerate and as rpms increased that third cylinder would
kick in but would kick in and out a few times intermittently before firing
normally.

I changed the plugs. Bottom plug didn't look as "dirty" as the other two
cylinders. My mechanic was busy and thought there might be water in the gas
so I ran chems in the gas for water removal. Still did it. Intermittent in
and out of firing all three cylinders and once it increases in rpms or warms
up it smoothes out Oh, and it always starts right up. I looked at the new
plugs and the bottom spark plug was much cleaner. I showed it to my
mechanic and he said it was "washed," from unburnt fuel. He changed a coil
in it for that cylinder and it appeared to have corrected it however he was
not running the engine with a load in the water. I took the boat out today.
On the way out it seemed fine. Ran aboout 5 0r 6 miles. Shut the motor off
for a while and started it up and the same thing happened again. Running on
two cylinders. Only this time the motor would stall as I tried to give it
gas to accelerate it. I know it's the bottom cylinder. What do you guys
think the problem could be? If I had a bad cylinder wouldn't it happen all
the time? Help... Thanks









My guess would be the bottom carburetor needs cleaning. Clogged jet. Running
lean might clean that plug. Especially if it's set up for a while.

Barry