Johnson 70 HP outboartd cylinder misfire?
 

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

Johnson 70 HP outboartd cylinder misfire?
 

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

Johnson 70 HP outboartd cylinder misfire?
 

Well Doc, I'd say it's time for exploratory surgery on the carbs. LOL

ie They may have to be taken apart and cleaned. :)

And don't bet entirely on it not being a bad cylinder, a hole with weak
compression will sometimes run OK after it's warmed up. A compression test
is an SOP diagnostic.

Lastly, the switchbox may have a thermal intermittant on the SCR for #3.

-W (Merc dood)

"Dr. Robert P. Richeson" wrote in message
...
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

Johnson 70 HP outboartd cylinder misfire?
 

Dr.

I'll be yet another echo for checking out the carbs. First thing
I suggest is a visual check comparing the three. i.e. look at the
linkages to all three carbs and make sure all are connected. Check
your fuel line to the last carb for any kinks in the line. Next,
remove the airbox and apply the choke to verify the butterflies are
in sync and then apply throttle (without the engine running) to see
if those butterflies (deeper in the throat) are in sync. If all
appears well, then the next step is to tear into that carb. As
F330GT and Clams suggest, it really does sound to me like a carb
problem. Not that I have great experiences with outboards, but went
through a lot of aggravation trying to figure out what was wrong
with my '77 Johnson, which ended up being a bad carb.

I strongly suggest you try to get to the bottom of this before
taking that boat out again under any kind of load and rpm. I
can't help but think of that plug being too clean from NOT getting
gas rather than too much gas.

-JimL


Dr. Robert P. Richeson wrote:
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