84 Johnson 40 Missfiring

ldurmon

Cadet
Joined
Apr 19, 2016
Messages
20
I have an 84 Johnson 40hp outboard and it misses. I just rebuilt both carbs and all passages have been blown out. What are some of the things that can cause a miss in outboards? Timing, plug wires, etc... I just need a punch list if you will to try and get it straightened out. :blue:
 

spirtS niggeB

Seaman Apprentice
Joined
Oct 13, 2012
Messages
40
Check plugs and ignition coils for spark. Get the motor running and hold a pair of pliers with a towel. Then pull the boot off of the plug, you should see or feel spark jump between the plug and the boot. If you pull the boot away and the engine doesn't stop it shows that specific coil as being bad. This is how I troubleshoot my 3 and 4 cylinder motors.
 

ldurmon

Cadet
Joined
Apr 19, 2016
Messages
20
It's not a steady miss. It's defiantly not running on one cylinder but it does effect the idle. I noticed it runs better on muffs than in the water for some reason. I gotta go back into the carbs to check the float settings and going to decarb it.
 

oldboat1

Fleet Admiral
Joined
Apr 3, 2002
Messages
9,598
If recleaning carbs --

No carbon in the carburetor, and using seafoam or similar isn't the right approach. Disassemble the carbs, including the domed plug at the top (idle passages underneath) -- leave linkages in place. Soak all loose metal parts in lacquer thinner along with the two carb halves. Leave the h.s. and l.s. jets in place, removing the bolt at the bottom front. Remove the float seat (soak if reuse). After soaking, run light wire or monofilament through all openings, and spray them out with carb cleaner. Set the floats level, and make sure they are reinstalled right side up. Use a new gasket between the carb and manifold. If using the black tipped float needle, use the wire clip as well. Upon reassembly, can use WD-40 on the springs and linkages (lacquer thinner will strip all lube).
 

ldurmon

Cadet
Joined
Apr 19, 2016
Messages
20
Not sure if it was the decarb or rerouting the plug wires so they aren't touching a surface to ground out on, but got it running a lot better (no miss). Now I have to periodocaly squeeze the primer bulb to keep it going or to start it after stopping for a few minutes. Fuel pump?
 

ldurmon

Cadet
Joined
Apr 19, 2016
Messages
20
Well I finally got around to getting a compression test. The top cylinder gets 60psi and the bottom gets 120psi. So I pulled the head off and the head gasket is blown into the water jacket. Now I gotta replace the head gasket. I found some pieces of I guess gasket material in the relief valve. Hopefully it will run like a new one soon.
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
make sure you retorque it to the correct spec and using the correct tightening pattern.Also retorque after first heat up and cool down cycle
 
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