1980/81 Blue Band 50 HP Mercury Outboard Rough Idle Issue

[Tom in Va]

Hello Again.
​ I've been working on my Mercury outboard trying to solve a rough idle issue. I have removed and cleaned both carbs, replaced the stator and trigger which had corroded wiring and changed spark plugs and wires. I finally I checked cylinder compression with my Bosch tester. The gauge showed a consistent 120 psi across all four cylinders. I'm not sure what the specs on compression should be but a consistent 120 psi shouldn't be bad. I have ordered a coil assembly. The main issue is that the motor runs rough at idle, but powers up and runs smooth at full throttle. Does anyone have any other ideas? Thanks a bunch.

 

[flyingscott]

Check to make sure you have spark on all 4 cylinders. An air gap test to jump at least 5/16" or a cylinder drop test.

 

[racerone]

Serial # is ?-----I believe a 80 / 81 model would have 4 coils.

 

[Tom in Va]

Yes. You are correct it does have 4 coils. Serial: 5843942. The last time I took the boat out, the Lake I took it to was a no wake area. So I never got to open it up and the motor ran rough the whole time. When I did the compression check today, I noted that the number 4 plug was cleaner than the other plugs like it hadn't fired much. I think it may be firing intermittently on 4 at idle but runs well when throttled up. I'll do the air gap test. Thanks

 

[racerone]

No fire on that bottom plug .-----Or possible water intrusion from lower crankshaft seals-------Bad / stuck open reed valve is also a possibility.

 

[Tom in Va]

Thanks Racerone I'm hoping that a new coil will fix the problem. As far as water intrusion, I haven't noticed any, although it is possible. The reed valve is a definite possibility. Seems that you have been around a few outboards. I'll let you know things work out.