6 hp Missfiring on 1 Cylinder

[Rob Whit]

Hi All,
I have recently aquired a 1975 Model 6504R 6HP. It starts and runs OK at first but after a time under load starts to missfire on one cylinder. The problem gets gradually worse until it runs on one cylinder only but with occasional short bursts from the bad cylinder. I replaced the plugs using NGK B6S. This did not cure the problem. I have removed Flywheel and all looks in good order. Point gaps look correct (.020"). It looks as if one coil has been replaced at some stage as one has an insulated earth wire while the other is bare.Is it possible that a H.T. lead is breaking down with temp. increase? Leads are subject to a lot of movement with advance/retard. Is there a chance of fatigue breakage?
Any help or advice would be appreciated.
Rob.

[boats.net]

First verify that you are loosing spark at the dead cylinder. If you are could be a bad condenser, loose connections or possibly bad coil. With a service manual, the coils can be checked.