I was so happy with my old 1983 mercury 80 hp motor, it ran so good but the last time out fishing she started running rough. i thought it was just bad gas or dirt in the carburetors and will fix it when i get home. I tested it and found that it was running on 1 cylinder. i cleaned all the connectors i could reach also swap wires at the coils to test the coils with the wires that was giving spark. all the coils was sparking with that set of wires. took a break and connected all the wires back start her up and she was running like a champ. started her up this morning and was back to rough running i let her run a while and the hotter she got the better it was. tested the spark and there was good spark on all 4. Question is why will i lose spark when the motor is cold and recover spark on all the plugs when it got hot. I can not through parts at it, the old girl keep me on a tight budget.

Just thinking logically through the problem... When metal cools, it contracts. When metal heats up, it expands. I think there should be an ohm spec for the coils. I would check those cold and hot. Then I would run an ohm test on the wires cold and hot and look for a change. Have you pulled the plugs to see how they look? I would check them too. It sounds like something is losing connection until it heats up and expands.

I'm sure another expert will be along to point you in a direction. I was just thinking through what steps I might try to troubleshoot it before throwing parts at it.

It is more than likely the switchbox. It is breaking connection internally when cold but when hot it must be making the connection it needs. Also check all of the grounds on the motor.

2nd the grounds

I found some flaking wires on the inside . I will hopefully find some good wireing under the covers so i can replace the bad parts then take it from there. Thanks for the replys i will need some more guidance after the wire job.