20 hp merc runs on one cylinder until it gets hot. Anyone know why?

[deejathome]

1989ish 20 hp merc. It starts when I launch it, but it only runs on the top cylinder. Very slow. If I take the top cylinder plug wire off, it wont start on that cylinder. <br />If I run it on one cyl for 10 minutes or so, the other cylinder seems to pop once or twice for a second, then all of a sudden it jumps up to full speed on both cylinders. Then it runs fine. Every start after that is fine. Even the next day at the lake it runs fine. But if it stands for a week or so on the trailer and I launch it again, the same thing happens. I changed both plugs and wires. The carb has been rebuilt and cleaned. <br />I would appreciate any help anyone could give me as to the root of this problem. Thanks alot from Winnipeg Canada.

 

[Chinewalker]

Re: 20 hp merc runs on one cylinder until it gets hot. Anyone know why?

Hi DeeJay,<br /> I am betting you've got a coil with a poor ground. Each coil has a ground wire going to the motor block. Check to make sure it hasn't been pinched or that the contacts haven't corroded somehow. Also, make sure all the plug connectors that connect the coils to the powerpack are clean and firmly connected. There are also two wires from the trigger under the flywheel that need to be firmly connected. If none of that does the trick, I would suspect a failing powerpack/switchbox.<br />- Scott