2005 Yamaha 50hp 2 stroke need help, asap please

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I got this pontoon boat 2 weeks ago. I cleaned the carburetors and it was running really good. The next day when I started the boat it started right up and was running great for a while, then the engine started misfiring, shaking, and dying. I checked the spark with the tester and two of the three cylinders look like they are misfiring. I pulled the plugs out and spinned the engine and the sparks are not constant on 2 bottom cylinders. How could this happen? Did 2 coils go bad in such a short time? I changed the plugs and it didn't help.

p.s. I do have experience with engines, carburetors and electrical systems. I also have the service book for this engine. The coils are firing sometimes, but not constantly and I'm not sure how to check them. I'm not sure what else to check.

[jeffersondodson561]

Start the engine and verify spark with a timing light on each cylinder, if you see a flash the spark is jumping the gap.

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Thank you for helping!

I just tested it with a timing light, they are all firing. Using an old mechanic trick of shorting the spark plugs by poking the rubber booth with the test light (12V), I realized that one cylinder was not running correctly.

I checked the coils with an ohmmeter, the one that wasn't firing had a completely different reading. I'm pretty sure it is bad.