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Hello, my name is***** will assist you.
There are a few things to check first. Number one is the compression of the engine, make sure that all 6 cylinders are over 90 PSI and with-in 10% of each other. Make sure that you do have the Champion QL77CC spark plugs installed, this engine does not run well on any other plug. Check the ends of the spark plugs to make sure that they are not overly fouled. Make sure that your battery is staying well above 8V when the engine is running. If you are having to charge it after a day on the water, either the charging system is bad or the battery is.
Let me know what you find out.
What all have you already done, so that I do not have you check something that you already have?