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Good morning my name is***** compression, fuel pressure, spark output, and timing advance been checked yet?
Got it. Is there a story to this engine. Have you ever personally seen it run correctly? Have you tried running it off of a portable fuel tank in order to rule out any kind of tank issue?
Well, since you have never seen it make full power this is going to be a lot more difficult and you are going to have to check the gear ratio on the sterndrive as well as the propeller sizes. The ratio will be stamped on the upper half of the sterndrive, and the propeller will be stamped a size someplace on that. Find that information and let me know what you get for numbers.
What you will need to do next is confirm the gear ratio is correct. If a gearing change was ever done the stamping on the unit might not match the actual gear ratio that is in the lower unit. Remove all of the spark plugs from the engine. Line up the mark on the balancer with the mark on the timing tab on the engine. Then go to the propeller and make a mark that goes from the side of the propeller over to the side of the lower unit. Turn the engine 2 full turns by hand and see if the propeller makes 1 full turn, or less than 1 full turn. Let me know what you find.