In conjunction with PNW's suggestions, next time you go out on the water bring with you a spray bottle with fuel-oil mix in it. Doesn't have to be full maybe 1/3 or 1/2 full. With the cowl off, ran the engine and when you try to go WOT and engine bogs down or would not go above 2600 RPM, spray fuel oil mix directly into each carb. (You will need to remove the air covers to do this and make sure you are clear off shore and traffic. Base results on the following for each carb's reaction.
1. If there is no effect on the carb, i.e. no increase in rpm or no decrease in rpm - both cylinders fed by the carbs are not firing. Troubleshoot for sparks. However, there is also a possibility the throttle is not synch properly (it happened on one of the threads not too long ago) wherein it is wide open even at idle causing too rich of a fuel air mixture to start with. This can be indicated by overly fuel wetting spark plugs.
2. If there is an increase in rpm - there is fuel starvation on the cylinders fed by the carb
3. If there is a decrease in rpm - it means the carb is operating properly and due to over rich fuel air mixture starts to misfire.
Now this is where it gets tricky. Since there are two cylinders fed by each carb, you will need to isolate the one from the other. And you can do this by pulling the plug wire on one of the cylinders affected and do the spray test again. In this manner, you are testing one cylinder at a time. You will do this only after you've done the preceding so you know at least where the problem area is. Try not to keep the plug wire disconnected unnecessarily or in short replace it as soon as possible to protect the ignition system.
1. If there is no effect on the carb, i.e. no increase in rpm or no decrease in rpm - both cylinders fed by the carbs are not firing. Troubleshoot for sparks. However, there is also a possibility the throttle is not synch properly (it happened on one of the threads not too long ago) wherein it is wide open even at idle causing too rich of a fuel air mixture to start with. This can be indicated by overly fuel wetting spark plugs.
2. If there is an increase in rpm - there is fuel starvation on the cylinders fed by the carb
3. If there is a decrease in rpm - it means the carb is operating properly and due to over rich fuel air mixture starts to misfire.
Now this is where it gets tricky. Since there are two cylinders fed by each carb, you will need to isolate the one from the other. And you can do this by pulling the plug wire on one of the cylinders affected and do the spray test again. In this manner, you are testing one cylinder at a time. You will do this only after you've done the preceding so you know at least where the problem area is. Try not to keep the plug wire disconnected unnecessarily or in short replace it as soon as possible to protect the ignition system.