My 02 Yamaha 115 4 stroke will run great for a while and then when I crank it back up to move spots it won't plane out and I can tell the motor just sounds different. Like it's not catching. The gas is good, it's getting fuel, but it sounds bogged down. Again it will do this, then out of nowhere it will run great for 4-5 trips in a row. I can't figure it out. Could it be a spun prop? Usually I can just gun it and it will get on plane. If I ease into it, it never will.
Doesn't sound like a spun prop unfortunately, that would be easy. Being so intermittent lends to a fuel problem. I've seen electrical problems be intermittent.
You say the gas is good, are you sure it's getting enough gas? No real way to check except to inspect the fuel lines for leaks. Any intrusion of air can cause issues. Especially with a Yamaha. The fuel system pressurizes everything to like 5k psi. Because of this it's very sensitive to air fuel mixture.
If you convinced the fuel system is good, then you're going to need to have it hooked to a computer and check the coils and spark system. I'd change the plugs before I paid someone to put it on a computer, just to make sure.
So a bad plug could cause the bogging down? It starts fine and will rev in neutral, it's just under a load.
It could. The can also tell you a lot. As you pull them out, check them for moisture. If they are wet with fuel then they haven't been firing. This could be the plugs fault, a bad plug wire, or a coil pack... The plugs can tell you a lot.