I had the same problem with my Volvo Penta. Changed filters, made it better for a bit. Cleaned and drained fuel system, made it better for a bit. Sometimes it would go good, sometimes like it was starving for fuel. Marina even changed prop, no difference just wasn't getting RPM's. For 2 years my buddy at work kept telling me he thought it was timing. He ended up taking a ride with me and he was right. He fixed it in 15 seconds.