[Babin]

I have a 1970?s-ish Mercury 9.8 thunderbolt, motor starts fine and purs like a kitten. With just me in the boat it will start to bog when wide open but then eventually get up on plane to about 21 mph. When under load with 2 people in the boat, the motor will bog when wide open and stay sputtering only going maybe 5 mph. Any ideas what may be the issue? Thanks guys.