I have A 1974 cheetah mini daycrusier with 455 olds. Aluminum heads. 850 cfm carb and log style through transom exhaust. I have just rebuilt the engine which should be in the 450 to 475 hp range. Since rebuild I get a strong vibration at 63 mph on the gps @ 4900 rpms. 12 jc pump w/ A impeller. The impeeler showed a bit of perosity so I took it apart and went through it. I installed a bowl stuffer and a B impeller. I had no rpm change and the vibration did not change. I found that the rear of the hull was not flat so i had it repaired at no small expense I might add. I picked up about 200 rpm's but the vibration did not change from the 63 mph. At the time of that test run i lifted off the throttle and the cleanout cover on the pump broke in two. Fortunately I was able to get it the beach before it went down. I have now done some research and would like some advice on what direction to go next. It has the origional intake grate and a place diverter on the nozzle. It seems the boat doesn't get much lift. I am wondering if I need a loader or A ride plate, or both? I'm not so sure I fully understand exactly what or how this is happening. I question if the pump is starving or it has something to do with the fact that the boat won't air out. What just happened was neither cool or safe. any insight or advice would be much appreciated.