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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.
 

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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.
My brother had a similar vibration problem a long time ago, his bahner felt like it was going to raddel apart at high speeds. After ripping everything apart chasing the problem it turned out to be his ujoints, change them out and it might go away.
 

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Thanks for the advice. At the time I went through the pump. I checked the engine to pump angle and it is ok. This boat has been in the family since new and I know the u-joints are OEM. I did however inspect them and they are tight and move freely. But I cant load them up by hand and for the few bucks it costs to replace them it's A no brainer. I have to ask, can A driveline vibration cause the pump to unload/cavitate, I'm not sure what the term is, And actually blow the hand hole cover off the pump? thanks again.
 

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Thanks for the advice. At the time I went through the pump. I checked the engine to pump angle and it is ok. This boat has been in the family since new and I know the u-joints are OEM. I did however inspect them and they are tight and move freely. But I cant load them up by hand and for the few bucks it costs to replace them it's A no brainer. I have to ask, can A driveline vibration cause the pump to unload/cavitate, I'm not sure what the term is, And actually blow the hand hole cover off the pump? thanks again.
When my brothers went out the u-joints looked physically fine and would have never thought they were bad but they were. I would be willing to bet its the u-joints.
 

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I have to ask, can A driveline vibration cause the pump to unload/cavitate, I'm not sure what the term is, And actually blow the hand hole cover off the pump? thanks again.
Not likley D/L problems had any direct effect on the cleanout... cleanouts can "pop",crack and blow from jets taking or having trapped a gulp of air , air trapped at the high reaches void around the cleanout gets "shock loaded" during operation and the combination will blast the cover off (sounds like 12 guage) , this can be due to a multitude of problems such as flying the boat , or high speeds in very rough water even loader issues tom
 

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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.
If it still has the stock finger grate, thats your problem. After rebuilding your engine your boat now has more top speed and now the pump cant be loaded properly. A good loader from MPD will take care of it and probably gain some top speed too.
 

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If it still has the stock finger grate, thats your problem. After rebuilding your engine your boat now has more top speed and now the pump cant be loaded properly. A good loader from MPD will take care of it and probably gain some top speed too.
X2 the stock figer grate will rattle the hell out of your boat. Its a harmonics thing. Get rid of the finger grate and put a good loader in it and all will be fine. My cruiser did the same thing. felt like the bottom was going to vibrate apart.

U joints will usually do it at any constant speed as long as your not going up the power band and just held at a constant rpm.
 

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If it still has the stock finger grate, thats your problem. After rebuilding your engine your boat now has more top speed and now the pump cant be loaded properly. A good loader from MPD will take care of it and probably gain some top speed too.

You are correct, I just had to do the same thing to my Tahiti, at 68mph,it started to chatter and shake, took out the stock grate,and installed a loader,and took the boat back out!! the problem was solved boat ran out at 73.1mph!! Get rid of the stock grate, that is your problem!
 
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