Sudden jump in RPM's and loss of acceleration

[boatinginca]

Hi - I have a 1977 Johnson OMC 40HP. When I take off accelerating the boat starts to come up on plane then the RPMs keep going up the motor is reving but not getting any spped It feels like something is slipping. If I throttle back and ease into it again - starts to go then slips - Once in a while If I ease the acceleration it finally it gets up on plane and runs great. The one thing I have noticed is a chunk of my cavitation plate is broken off. About 3 inches by 1 inches on the one side. could this create cavitation or ventilation??? <br /><br />I hear no loud thunks it just feels like it lost grip like spinning tires.<br /><br />Would a hydrofoil fix the problem?<br /><br />TIA<br /><br />boatinginca

 

[rickdb1boat]

Re: Sudden jump in RPM's and loss of acceleration

Your prop hub is spun. Take it to a prop shop and have them insert a new rubber hub. They may also be able to fix the cavitation plate. If not, a welder can..

 

[boatinginca]

Re: Sudden jump in RPM's and loss of acceleration

Is that inside the lower unit? <br />My prop has a regular hub with no spline. You just put the prop on and insert a shear pin then the nose cap.

 

[rickdb1boat]

Re: Sudden jump in RPM's and loss of acceleration

No. Just the prop may have sheared the pin. Have you checked it out? I believe they also have the rubber hub, in addition to the pin..

 

[boatinginca]

Re: Sudden jump in RPM's and loss of acceleration

The shear pin is intact I'll see if there is also a rubber hub... thanks

 

[onebohemian]

Re: Sudden jump in RPM's and loss of acceleration

I had the exact same symptoms from a 1987 90 horse after I put a tiny, two-inch screen over the end of my live-well intake pump, right ahead of the prop on the bottom, back of the boat. Did exactly what you describe, spinning with high rpms when trying to get up on plane. Thought my prop hub was spun as well, but when I scratched a mark on the prop and the hub, and experienced the same condition, the lines were still lined up so I knew the prop wasn't spinning on the hub. Turned out to be solely related to the cavitation/ventilation caused by the addition of the live well screen. If you didn't have the problem before the plate broke, but you do now, it's probably the cavitation plate. If this is a new problem, check the hub by marking it and the prop, and then also check for anything else that may be new which could be causing ventilation in front of the prop. I was amazed at how delicate a system the prop biting the water is. <br /><br />Good luck, <br />Mark<br />Minneapolis

 

[voodoo]

Re: Sudden jump in RPM's and loss of acceleration

What tore off the piece of plate? Sure the prop isn't damaged too?

 

[boatinginca]

Re: Sudden jump in RPM's and loss of acceleration

Marked the prop and hub and it moved..prop hub is spun. thanks for the info.<br /><br />Going to take the pro in and get it fixed.