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Evinrude 50 problem acts like spun prop

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georgeg

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"I have an 1998 model evinrude

"I have an 1998 model evinrude 50HP outboard on a 20 foot pontoon boat that acts like it has a spun prop (boat moves OK at low RMP - 1500 to 3000 but when you increase to full throttle the motor revs up excessively 6000 RPM with little or no additional forward motion). This usually starts happening after the boat has been run for about 10 minutes. The prop is new, has been checked and is not spun. The trim is all the way down so the eigine is far enough into to water. Any ideas on what else could be causing this problem?"
 
"Providing engine is deep enou

"Providing engine is deep enough (and not too deep!)and nothing preventing water from entering the prop, and hub not spun,the problem might in fact be the gear-case design, or more accurate the design of the anti-ventilation plate.
Be sure You use the correct prop. Too high pitch, and the prop will spin loose, A rough guess would be 12 1/2 x 13 or 11 ."
 
"Morten, Thanks for the inform

"Morten, Thanks for the informaton. The boat has the correct prop 12 1/2 x 13. I checked two sources and they both specify this prop. There is not anything preventing the water from entering the prop. There is a slight buildup of alge on the lower unit of the engine from sitting in the water. Could this disrupt the flow of water over the prop enough to cause a problem like this. If not, could there be something slipping in the lower unit of the engine? Thanks for your help."
 
"The symptoms are those of a s

"The symptoms are those of a spun prop. Have you marked the end of the shaft and the prop, run it until it slips then rechecked that the marks still line up?

HERE IS A DIGRAM of the gear case and its internal gubbins. Consider what might slip to create the problem you have if it is not the prop on its bushing or the bushing on the shaft. The clutch dog on the shaft, the pinion gear on the drive shaft ... it does not appear to be pinned or keyed, or the driveshaft in the crankshaft."
 
"Vic, Thank you for replying

"Vic, Thank you for replying to my request for help. I have tested for a spun prop using the method you mentioned. The marks still line up. Thanks for the diagaram it may help me locate the problem."
 
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