engine races, prop doesn't

[capt.revere]

3 HP 2-stroke Yamaha, older, at full throttle engine races but prop thrust diminishes, as though someone stepped on the clutch, then when I slow the engine it returns to normal. Have ruled out cavitation, nothing in prop, no funky noises or clunks, just engine rpm increase and prop rpm decrease.

 

[eavega]

Sounds like you spun the prop. Scribe a line across the prop, hub and shaft. Run your engine until you feel the "racing" again. Stop, look at the scribe mark. If they don't line up anymore, the rubber hub inside the prop has spun (or if its old enough, you may have sheared the pin in the hub) Either way the prop is no longer spinning in concert with the shaft.

-E

 

[capt.revere]

Thanks, I'll try that tonite, that would be an easy fix.

 

[capt.revere]

Okay, I can't see the prop shaft. My 3 has a black plastic shaped nut with a stainless cotter pin. How does that nut come off. I assume first take out the cotter pin, then unscrew the nut? How to keep the shaft from turning, should I somehow jam the flywheel? Right now turning the nut does little, aand the cotter pin hole stays lined up.

 

[eavega]

So looking at the setup for Yamaha 3 HP (actually if you can indicate what year motor it is, it might be more accurate) the prop nut holds the propeller in place, and the nut is held by a cotter pin. looks like it is a splined shaft, though. So, remove the cotter pin, nut, and washer. Scribe you line across the prop, hub, and shaft. Replace the hardware and go for a run. When it revs without moving take off the pin and nut, and see if the scribe line still lines up. If it doesn't, you've spun the rubber hub inside the prop. Replace the prop and you should be good to go. If it all still lines up, then you may have a problem somewhere in your gearcase.

Rgds

E

 

[capt.revere]

Okay, Thanks

 

[99yam40]

the nut should be on the splined shaft and will not move unless the cotter pin shears off ,
so no need to take it off, just mark it and the prop to see if alignment moves after running it

 

[capt.revere]

So here is where I am. I removed the cotter pin, held onto the prop and tried to turn the nut. The nut does turn while I hold the prop from moving, but the cotter-pin hole still lines up. So I must be turning the shaft and like eavega indicates, perhaps this means that the rubber? inside the prop is spent or spun, and if so, needs to be replaced. If so, how do I keep the shaft from turning, put the engine in gear and hold the flywheel?

 

[99yam40]

use an air or electric impact gun.
Normally you set a 2X4 or 4X4 above the prop to keep it from turning while loosing or tightening the nut

 

[eavega]

put the motor in gear, strap down the flywheel (get someone to help you, it will be easier). That should keep the prop shaft from turning while you back off the nut.

Rgds

Eric