Yahama 4hp Outboard

[north port]

Have a 4 HP Yahama OB 2007..runs great until last week...starts fine runs fine until i crank it up and the prop no longer works and the engines just revs up...cut back on rpms and prop kicks in again...never has done this before, i could go max rpms and it would go but now put in gear and once i hit a certain point no prop and engine just revs up..cut down on rpms and prop engages....

[FSMike]

I'm not familiar with that particular model but it sounds to me like the propeller hub is partially stripped out and on its way to being completely stripped. If so you will probably need to buy a new one.

[kmacdonald]

Check the shear pin on the prop. Small outboards like that dont have hubs on the prop, they use shear pins instead.

[marshmat]

IIRC, the newer Yamahas have a rubber hub... the older ones having a conventional shear pin (don't quote me on that, though). If a shear pin goes, the prop slips at all speeds, so I'm guessing partially stripped rubber hub as FSMike suggests.
Similar symptoms can be created by weeds wrapped around the prop or lower unit, but I presume our OP has already cleared any vegetation from his engine.