Yamaha F90 'bumping at WOT'

[pckeen]

Hi,

The 2010 Yamaha F90 4 stroke I have was bumped while on shore. The trailer tipped back, and the back of the boat and motor hit the ground. It wasn't a particularly hard hit, And there was no sign of damage.

When running the boat at WOT 6100 rpm today the boat felt like it was hesitating. I looked back at the engine, and it was visibly bumping or shaking every few secinds on the transom. I was in calm deep water, so there was no chance I was hitting something.

I backed it off, and the boat ran OK.

Any ideas what this might be?

 

[MARV1]

Re: Yamaha F90 'bumping at WOT'

Over revving the outboard, get a next higher pitch propeller. Or maybe 2" pitch higher and should bring it down to 5700 rpm which I think is ideal...

 

[pckeen]

Re: Yamaha F90 'bumping at WOT'

Hi Marv,

Can you explain this - how does over-revving it make the outboard 'bump'

 

[pckeen]

Re: Yamaha F90 'bumping at WOT'

The reason I asked is I posted the rpms of the prop, and several people (and the dealer) said keep the existing prop, as I was very close to proper levels.

 

[82rude]

Re: Yamaha F90 'bumping at WOT'

sounds what it happening is you are hitting the rev limiter at wot and the motor is cutting out cylinders to prevent itself from blowing up.you have a max of 6000 rpm wot so your hitting the rev limiter no doubt about it.for an example, if you have a 13x15 p prop go to a 13x17 and you will be a ok.