2 yamaha 9.9 with same problem

[2rivers]

I have two fourstroke 9.9's on a pontoon, both same year, 1989 or 1990. They start fine, idle fine, run slow to half throttle fine. When I go full throttle on either one or both they act like there's a clutch that's slipping. The rpm's stay high, or even go higher, but the propulsion goes way down. Not a jerking in and out of gear thing, just like a clutch slipping. When I throttle down the propulsion comes back. I had both props rehubbed, that wasn't it. They both run off of one fuel tank with seperate bulbs and I've noticed it's difficult to get both bulbs hard, like there's an air leak on the tank. It seems wierd they would both have the same mechanical problem so I'm focused on the common element, the fuel tank, but the symptom doesn't seem like fuel related. Ideas?

 

[JB]

Re: 2 yamaha 9.9 with same problem

Your problem may be in the rigging or props, not the engine.

When underweigh the anti-ventilation plates should be right at, not above, the water surface. If they get too high the props will ventilate and try to run in foam.

If the prop pitch is too great for the load they can cavitate. . . creating bubbles of water vapor. The effect is the same as ventilation, trying to run in foam.

I take it this has always happened with this 'toon and these engines. Is that correct?

 

[2rivers]

Re: 2 yamaha 9.9 with same problem

No, the problem is new this year.

 

[JB]

Re: 2 yamaha 9.9 with same problem

I would still check those possibilities out, 2rivers.

The possibilities that two engines would have the same rare problem at the same time seem beyond my imagination.

 

[garyN]

Re: 2 yamaha 9.9 with same problem

Want to know what it was when that happened to mine_

It was the rubber hub in the prop - it works at lower speeds, but around 4000rps the prop spins, the engine over revs and the whole thing repeats itself. I was told by Yamaha these prop hubs only last a few years !!! Cost about $90.00 each for new, or you can send out to rebuild -for about $90.00.

The sun apparently eats the rubber, and Yamaha doesn't have enough UV filters in the material. That is my opinion. When you take it off you can sometimes feel it or see broken rubber at the spline.

Get out your wallet