1999 Yamaha 40hp with loss of power at full throttle

[riverghost]

Hello, I have a 1999 Yamaha 40hp outboard jet that is losing power and RPM at full throttle. The model number 40MJHX with tiller. When I give the motor full throttle max power and RPM is achieved and after about 20 seconds the engine slowly loses power and RPM. If I reduced the throttle to nil and crank it back up to full throttle max power and RPM are again achieved. There is no problem at slow speeds or idle. I figured it was a fuel starvation issue and replaced the fuel primer bulb in the gas line and checked all lines/connections for leaks and found none. I have also cleaned out the fuel filter. When I remove the gas cap while running there is still a problem. Also pumping the primer bulb while running does not help. Is it the fuel pump or carbs? Searched and tried everything on this site. Thanks any help is appreciated.

 

[troutrvr]

Re: 1999 Yamaha 40hp with loss of power at full throttle

I feal your pain, I have the same problem with a DT75. I have done all those things and more. I'm going to run another portable gas tank right to the fuel pump by passing all connections and filters. If that doesnt work I'll rebuild the fuel pump even though I could find anything wrong with it. You can try taking off the fuel pump and pump the bulb to see if fuel leaks through the pump diaphram and out the hole to the block. Good luck.......

 

[Ray Neudecker]

Re: 1999 Yamaha 40hp with loss of power at full throttle

If it is a factory installed tank, check for a sticking anti siphon ball in the tank fitting.

 

[riverghost]

Re: 1999 Yamaha 40hp with loss of power at full throttle

Thanks. Troutrvr, let me know what happens. Ray, I have two tanks that I use they are both Yamaha factory tanks switching between the two does not stop the loss of power at WOT. I have read other posts about problems with the anti siphon ball, where is it located and do I remove it all together or clean it?

 

[netman56]

Re: 1999 Yamaha 40hp with loss of power at full throttle

Hi

New to this forum and was looking thru the posts. I have this same motor with the same issue. If I am moving along at full throttle it will lose rpm and slow down to half speed. If I back right off the throttle and hit it again it is fine. It does this continuously now, but it never used to. Has anyone solved this problem as it is driving us nuts. Redone carbs, replaced fuel filter and primer bulb but it still does it.