87 Yamaha 40 hp dying?

[lifestone]

I'm at a loss here guys. I have a 87 oil injected Yamaha 40hp. I take it out it runs great. The next time I take it out, I'm going wide open or close to it and all power is lost. Well all but maybe 1/10th of power, but the throttle is wide open. Then sometimes it will snap back and speed up. Well I get it home, check for a fouled plug, basic things. Nothing, Next day I put it back into the water and it runs like a champ. Great trip. Later the same week I run it a couple miles. And on the way back, it looses all power except for a small slow amount of power... I wait 5 hours hook it up to the muffs and garden hose, and its works fine again..I'm really stumped here guys.. any suggestions, advise? Thanks.

Ps I just filled the gas tank today so I know the gas is good.

 

[Yamajoe]

Re: 87 Yamaha 40 hp dying?

You need to check for clean spark on all plugs; while you there check the compression; and then move onto fuel;

Taking a punt, it appears you are getting a restriction to your fuel supply when you loose power. when this happens, is the primer bulb firm or flat? try squeezing the primer when you experience this problem and if the power picks up, you can suspect the fuel pump. try to isolate the problem at the tank pickup and connectors by using a known good tank and work your way through from tank to carbs to ensure their are no air leaks or restrcitions and filters are clean/ replaced. more than likely though, your carbs require cleaning, float intake and jets checked and nozzles cleared with air and the float valve hieght adjusted to specs.
When you remove carbs, dont foget to check timing and link and sinc. Now is a good time to get a maual.

Dont get fooled into thinking that just because it runs well on the ear muffs all is well, the surest way to check for sound operation is in the water at WOT.

 

[lifestone]

Re: 87 Yamaha 40 hp dying?

Thanks Yamajoe, I will try your advice.