Yamaha runs like a banshee then dies - "now it happens to me"

[frozenokie]

After this weekend of giving this 97 Yamaha GP760 another tryout, I have experienced something new. I ran it on the trailer while in the water to assure my carb settings somewhat. After I felt good about them, I took off for a test ride. WOW!! She really zips down the lake and after about 15 minutes of riding, whaaaaa... completely died. Then the battery wouldnt even turn it over. Wierd - I just charged that new battery the night before so it isn't that. After floating for about 2 to 3 minnutes, I tried again. Wah, wah, wah, then vroom! Up and out she came again. Zipped down the lake for about another 100 yards, then same thing again, and again, and again!!! And again, the battery "acts" like it's dead - but it isn't. Hmmm...

Then I remembered... when I have been taking her to the pump for fill up, each time when I unscrewed the gas cap... PSSSSSSSSSSSSSS. Out came all the pressure from the shaking of riding on the trailer. Both times filling up it has done this. I'm now thinking that the tank is over-pressurizing while riding and creating a reverse vapor lock for the lack of better terms. Pressure instead of vacuum. Does this seem to be my problem I can contemplate how the extra pressure may be shutting off my fuel supply at the needle/seat. Then after a few minutes resting, the pressure escapes just enough to get it started again.

I took off the vent supply hose after I got home and blew through it - completely clear and you can hear it thrushing through the check valve. This didn't happen on the maiden voyage a few weeks back. So I'm confused as to where to start looking for the problem. Carb HI/LOW settings are right where the book says they should be.

Am I looking at a "pop-off pressure" issue here?

Any help is appreciated - Even from slag... Dude - no hard feelings by any means And you too smoofers - you seem to have that kind of experience likewise

 

[frozenokie]

Re: Yamaha runs like a banshee then dies - "now it happens to me"

I took off the vent supply hose after I got home and blew through it - completely clear and you can hear it thrushing through the check valve. This didn't happen on the maiden voyage a few weeks back. So I'm confused as to where to start looking for the problem.

I want to clarify that when I stated "this didn't happen a few weeks back" I meant that this problem just arose. I didn't have this dying problem a few weeks back. This was the first time I have checked the check valve.

 

[frozenokie]

Re: Yamaha runs like a banshee then dies - "now it happens to me"

Just found a broken front tank mount. The other three are still good. Could this cause my dying problem?

 

[Fireman431]

Re: Yamaha runs like a banshee then dies - "now it happens to me"

I had the same issue. It was a bad tank vent.

 

[frozenokie]

Re: Yamaha runs like a banshee then dies - "now it happens to me"

Hey Fireman, do you mean the little white "box" under the yellow hood? Or the clear white cylinder shaped contraption that has the screw-out bottom?


And thanks for your help

 

[frozenokie]

Re: Yamaha runs like a banshee then dies - "now it happens to me"

the little white "box" under the yellow hood

I found that the plastic tube with the port holes in the sides was all the way down in the ballast. Meaning the port holes didn't have the chance to let in air. So I just took the tube out. If I have problems from doing that, like water intrusion, I will jab a pin through the tube beneath the port holes to keep it from falling all the way down again.

Sound right to you Fireman431?

 

[orangeman]

Re: Yamaha runs like a banshee then dies - "now it happens to me"

You should be able to blow through the check valve in one direction but not the other. If you can blow through both ways it is bad. This is a common problem. When mine went bad I would have problems starting and idling when tank was half full or less. Generally it is not a good thing when A two stroke engine cuts out at speed. This usually indicates some kind of seizure. I would not run this ski at speed till you figure out what's going on. I would do a compression test first.

 

[frozenokie]

Re: Yamaha runs like a banshee then dies - "now it happens to me"

Yep. I did the compression tests this evening. 140 both cyls. And did blow through into the tank but couldn't pull any air out, so it was OK. So, I think all is fine now that I found that the air inlet tube was sitting too low inside the ballast. Stand to reason that the ski didn't give me problems for the first two outings which gave it enough wave bumbing to slowly "knock" it down past the two little inlet port holes

Good information to know on your behalf nonetheless with the bad check valve symptoms.