40 hp Yamaha cuts out at high rpm

I just got this a few days ago and have taken it out twice now. It starts and runs great but if you go past about 2/3 throttle it starts to sputter and cut out. Yesterday when I had it out, about half the time when I went pushed it past 2/3 throttle nothing would happen, the revs would stay the same. So it would either cut out past 2/3 throttle, or nothing would happen. The guy I got it from said he'd had two "experts" look at it and they don't have any idea. The cylinders are all within 2 lbs of each other. I'm thinking it's a fuel delivery problem, but I really don't know where to start from there. Bear with me guys I'm a little green but I'm more than willing to learn so any advice you guys can give is very appreciated.

Btw it's a 1984

I have a 2003 yamaha 40hp. The the guy I bought it from said it did the same thing. He said it ran great but if he went to full throttle it would cut out and run slow for a while, then after about a min or so it would pick back up run fine for a few min and do it all over again.

I talked to my motor guy about it before I bought it. He said Yamaha 40hp 2 strokes are famous for that and that its usually a fuel delivery problem. He said more than likely the fuel system is sucking air somewhere, or the carbs need cleaned or both. There are a bunch of threads on the net about this problem and many of them have a different solution to it.

He also said the fuel pump could be worn/wearing out. When you are at full throttle, the fuel pump cant keep up with the demand the motor is putting on it, so it's starving for fuel and slows down. After a while the pressure builds back up and your good for bit.

I think that is the most expensive thing that could be wrong. You don't have to worry about efi issues, so thats out.

Hey reed thanks for the input. It's nice to hear this is commonly dealt with. I can check the fuel pump but I can't get to full throttle even if I do wait a while. I also forgot to add that this didn't come with a tachometer so I don't know the rpms. I plan on getting one soon though.

See if you can get somebody to pump your primer ball while out to see if that helps.That will override the fuel pump.

crazymanme2 said:

See if you can get somebody to pump your primer ball while out to see if that helps.That will override the fuel pump.

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I'm taking it out today so I will try that out. Any other on-the-water tests I should try while out?

Took it out today but the small lake I was on had too many hidden trees for me to throttle it up. Problem is the rivers are way too low and the nearest lake I could open it up on is 50 miles away... But it ran great while it was out, except while going back to the boat ramp at just over idle speed the alarm came on as I guess it was getting too hot. Don't know what caused that. But anyway I brought it home and ran it on muffs at a fast idle, checked the primer bulb and it was not hard. Not collapsed, just not hard. Should the bulb stay firm while the engine is running, or does the vacuum collapse it? -Tucker

There are several possible things that could be the problem. My first thought is that the high idle jet or the main jet is plugged. With a low idle jet open you can still run the motor, but not rev it up. I would start with a carb. clean, but I would first like to know, do you have an inline fuel filter? If not I would not clean the carb and then take a chance in pumping in more dirt that may plug it back up. WIth the fuel filter inline if there is any trash in the fuel, the filter will catch it. Clean the jets on the carb. Make sure you run something through them. I use a tip cleaner. Then I spray them with Berrymans carb cleaner.

I have a 2000 40hp Yamaha and I have not had any problems....yet.

One thing I do is always take my fuel line off and run all the fuel out of the carb, before I take it out of the water at the lake. Then I have no fuel in the carb to gum up and require a cleaning.

Good luck and let us know what y ou have found.

The reason I haven't pulled the carbs yet is because the guy I just got it from said that he'd cleaned the carbs. He said it's been like this for about 2 years :|

If I had a dollar for every time I heard " I just had the carbs cleaned and replaced the impeller" when hunting for a boat I would be rich.

I know everyone I buy I clean the carbs and replace the impeller though!

reedjj said:

If I had a dollar for every time I heard " I just had the carbs cleaned and replaced the impeller" when hunting for a boat I would be rich.

I know everyone I buy I clean the carbs and replace the impeller though!

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Haha yeah I know what you mean but the guy was honest and told me what was wrong the boat, motor etc. He said he had no idea what the problem was and he drove himself nuts trying to figure it out. I believe he cleaned them, but who knows how well. :roll:

I clean carbs all the time. I personally have cleaned carbs, ran a motor and they acted like I had not cleaned them. Took them back off and they had picked up some trash somewhere and needed to be cleaned again. Also, you never know when one piece of trash gets sucked up into the jet and that is the only problem. That motor should be fairly easy to pull the carb. At lease if you did pull it you could eliminate that as the problem.