Yamaha 4stroke stalls at full throttle

[DougieS]

I have a brand new Yamaha 4hp 4 stroke outboard, but cannot run it at full throttle.

1. From cold, the motor starts easily and I have to push in the choke right away.
2. At low throttle, I have run the motor for 40 minutes with no problems.
3. Within 30-60 seconds of accelerating to medium/full throttle, the motor dies over a few seconds.
4. After this, the motor will only start with full choke, and seems to run OK like this - but as soon as you put in the choke, the motor dies, at times with some backfiring.
5. Sometimes within a few minutes, other times leaving the motor overnight, it starts and runs fine (at low throttle) again.

The fuel cap vent is definitely open. This happens using either the internal or an external fuel tank, and whether the cap is on the tank or not.

My dealer says that they took apart and checked the fuel system, and ran the motor in a tank for a couple of hours without a problem.

 

[CaptnKingfisher]

It's running lean. If it's not related to your fuel tank as you said then next I'd try cleaning the fuel filter and checking your fuel pump pressure. After that if be thinking fuel restriction in the carburetor. You could also verify your idle mix is set correctly but i have a feeling you'll be unable to improve performance since your fuel is likely restricted somewhere

 

[CaptnKingfisher]

I'm surprised the dealer wasn't able to replicate the issue, did they hook up their own fuel line and tank?

 

[DougieS]

I'm surprised the dealer wasn't able to replicate the issue, did they hook up their own fuel line and tank?

I don't know - there was a certain amount of hand waving on their part, I was pretty sure when I left that they hadn't fixed the problem. Thanks for your suggestions, I'll give them a try...

 

[mike_i]

Are you able to take someone from the dealer out in your boat to duplicate the problem? You should not have to do anything other than let the shop fix it, don't let them make it your problem it's their problem.