New 6hp 4-stroke missing at high RPM

rnorman3

Seaman
Joined
Oct 2, 2012
Messages
52
Hi all, I have a my 2012 6HP which I just bought seems to be missing at high rpm. On the boat at about 3/4 throttle it'll start to buck like it's missing. I thought it was cavitating but no, because when I take it out of gear, and just rev it up, it does the same thing. What would cause this? It's running off the external tank, maybe it's not getting enough fuel? I haven't tried the built in tank yet.

Of course it's still under warranty, but if it's just something simple I'd rather solve it myself...

Thanks!
 

rnorman3

Seaman
Joined
Oct 2, 2012
Messages
52
Re: New 6hp 4-stroke missing at high RPM

Guys, any idea on this one? It's at the shop, they at first couldn't reproduce the problem, I had to go in and show them. They had it in a tank that was cavitating, so I turned the motor to the side to allow more water to flow past the prop. Anyways, when they seen what it was doing they looked pretty puzzled.
Thanks!
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: New 6hp 4-stroke missing at high RPM

they should be able to at least test that the spark is not dropping when the missing is happening, then it would be a fuel problem to chase if spark stays good
 

rnorman3

Seaman
Joined
Oct 2, 2012
Messages
52
Re: New 6hp 4-stroke missing at high RPM

Thanks 99yam40, but how would they do that if the engine's reving at 3000rpm? Some kind of digital scope? Cause I don't think these guys know what that is.
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: New 6hp 4-stroke missing at high RPM

They can monitor voltage going to coils, monitor KV out of coils, or hook up a timing light to plug wires.
Scope would also do it
 

rnorman3

Seaman
Joined
Oct 2, 2012
Messages
52
Re: New 6hp 4-stroke missing at high RPM

I spoke to yamaha, they said it's probably reaching the max 5000rpm at which point the engine will cut the spark off and not rev any higher. does this sound right?
Thanks!
 

99yam40

Fleet Admiral
Joined
Sep 7, 2008
Messages
8,851
Re: New 6hp 4-stroke missing at high RPM

Most motors have a higher WOT RPM than 5000, but have no idea on that motor.
Yamaha service and owners manuals should give that info
Need to hook up a tach to see what it is turning
 
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