yammy 40 missing???

[rando]

6 yr old yamaha 40 2st, is running rough at wot. Back off a bit , smooth as silk.
Where do i start looking.??

[Noelm]

OK, here goes my usual reply, can you use another complete tank,hose and fresh fuel? remove the plugs, do the all look the same? replace them anyway, do a compression test while they are out, does the motor have a fuel filter in line anywhere? if so replace, do NOT start by pulling things to bits looking for maybe's, do the very basic non invasive trouble shooting first.

[rando]

Thanks Noel.
with the spare fuel system,? am I looking for a delivery problem,,, ie poor pick-up ,,damaged hose etc.

I have been meaning to install a proper primary fuel filter, and attach the hose clip to the inlet. Seems like I better do it now.
What are they worth those glass filters??
Had a compression test two months ago, all good,, do I need another?

[PADDLES]

hey rando, what does "rough" mean? if there's a fuel issue with a 2 stroker at high revs it normally feels down on power as it leans out but still runs smooth. it can however make it hard to start and idle like krap. for sure check out all the basics that noel has said as a good starting point to make sure there's nothing silly wrong with the fuel, but i'd almost bet my left one on a broken spark plug or some sort of ignition issue for it to break down at high rpm and then come good at lower rpm. throw a couple of new plugs at it and also check the spark plug caps are fitting nicely to the plugs.

[OPTI]

im betting dirt in the carbs .you could clean them out your self .they have rubber orings instead of gaskets so you wont need any parts,just give them a really good blow out when they are dissasembled.[with compressed air];)
ive had this problem before ,and didnt really find anything in the carbies when they were dissasembled ,gave them a through blow out after being in cleaning acid,and the problem was fixed.

[rando]

PADDLES.
I had that thought first up. plugs?/leads?/coils?, just relating back to my bike days.

But I cant answer your question directly. My boat is at my brothers,he is reporting it as "missing" at wot.

I had a problem with dirty fuel a couple of months ago and replaced the fuel filter and my brother had the carbies cleaned. I was'nt too happy about the service though.

[PADDLES]

yeah, i've got a bit of a bike background too. from trouble shooting in the past, i'm figuring that if it's running fine at lower rpm the fuel/carby hopefully is ok, sure clean it out with some carby cleaner to make sure, but at WOT it's on the main jet and so a fuel issue should just make it run lean and lose power. the fact that your problem seems to be rpm dependent does still point me at an ignition issue. plugs are cheap and easy to fit, tell your brother to chuck a new set in and make sure the plug caps are clean/not corroded and on nice and snug and take it for a spin. there could still be some sort of balancing issue with the carbs if you've recently had them played with, but i would have thought it'd show up at low rpm as well as high. good luck with it, hopefully it'll be something cheap and silly, it usually is with a 2 stroke.