No power at WOT on Mariner 40

[SimonStDavids]

Over the winter I thought I would tackle a service on my Mariner 40 and try and make it run a bit smoother at idle.

The naughty mariner in question is a 1990 ish 2 carb model made by yamaha

After talking to a good few people, i have a managed a rudimentary service and disassembled and cleaned down the carbs and the engine now runs like a peach at idle and low revs, the problem occurs when i try and use anything more than half throttle!!

It either runs briefly at open throttle and then sounds like it is being strangled or refuses to go past half throttle and still sounds like it is being strangled!

I am guessing this is a fuel issue, but i can't find the fault. The fuel is brand new and a second batch has been tried. the fuel lines are clear, the filter is clean and the fuel pump is pushing out a good amount.

Have i missed something obvious when i put the carbs back together?

Also does anyone know the correct setting for the brass screw at the top of the carbs? and could incorrect adjustment of these be my downfall...

[Downhilldai]

Did you use new gaskets when re-assembling? Sounds like it may be drawing air.

[Turbodiesel]

Originally Posted by SimonStDavids

Over the winter I thought I would tackle a service on my Mariner 40 and try and make it run a bit smoother at idle.

The naughty mariner in question is a 1990 ish 2 carb model made by yamaha

After talking to a good few people, i have a managed a rudimentary service and disassembled and cleaned down the carbs and the engine now runs like a peach at idle and low revs, the problem occurs when i try and use anything more than half throttle!!

It either runs briefly at open throttle and then sounds like it is being strangled or refuses to go past half throttle and still sounds like it is being strangled!

I am guessing this is a fuel issue, but i can't find the fault. The fuel is brand new and a second batch has been tried. the fuel lines are clear, the filter is clean and the fuel pump is pushing out a good amount.

Have i missed something obvious when i put the carbs back together?

Also does anyone know the correct setting for the brass screw at the top of the carbs? and could incorrect adjustment of these be my downfall...

Check the Mixture screws, The brass ones you have talked about. Screw them in until the lightly seat, then take them out by approx 1.5 turns to give you a base setting. If its not picking up then take them out by 1/8th of a turn. Do the same on both carbs. You might end up with them at 2 turns out but its ok if they work. Down side is the further out you take them usually the idle will get a bit worse as the mixture is richer. Also just check you are getting Max timing when you go to full throttle. Its a fiddle to set the timing up on those as its called static timing so its all done by adjusting the length of the link rods.

[SimonStDavids]

After a week of tinkering I still have got no further, I have taken a short (probably the least interesting video in the world) so that someone with a bit more knowledge might be able to help identify the struggling sound!

http://www.youtube.com/watch?v=rufDK...yer_detailpage

I didn't renew the gaskets when i put the carbs back on as the ones that were on there looked ok? Would an air leak there be sufficient to explain the struggle?

I have also been adjusting the brass screws, it does make a small amount of difference the further out they are, but even when they are 4 or 5 turns out (and about to fall out!) the open throttle still isn't there? It does make it very rough at idle though!

As per the timing, the linkage is pushing it right round until it hits a stopper, i could move this stopper as it is only held on by a bolt but there is no indication of where max is? just lots of lines!?

[uncle al]

have you tried keping preasure on the fuel pump priming bulb? did it improve? if so go and replace your fuel pump diaphrams,have you moved any of the lock bolts (the ones that the timing goes to on mi/max revs.If so i would suggest you purchase a factory service manual from any merc/mar dealer (they were available when i retired last dec) cos the set up is complecated if you have never done one before ,did you change the spark plug?if so are they the correct ones as per the factory reccomendation?It sounds like carbs or set up.
regards
alan

[uncle al]

what i forgot to mention in my earlier reply there is a metal bracket that restricts the revs in rev it might be worth checking that the throttle linkage in fwd does not touch that shield

[SimonStDavids]

Originally Posted by uncle al

what i forgot to mention in my earlier reply there is a metal bracket that restricts the revs in rev it might be worth checking that the throttle linkage in fwd does not touch that shield

Thanks Al, no i checked that and the throttle can open fully. My first suspicion was the fuel pump and as luck would have it someone local was breaking a 40hp yamaha, so i bought the fuel pump, it looks identical except that the fuel "in" is in a slightly different place. I don't have any way of measuring the fuel pressure, but if i bypass the pump and hand prime it and let it suck fuel out of a jam jar it seems to pump a good amount, ( I am sure more than the engine could use?)

The problem still happens when i hand prime too, that is why i think it is something else?

I think i am going to have to hand it over to a professional,