i gotta 600fx , its sat over winter and i've put an acewell digi dash on.....
took it to snett and it wouldn't run prop..took the acewell off and its no diff....
i,ve changed the plugs cleaned the carbs & air filter...still the same.
it splutters at about 50ish and wont go any faster.... but it revs ok at standstill????? any ideas...

pump seems ok....

 

it might be a silly question, but did you replace the fuel ?

the one thing I can say other then this is it probably isn't a specific cylinder issue (like a bad plug). this bike can run fine on 3 cylinders and you'd barely notice it.

is it possible the air box or one of the air ram ducts isn't installed properly and there's air leaking in ?

the other alternative is the pump, which also flows fuel to all 4 carbs.
one other (silly) thing that came up, did you check the fuel valve (petcock?) on the bottom of the tank.

hope this helps.

Avi