Inconsistant idle and splutter central at low throttle

[e-a-m-o-n]

Yamaha FZX 250,

So yeah, idle has been really inconsistent on my bike lately. At one set of lights it might be at about 2300rpm, at the next lights it might have dropped down to 1500... or even cut out completely.
have checked throttle cable, plugs are a bit sooty but only a few months old so shouldnt be a problem. air filter's clean. there are a few external cracks around the rubber boots which conect carbs with engine but nothing that i think could cause a leak.. have given carbs a good clean out. Is there anything else which I could check easily??

Does anyone know the correct float hights for a zeal? would prefer to set them with a ruler (rather than the old rubber hose trick) as the carbs are in pieces at the moment anyway.

The other issue is that the bike splutters like hell if im riding at low to mid throttle, and is sluggish/hesitates on takeoff. If im accelerating hard with wound back throttle the bike sings and reacts just how it should. Could the idle and this rough running be connected??

Any advice or ideas would be appreciated, cheers!

[MSTRS:Me]

Mixture screws set right on all carbs? Low speed jets clean? Choke operating correctly? Mainjets and needles not worn?
Sooty plugs are a sign it is running too rich, so an airleak is unlikely.
Another thing to check is that the needles are not lifting in the slides, and that they are set on the right clip/groove.

[Rhys]

have you tried giving it a good hard ride, a lot of full throttle that often helps the 'high performance' 250s
as a plus its lots of fun