when i pulled my bike (H1 zxr750) outta the shed today to go for a bit of a run, after warming it up for a lil bit, i noticed when stationary, if you wind the throttle on say, 1/4 - 1/2 a turn, it'll rev smoothly right thru the range, but if you roll it on much further past that, it coughs n splutters n carries on like a rife ole mole...back it off again, and it'll pick up smoothly past 12K again

ive only had the bike a few weeks, and I'm gonna do the valve clearance s/ balance carbs etc etc next weekend
the end of the can is a darkish kinda grey, as opposed to what it should look like (light grey), i guess, were it to be running right...
so running rich is what im thinkin, but im only new to this bike, and reckon someone here might be able to confirm this for me or perhaps give me some other idea/s as to where i might go looking for the trouble...
thanks in advance..