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I bought a 1995 ZXR400 track bike but have since put it on the road, I have a bad rough spot between 4 and 5k , when cold the bike runs fine but after a mile or so the rough spot appears , it bogs down and splutters between 4 and 5k, if i roll the throttle very slowly it is possible to ride through the rough spot , at all other revs its fine, the bike has a beowulf exhaust and K&N type air filter, other than that its standard.
The carbs fitted when I got it were dynojetted so in the hope of getting rid of the rough patch I refitted the original carbs which the seller gave me, they have standard jets (I cleanedthe jets and checked the float height, 11mm).
The rough spot is still there, the exhaust has a black sooty deposit but the plugs didnt look too bad, maybe a bit on the black side, the mixture screws are set at 2.25 turns out (i tried 1.5 turns but the revs hung for a couple of secs at 2k), the needles are on the highest groove and have 2 shim washers between the needle and plastic spring carrier, I have fitted new spark plugs but unsurprisingly that made no difference.
The carb mounting rubbers all look good ,no splits, I'm still thinking it's rich at 4 to 5k as it runs fine when cold, does anyone have any ideas what the fix could be, thanks in advance, Mark.

Have u tried new plugs?

Yes tried them but made no difference, im reasonably sure its a fueling issue.

when mine was hanging around there it was due to slightly rich pilot before moving onto the main needle, transition happens at 4-5k rpm.

i would wind pilot screw out a bit more, what main jets are you on, and i take it thats its a dynojet needle? maybe switch back to the standars keihin n77s jet needle?

wouldnt harm to take it to a dyno place as well, only real way to test the fueling through the rev range. then once its dyno'd you know where to start tinkering.

hesitation under load would suggest knackered plug leads to me

gem

Thanks for your replies, I will continue tinkering today.

After much messing around with needle heights etc I fitted new coils today and the problem went away!