ZX6R J1 running badly at low/mid revs *Fixed*

[Fullers1845]

Morning all, I took the ZX6R out to work yesterday and in the morning absolutely fine then by the evening it had started running like a bag of shit under load at low to mid revs, starts ok and tickover is absolutely fine, revs up/down in neutral same as usual and settles ok. Cruises along fine.
There's like a hesitation/stutter when opening throttle almost like it's just about to run out of fuel, acceleration is poor and then as the revs pick up to around 8K it suddenly clears and goes like a rocket as usual, almost like a bad case of turbo lag, though when you ease back off and slow down it runs a bit rough and feels like it's then dropping a cylinder or two. Then cruise a bit and it runs ok again.
The fact that starting/idle and higher revs are really good makes me lean away from electrical.
I had the tank off last night and all vacuum hoses are ok, did notice that the back of the airbox was quite wet with fuel, maybe a carb is spitting back?
This is one of the yr 2000 600's with Mikuni carbs (cleaned/balanced 600 miles ago)
I guess the carbs will need to come off next but any ideas most welcome.

Edit: Finally got the blooming thing sorted today.
I took the carb manifolds off and the o-rings that seal them to the inlet manifold were flattened, one of them was actually thinner in one area than the groove depth!
Gave all the carbs another good clean out before refitting.
I also removed the secondary air system ‘Kleen’ as the vacuum valve was sticking and the rubbers starting to go; made up some blanking plugs in the workshop at work for the reed ports; I don’t have a cat in the exhaust to worry about overheating.
Running the best it has since I got it, really picks up nicely at lower revs too which is a bonus.

TLDR: did lots of stuff so not sure what finally fixed it!
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Honda CB125F, Kawasaki ZX6R J1, 1922 Sheffield-Henderson Blackburne grasstrack/sprint

[A100man]

[stinkwheel]

Fuel pump glitching? I think they have the electromechanical type of pump which have wearable parts in them, maybe pop the top off and inspect the points, maybe give them a clean up/file if they look ok.

You can get service kits for them. You can also buy cheap replacements for less than £20.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[MCN]

Ignore your first assumptions.

Spark plugs are not dead expensive and should be replaced every (couple) of years anyway. Whether they 'look' OK or not.
Insulation breaks down, microscopically and some of the magic leaks out.

Easier to change than carb phaphery.
And in shitey issues like that, wee elimination jobs all add up.

The rule of do easy-shite-first.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Fullers1845]

I'll hopefully get a look in this evening, I've also read that the earlier Mikuni's are known for occasionally dropping a 'metering rod' (assume this is main jet needle?) leading to these symptoms.

Is the fuel pump likely to be tied in with the throttle position sensor on the end of the carb or is the TPS only for ignition advance?

Edit: I'll check the plugs for colour, they are only 600 miles old though.
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Honda CB125F, Kawasaki ZX6R J1, 1922 Sheffield-Henderson Blackburne grasstrack/sprint

[stinkwheel]

[MCN]

[Fullers1845]

Yes definitely looking at that pump.
Just got to find some time , supposed to be working on the 101 year old bike for next year.
I really need to retire or win some money so I can get all this shit done...
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Honda CB125F, Kawasaki ZX6R J1, 1922 Sheffield-Henderson Blackburne grasstrack/sprint

[Kawasaki Jimbo]

If it runs fine above 8k revs it’s not going to be fuel pump related, is it?

Ignition coils came behave really weirdly when they die. In my limited experience they’ll allow high speed and revs but deliver bogging under load.

In these moments I always say, “carb-icing” and I’m usually wrong but it’s worth consideration. That can also behave weirdly in terms of speed and throttle-opening.

My ‘99 (G2) ZX6R slowly developed a big ‘ol flat spot at 8-9k until I did a big refurb/clean/service at 60k miles. The carbs were clean but the Dynojet needles had some corrosion (albeit the shiny region might be the working part). I changed a lot during that refurb including the needles, but the problem was solved. However I can’t be sure what fixed it.
https://i.postimg.cc/nV6YJ91Q/IMG-0185.jpg

Incidentally the slide looks horrible but was quite smooth on all four.

[stinkwheel]

[CaNsA]

[Fullers1845]

Well Stinkwheel's suggestion of looking at the pump was a good one; the brass points were black and pitted and had worn at an angle so I spent some time with a fine needle file and they are much better.
So this has improved things but not much; it did suddenly dawned on me riding in today that last tuesday when this issue started the bike was left out in torrential rain for over 4 hours; being that the cap seal is 23 years old I'm a bit suspicious that I may have a little water ingress into the tank or some residual moisture on electrics.
It does seem to be gradually improving with running.
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Honda CB125F, Kawasaki ZX6R J1, 1922 Sheffield-Henderson Blackburne grasstrack/sprint