So I fitted the bigger cables, had no effect.
Tuned the bike several times. Made a small improvement but the issue still persisted.
So I took the starter off and stripped it down, I found the back bush near the brushes had some play in it and the front thrust washer had some heat scores on it. Basically nothing shouting massive issues.
So I thought I would try another starter motor. After ordering a new second hand one, I was looking at trying to buy new (not available) but I came across new ZZR600 ones that looked the same with the same 10spline shafts.
I found an auction site special for £20 inc delivery so worth a punt.
When it came I checked it against the original and the collar and the rear fixings are identical but the diameter was about 1/4 bigger. So I tried it anyway. It fitted and wow
what a difference. It now starts Very quickly and when very hot too.Starter motor from 1997 ZZR600
Denzo starter
Part no. 21163-1211
228000-1820
12v
Worth getting a set of uprated cables if you fit the bigger motor as standard ones will cause issues and get hot.
I can’t say if there’s any issues with using the above but I am trying it out myself. I hope the above will be useful to others but I can’t be responsible if it causes issues for other people just sayin what I have done and it works for me.
IMO the starter as fitted from factory is a bit light on power and as soon as wear sets it then things that’s when it becomes a problem, looking at my starter it looks like a combo of the tail bush wear and the front thrust washer taking a heavy load is causing it to loose power so when things are hot and tight then it can’t overcome the friction of the engine.