hi and welcome to Ninjette!
You have been inflicted with most common issue with these bikes! Over 80% of new owners encountre it and many are defeated by it and dump their bikes. You have "
Ninjette dirty and clogged carbs syndrome!!!". Reason it took while to act up is rust and debris took time to work their way from tank to carbs to combine forces with gunk at bottom of float bowls. When their levels built up enough, they got sucked up by jets into passageways that accumulated enough to block flow. Then bike stutters worse and worse until it dies completely. You will
NOT be able to fix this issue without drastic measures...
Due to their small size, these carbs have some of tiniest and easiest to clog passageways in existence! To make them, Kawasaki had to employ specialised Kaze elves from Yakushima forest for their thousands of years of smelting and wind-harnessing experience. No one really knows how these carbs are made, just that there's lots of voodoo and black arts involved.
Many, many extremely skilled mechanics, who can tear an engine apart, rebuild it and put all back together in less than 1 day, have admitted defeat at cleaning and fixing these carbs. They've had to repeatedly pull these carbs 4, 5, 6 times! for ever deeper cleaning until it was factory-fresh clean. Sometimes, there's blood sacrifise needed! One person had to pull them 9 times!!! Then finally, bike would run like brand-new off showroom floor.
So... I suggest you deploy ultimate nuclear WMD operation with full restoration 1st time around. Otherwize, you'll be pulling them over and over again. What's involved?
1. complete tear down of carbs to every last nut, bolt and individual component. If it can be futher separated, take it apart even more!
2.
scrub out every secret hidden passage with acetone and PEA-based fuel system cleaner. Spray carb-cleaners no longer work due to removal of clhlorinated compounds (thanks! crunchy granola bark eating crystal worshipping tree hugging smelly hippies!).
http://n4mwd.blogspot.com/2013/10/se...-passages.html
3. poke out all microscopic bleed holes in jets, emulsion tubes, carb-venturi, choke circuit with soft copper wire. In one recent case, non-removable choke jet had to be drilled out with micro-bit, it was so clogged with dried petrol that NOTHING could clear it.
4. soak everything in ultrasonic cleaner for days on end using caustic radioactive solvents.
Amazon - 4L Ultrasonic cleaner
5. micro soda-blast everything @ 10000psi! All that scrubbing leaves tonnes of dried petrol plastic flakes everywhere to clog things up.
6. replace all rubbers: float-valves, fuel-rail O-rings, pilot-screw O-rings, float-bowl seals with modern Viton & fluoroelastomer versions to deal with modern ethanol petrol. Perhaps even slide diaphragms if needed
7. set float levels and wet-test
8. sync carbs
If you leave any step out, you can't be guaranteed that carbs are factory-fresh clean and will have to pull them yet again to do missed step. Luckily, we have wizard who has has worked on millions of these carbs and has reversed-engineered the secret elve's black magic and is able to get them factory-fresh clean 1st time around. Do search on here for "clean carbs ducatiman" to find posts with photos of what's involved.
Some related threads:
https://www.ninjette.org/forums/showthread.php?t=365823
https://www.ninjette.org/forums/showthread.php?t=354327
https://www.ninjette.org/forums/showthread.php?t=352293
https://www.ninjette.org/forums/showthread.php?t=366273
https://www.ninjette.org/forums/show...13#post1225613
That 1st link above was by extremely skilled guy who could do this kind of custom work:
https://www.ninjette.org/forums/showthread.php?t=117501
https://www.ninjette.org/forums/showthread.php?t=117957
But in end, even he was defeated by dirty clogged carbs and sent them to "The Wizard" to work his magic.