New plugs just confirms that previous plugs were not problem, since behavior did not change. Therefore issue is elsewhere.
Real problem is your carbs have clogged fuel-passages. When you open throttle, you increase rate of air-intake through engine & carbs. However, due to clogged fuel-passages, your carbs aren't able to increase their fuel-delivery as fast as increase in air-intake. Super-lean mixtures doesn't burn and engine dies.
Yes, to clean carbs, you need to disassemble them down to every last nut, bolt, washer, o-ring, jets, etc. With loss of chlorinated compounds in modern carb "cleaners", you might as well pee in your carbs to clean them. Every single jet, fuel-circuit, passage where fuel flows needs to be mechanically scrubbed and soda-blasted to ensure sufficient cleaning to restore to factory-fresh condition. Some related threads for your review:
https://www.ninjette.org/forums/showthread.php?t=330960
https://www.ninjette.org/forums/showthread.php?t=321223
https://www.ninjette.org/forums/showthread.php?t=317977
https://www.ninjette.org/forums/showthread.php?t=317810
https://www.ninjette.org/forums/showthread.php?t=316395
https://www.ninjette.org/forums/showthread.php?t=315193
https://www.ninjette.org/forums/showthread.php?t=280658
Easy way:
Custom Carb Service by ducatiman