K&N lets through lots of dust. Ends up wearing out your engine quicker than factory filter. Factory filter and airbox poses no restrictions anyway, so no benefit in removing.
Rejetting will do nothing because air-flow with K&N is same as stock. Most likely bike has already been rejetted and that's why it stumbles.
Verify carbs have all stock parts: jets, needles, etc. If not, put stock parts back in and give it thorough cleaning. Not just spray useless carb-cleaner, but actually use brushes and scrub out hidden secret passages from end-to-end with PEA-based fuel-system cleaner.
https://n4mwd.blogspot.com/2013/10/s...-passages.html