air box or k and n's

my 1980 kz650 ltd is running on k and n air filters, do they run better with standard airbox?

A stock KZ650 engine will run perfectly fine using the stock airbox and stock jetting. That configuration will give you a smooth powerband from idle to redline. That has been the case on my bike for 36+ years. If your carbs were rejetted or otherwise fiddled with to match the individual filters you will need to rejet back to the stock specs for the airbox to perform at its best. Ed

Unless you feel like spending a lot of time adjusting the jetting to make the bike run right, I'd suggest staying with the air box. I didn't notice much of a benefit when I had pods other than it made the carbs easier to take on and off.