If the tamper caps are still on the carburetors, then you couldn't have cleaned the idle circuit.
Did you physical remove the idle and main jets? And main jet holders? Did you verify all the fuel circuits were clear with a air compressor? Set the float height? Etc......
http://faq.ninja250.org/wiki/Changin...rands_to_avoid
NGK Iridium IX® plugs are the way to go personally, I run them in both the Ninjette and EX.
http://www.ngksparkplugs.com/product.../iridiumix.asp
NGK (7669) CR8HIX Iridium IX Spark Plug, Pack of 1
https://www.amazon.com/dp/B000HDF852..._3P6Zvb2PBX9FX