Ngk plug info for you! With karts we always run a number 10 in dry and then a hotter plug for the wet which is a 9
http://www.ngksparkplugs.com/tech_su...k_plugs/p2.asp
The varnishing is from the blowbuy from the rocker cover so is normal
Have you checked the oil level or changed the oil?
When you spray stuff around the carb the revs should increase if you do have an air leak - this will highlight the area to look next