Many things can cause buildup, however the most common cause is the carburetor. Often the carburetor is jetted too rich to balance out the use of restrictor plate in the mid range [where 95% ride]. Even though the main jet is a #80, it is possible the needle is too high and running rich, because most of the time the throttle is less than 3/4 open. The carburetor should be jetted to match your riding style. Another cause is compression, if the compression is increased the buildup will most likely decrease. If the valves aren't seating completely it could also cause reverse induction [blow by through the carburetor], if the lifters are set way off the motor could run rich[too much clearance], and last the camshaft should be set one tooth advanced to lean the motor out somewhat.
I am somewhat confused by your use of a #80 main jet, most NE motors use from a #84 to #88. If you have the earlier WC-1 motor a #80 is too rich, most use #74 to #78.
Hope this infromation is helpful,
Whizzer OuterBanks LTD
A North Carolina Corporation