lt can be either, a carb tuned to rich or a mix ratio to rich. One culprit also is how operators use their ope, lots of idle, half throttle ect. Carbon is mainly evil to those not rehearsed in removing/checking for it. Don't forget 99.9% of 2cycle equiptment users don't do the maintenence that enthusiasts do, they just run their two cylces as set up from dealer and only check things after a failure has occured and for this reason full synthetics come into play leaving combustion chambers, pistons, sparkplugs cleaner with less build up. That is the main reason oem's reccommend them, not for their lubricating properties.