Had this with mine. After a lot of fettling with new carb parts, now have brown/sooty around city, grey on a run which is better.
Don't really understand the dashpot thing to produce more mixture for sudden acceleration , as its slowing down the main piston (which will let less air, but also presumably at the same time, let less fuel through the jet hole?).
Hi Dave, with the airflow a lower pressure is produced in the intake side of the carb piston giving a higher velocity which literally sucks extra fuel out of the jet even though the needles upward movement is delayed by the dashpot oil.