In theory, if the idle side was set extremely rich, the idle stop screw (plastic head) would be screwed in a fair way to compensate for the idle mixture wanting to stall the engine. I would lightly seat and unscrew 2 & 1/8 turns (as max has posted), then undo the plastic idle stop screw until you get around 2000rpm and only then have a decent play with the mixture screw
One thing I do recall is when honda fitted their snorkels, they recommended changing the main jet in some instances. Similarly, when victa did their snorkel kit for the gcv, they managed it by changing the size of a nozzle stuck in the motor end of the snorkel. Most used black nozzle, some used another colour.
As Norm said, if it was running right, you should be able to go years between cleaning plugs. If it was carboning up, maybe your muffler is also now full of carbon. I have faced this issue with my big bob as the previous owner ran it with a clogged filter (gxv120). Once I sorted out the mixtures and filter, the first hard run saw me nearly light up 100sqm of dry grass by the amount of black and red hot carbon mixture shooting out the muffler
