Originally Posted by Venice Motor Bikes
I've found on my bikes that the 'surging' is from the engine being very close to being the right mixture but a little too much on the rich side. it feels like the bike is bogging & then it will get a little 'crisp' burst of speed, then go back to bogging... re-jetting a little smaller has always fixed it.
This is also how I know when I have the jetting right, it will run real crisp & clean with just a small bit of 4-stroking. (the little bit of 4-stroking tells me that it's not too lean!!)
Could it be possible that the smaller jet uses less fuel and allows the restricted fuel flow to "keep up", also allowing for the right mixture at the same time? (slightly too rich+restricted flow=surging) (smaller jetting+restricted flow=just right).