Yeah, it never even ends up being half a main jet size. read some works by men like Gordon Jennings and you can see the benefit of the added oil, would be my suggestion. Oil has more wives' tails surrounding it than probably anything. whether it is two stroke mix or 4 stroke lubricants.
Going back to stock suggested jetting is always my way when I make changes, but I have seen so few jetting problems where bearing failures are concerned to wonder if I have ever seen one. Air leaks are a little different(they do change effective jetting of course), and they come with failing bearings anyway. no mix ratios are going to alleviate that in a worn engine nor was it my intent to suggest it. So I would go stock, choose a mix, and find your jetting in the traditional means. FYI an increase in displacement on the previous carburetion will cause an increase in vacuum, air velocity follows that, and your bike would actually run richer than it was on the smaller displacement. the boring process on a two stroke tends to retard at least the transfer timing when material is taken away from the cylinder walls. in times past boring also retarded the exhaust port timing as well. with more modern arrangements including power valves the exhaust timing is now changed little and may even be advanced during boring. if your exhaust timing and transfer timing is retarded you get an increase in compression, which may necessitate a richer jet than you needed before the boring. if you go so large on the bore to affect your squish area you may end up needing some really big jets to keep the bike from detonating to the point of preigntion and "death". I have seen some bigbore jobs that turned into total dogs because of things like that. how much change is determined by what the actual angles are of the ports. you find the change in timing with a degree wheel, if you care. you can use a pretty simple formula of .25 x the bore increase to determine the change in port height. derived from most modern transfer ports having 45 degree or close port angles. When you retard port timing you usually eliminate some overlap and that again increases vacuum which in effect richens jetting. FAR more so than I have ever seen in changing an oil mix. So rather than trying to use some slide rule method to determine what to start with, it is just pretty darn hard to go wrong returning to stock and going from there to suit your needs. Like Mi said, jetting or optimum jetting changes hourly in some cases even if nothing else seems to change.
Most of us are running what I call "chicken jetting" just to try to keep from seizing and engine that will be used in varying conditions, perhaps all on the same ride. there are two things on your carburetor that can affect the overall jetting of your engine: the air or fuel screw (in the case of most 4 strokes) and your float level. most of us opt for standard float levels and play up and down during the day if the need arises on the air/fuel screws to get what we want.
