Couldn't find it but maybe he is using a modified airbox? Or it could mean that he has dumped the airbox all together and has K&N filter pods which you would probably need around 127.5's. Be careful what other people are doing, they may be not understanding what is actually going into their bike if they haven't installed it themselves or may be doing mods that are not fully explained.
The easiest and safest way is to do the jetting in steps at a time. So say in your case you say "Ok I need new mainjets what do I have in there". So you know you have 100's so you look up the appropriate mainjet for your bike and know they come in steps of 2.5. So you first start out with 102.5's ride around for a good 3 hours testing throttle response, full throttle runs with a plug check etc. and test it basically. You look at the plugs and the base ring looks a little lean(Short of a full turn of light soot). So you go ok let's try 105's, hey this is better, More full throttle power, plugs look good, etc. But to be sure you try 107.5's for fun. Hmmm not so good power is lower when the bike gets hot. Sooty base ring. So now you know that 105's are good for your bike. Total cost $24 and a few hours of your time. Sometimes it's good to get what other people are saying on the forums to get sort of an idea of where you should be but every bike is different. Some need more jet, some need less, etc. Try and use common sense when experiment and your "butt dyno" when jetting.
Ask questions to yourself "Is the throttle more responsive?" "Does it run better cold than hot" "What are the plugs telling me?" I'd say %99.999 of the people out there can tell when a bike is running stronger or weaker and at what temperature.
For example if the bike pulls awesome when semi-warmed up then less so when fully warmed up then it's rich. Better warm but not great then it's lean. ETC ETC. You should also jet for the average temperature at what you normally ride at. So if you only ride when it's 30C or above it's going to be lean when running a 10C. For me I ride right up until it snows so I could be riding in 2 degrees or 35 degress so it averages out around 20 degrees so thats what I shoot for, so its only slightly lean in wintertime(sometimes I turn the mixture screws out a 1/4 turn for the winter) and in 35C its slightly rich but generally overall it rides perfectly. If you only jetted for the cutting edge of lean power at 35C at 10C it would run like a dog. But generally if your not modding the intake with K&N pods or something it should run perfectly fine with a slip on with 1 or 2 jets sizes up and a needle adjustment.
Put it this way I have a 1200 so my cylinders are around 270 CC or something and they are using the same mainjet size as your 100CC cylinders(I have bigger carbs of course)thats how much the 1200 is restricted. So on your little 100CC cylinders I would guess that 127.5's would be crazily rich unless you were going to K&N filter pods and even then I would be a little worried that it's too much jet. I found the post he made and he seems to infer that he has pods. Although he is using a 91 Bandit 400 which I believe had different carbs.
I would try a step at a time until you are rich then go one step down with the stock airbox.
Topic: B400: 50, 53, or 59 HP? (Read 13666 times)