I read the article you are quoting. What it is referring to is with a K&N pod type filter, you will need to increase the main jet size in the carb to basically increase the amount of fuel in proper relation to the amount of air. The article was also mainly in reference to 4 cylinder bikes and bigger although the principles are the same for small cc bikes.
So if you remove the airbox from the Ninja 250, you would have to change the stock 98 main jets to something like a 108-112 main jet. If you leave the airbox on with a Drop in type air filter, a large increase in main jet size is not necessary, a 100 or 102 would be the biggest depending on the brand. Shimming the carb needles and removing the snorkel can net very similar results as re-jetting. Also in the article it also states;
" Carbureted powersports vehicle manufacturers do not jet their carburetors perfectly, for two reasons. First, there are emissions considerations, mostly affecting the idle and midrange carburetor circuits. Consequently these circuits are jetted leaner than normal, and should be richened slightly for better performance. However, this rarely requires more than an adjustment to the idle mixture screw, and in some cases an adjusted slide needle height. Second, manufacturers ship their product to many different places across the globe. They purposely jet too rich on the main jet, the carburetor circuit that presents the most liability in terms of engine overheating. Therefore, all road-going powersports vehicles are jetted "fat" on the main jet. Contrary to what many people believe, but true nonetheless. Manufacturers do this because they cannot jet individually for each market, so they simply err on the rich side rather than the lean side, because the latter would present more problems. The fact is, engines are much more forgiving of a 3% rich condition than an equal lean condition, and just as importantly, few customers will notice a rich main jet, while nearly all will complain about a lean one."
Last edited by DaBlue1; 03-07-2012 at 05:49 AM.