I am guessing your reason for wanting to remove the airbox cover is to get more ponies right? Well, yeah - remove it or at least cut half of it away, throw in a high flow air filter (K&N etc)rejet to a 140 main jet, place 4 x3mm (YES 4!) washers under the head of the needle that is housed inside the carby slide (easy to do, just remove the fuel tank, remove the diaphram cover off the carby, pull the slide, needle etc out, place the washers in and put it all back.) You will have to remove the carby to re-jet though.

Also, you will need a better exhaust than the mo-ped style stock one. Throw on an FMF Powercore 4 or similar. If you've got the cash - throw on a bigger, better high flow header pipe also. Once you've done all of this, and only then, let me know what you think! If you do all of these small improvements you are pretty much most of the way to unleashing the trapped in power of this 250. Still, will never be a 450, but for a 250 mine hummms!!

 

mine barks or rips, but definately dosen't humm.