air filter

hello Im new to this forum malarky,I ve just bort a shineray 125 its well looked after does 65 mph on flat which is fine, that said I am going to change the drive sprocket first to see what happens then maybe wheel sprocket. what Im more concerned about is the bike is fitted with a k an n air filter which is causing a flat spot at the start of revving the engine, you can hear the air being sucked in so I am assuming this is making the mixture weaker and hence the flat spot (tick over is too low as well and it cuts out unless revved) could somebody tell me what has to be adjusted on the carb please and where the screws are located. thanks
Retro.

http://walbro.com/brochurefiles/3368...g%20Manual.pdf
What's ur current main jet size? The stock "90" is too small for modded bike.

ok thanks for help humanbeing, today I put the old air filter back on,(what a fiddle) and the bike seemed to run better and doesn't cut out now but if you put on full throttle from tickover it will die unless you de throttle quickly. Could the bottom of the float chamber have crap in it I wonder? I did used to have bikes years ago and Im starting to remember "the tinkering" lol , would you know what size jet would I need if I wanted to put the filter back on? as I would like to if its going to add to performance,I will have to take the carb apart and have a look to see what size the jet is.

Thanks for the really valuable Information.Can't write it better than this...!