which is the best air filter for the yzf 125r

Generally original. K&N are OK but costly.

if fitting a k@n filter would I need to get the fuel ratio fixed or is that only done when adding a scorpion exhaust has well

Yeh spurs, air filter will run without jetting, but with the exhaust mod, it will need adjustment,,, and its not easy to get right,,

would you kindly explain how the jetting is done has these bikes have no carbs

Assuming the filter is flowing more air the FI may need adjusting but some do this automatically but only within a small range, this only matters if a open exhaust system is fitted.

There are a few cans on the market that claim there is no need to alter the fuelling.

Hi. Im also in need of advice on this. If I was to fit k&nfilters direct to my carbs will I need to replace the jets or do any other carb work for that matter?

Yes the jetting be totally different, also if you are not using the original airbox the tuned length will be shorter and effect the whole engines characteristics.