The klx isn't lean from new just very restricted, look if all new bikes were as lean as most people think they are they wouldn't last too long as on a 4 stroke lean ( heat ) damage is cumulative and it would affect how long a new motor would last overall. There are examples of stock motors on this forum with over 20,000 miles, they can't be lean and last like they do.

 

I can agree with that, to a point. Perhaps they aren't dangerously lean, but they certainly aren't rich either. They need to pass emissions. And then there are those that unrestrict them with an aftermarket pipe and keep riding without a rejet. We don't hear about them burning up their bikes.

 

There's a lot of range between "so lean it burns up" and "so rich that it fouls the plug". A guy can happily ride on the lean side of optimum for a long time and not know what he is missing but also not grenade the engine. Cold starting, hot starting, throttle response, mpg, peak power, etc. can all suffer when jetting is not ideal.

Only an inexperienced person would say that OEM jetting is anything but way on the lean side. Surely not too lean for OEM equipment but dangerously lean when you replace the stock equipment with free flow intake and exhaust.

Note that in addition to the air filter, you also have an OEM restrictive intake snorkel. To derestrict the intake, that snorkel may have to go.