for a bike to run properly it needs a perfect mixture of petrol and air.
normaly biks are tuned at factory for that optimal range .
the ratio of air and petrol is 1:14
and if a bike is tuned to this ratio it will perform ok. however elevation affects this ratio. i.e. at differect elevation the carb cannot maintaint this ratio. so there is a provision in the carb to accomodate for this anomaly.
so you can play with the needle . try to raise it in the clip it will richen the mixture from half throttle to full throttle position.
then try lowering it ..
see for yourself ..
the stock setting is mostly the middle grove ..
if ratio is too low i.e. more air and less petrol .. bike will heat a lot and will damage the engine ..
too much petrol and less air will leave a lot of feul unburnt and that will foul the plug and head and valves .. thus in the end decreasing the performance .