missing might be due to the improper facing of carb inlet pipe and carb outlet .. the inside dia of the aluminium pipe connecting carb to head must be of same size as that of carb and both holes should be perfectly aligned .. i suspect in your case its not..
if they are and is your bike 78cc or 72 cc one ? if 72cc then try 68-70-72 main jet. it costs 35-50 rs each.
if its 78cc then try 75,78 mj.
buy any vendor jet and make sure that the hole in jet is in center ..
is your bike useing euro 2 style carb or normal one ?
lifan is very poor quality carb.. i had a thread about that as well...
increasing jet size will increase fuel consumption.. dec will reduce fuel consumption