wll guys thanks for all ur advise but i went to the mechanic and he made following changes in the engine
replace cluch box and its plates with its gear
install a new coil on it
install a more precise balancing gear
change it carburator with cd 70 to give better average
change its piston and replace with a bigger piston
the mechanic who done all these stuff is my friend he is still testin the bike he told me that bike sound is totally different from before .. he is still keeping the bike till its running is finished because he change its piston..
i also have two 70cc motorcycles at my house .. but this bike havent start for at least 8 months because i was using my other bike
now iam doing a little experiment on it....