wa'Salam
your mechnic is correct bike zayda petrol khainch leti hai .. usnay desi zubaan me he sahi problem ko diagnose bilkul sahi kiya hai...
But treatment ghalat advise kiya hai.. you bike do not need any other caburator or nay thing...
this is called Choking on redline due to rich feul settings...
You have lean your carburator a little .. put the bike on double stand, reve it to redline in neutral (bike should hit 11.5+RPM) if not lean more untill it starts to redline instantly..
zyada lean ker diya tu phir bhi it will not go redline or go very lazyily ... so one has to set it very carefully..
once done redline it for atleast 2 seconds.. bike will vibrate like hell on stand.. switch it off and remove plug instantlywith plud wrench it will be hot so do it carefully
see plug color it would be radiant and orange like red... with no white blisters... or no dark black blisters..
if there are dark black blisters lean it more.. if there are white blisters rich it a little.. once its proper red.. BINGO
not test bike will surprise you hope fully unless it has nothing else going wrong..
hope that helps...