ok..now my car is fine on petrol mashallah..
problem solved in weird way but still..
prob was wen i swithced to petrol ,RPM was too much,so ironically wen i unscrewed the AC idle speed screw,rpm went down ...its OK on ac n OK on petrol with out ac..now wen i use cng,i screw it up a bit n on petrol,i unscrew it...
dont knw y the ac idle spped screww had to do some thing wen ac is not running!
n petrol avg has gone better now too...like 10-12 kms per liter in city! although cng consumption has gone up a bit but i can live with tht
thts too all for inputs..really helped alot...specially FARHAN bhai whom i contacted on phone too
thnx farhan bhai