Belive me mostly mechanics are just stupid and they have no idea what they are doing.
first fill up your CNG tank and add 1000 rupes patrol on your car.
Ask mechanic to tune your car on petrol and remove air blockage (if any). Then run your car on petrol and try full throttle and see car's response. If car is not working good on petrol, then it can't work good on cng as well. Drive good 5,10KM and then confirm your car's performance, not just run it in first gear and decide.
Once you are satisfied with car's performance on petrol. It time to tune your car for CNG. If your car currently have some blockage, then this means that your car dont have a good venturi and you can't tune your car for CNG without air blockage. I believe that adding a good venturi is good investment if you want to drive your car on petrol as well. Otherwise your car can only perform good on CNG or petrol, not both.
Ask mechanic to advance timing little bit (by adjusting distributor) and then tune the car on CNG. After that run your car on CNG for good 5,10KM and make sure you use all of gears and check acceleration. And continue to ping mechanic unless you are satisfied with performance on CNG. (Don't expect rocket like performance on CNG but try to get it close to your car performance on petrol)