well i converted my carb from a nozzle drilled in it to venturi and i dont tune my car from cng specialists and i have never done that either, pal i am the guy u need;) first thing is u cannot advance ur vehicle more then 15 to 20 degrees max believe me cause i am a mechanical engineer who is in love with cars and work on them all the time if u advance the car to much it will stall on hi r.p.ms, the second thing is check ur plugs, power leads the ideal value for them lies between 10kohms to 20khms but less then 20kohms and above 10kohms ur filter needs to be clean! now we move to the carb remove the nozzle asap or close it in order to get more power we need to activate the second butterfly asap to get that u have to get a venturi the venturi should not have numerous holes in it just one rectangular opening which u would place such that half of it is on one barrel the other half on the other barrel, remove any tape the damn cng mechanics have placed over the filter to control the amount of air!!! then u reduce the intake cng pipe as short as possible and the power T as near to the carb as possible clamp the ends so that no cng leaks and tune ur car make sure when u tune it see if the second butterfly opens or not automatically the vacum operating it should work do this and report to me the venturi will cost u no more then 150rs max