Read this just after my last post. Now Ultima are locally assembled but good quality wires. Previously I used Seiwa Japan (they are called as such but are assembled in Malaysia as per my info). I found Ultima, though local, to be better in looks and build quality than Seiwa. Ultima are also more expensive than Seiwa.
Still, Ultima may not be better than the factory-fitted ones. They write the name "sports" whereas they are common replacement parts. A product cannot suddenly become "sports" just by writing "sports" on the box. I really doubt they will perform similar to or better than stock wires.
Real performance plug wires are quite thick, contain lots of copper. Popular brand names include Autolite and Splitfire, although American Silicon products are also being assembled by Fairsale Autos 0321-8250197 Saddar Karachi. (I have never bought something from them so can't comment on quality of service or products etc. so no guarantee there).
Now plug wires don't affect consumption that much so this is what I am thinking: Your spark timing has been advanced too much because of the weaker spark being previously produced due to worn out plug wires. (Just to make those wires work as they gradually wore out, slight changes had been made at every tune-up, this is nothing worrisome); now you can have your spark timing adjusted again so it will be better. Air/fuel ratio of both CNG and petrol would also have to be readjusted so as achieve like-new balance.
Hope I haven't made a typo since edit feature is off. I simply hate spelling mistakes.
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Thanks Ahsans for such a long reply...
The timing was advanced by the mechanic with the prev plug wires and it is not brought back when i changed the wires... so u r right this might bee the cause... 
