get a complete engine service done, including new plugs+PROPER and ORIGINAL tappet settings for PETROL, make sure distributor vacuum advance is running properly, check for any air leaks into the intake manifold etc. etc.
tune car for petrol first, make sure it gives good power and economy on petrol, that will tell whether engine is in good shape or not. Then change only timing for cng. And then adjust cng mixture. And ure dun.