visible black smoke coming from a petrol engine is just insane! It SHOULD NOT smoke.
Obviously your carbs are running too rich, even on idle! Probably the mech who serviced the carb (did he open it all up?) changed the float level settings, maybe he put the wrong jets inside, maybe vacuum lines are not hooked up properly. It could be any of these things...
I recommend you to get the carb checked from a specialist. Beware, everyone says i can do it. But trust only that who actually has a lot of carbs lying around and better yet only someone who has been recommended by a khyber/ suzuki owner who has had his own carb serviced.
If you'd rather not go through the hassles of carb adjustment or repair (and believe me once its opened up, theres probably no end to it), then just get a kaablee PETROL ONLY carb of khyber (or cultus if its a bolt on?), clean it up using petrol on the outside, use carb cleaner and/or liqui moly fuel additive to clean up carb's fuel passageways. Voila, you've solved it
A car running good on petrol will have less problems on CNG coz most rules applying to petrol as a fuel apply to cng as well, only a few changes exist that can be done by changing ignition timing and cng pressure settings.
Hopefullly i havnt confused you more.....