My hero dawn brand new bike still 2months....in the carburetor overflow srew get tighten by a mistake and now i cant unsrew it....what should i do

First shut water off at stop than flush all the water out of the tank next take a sponge and get all the water out of tank next unsrew the supply line from the valve under tank than move out of the way next unsrew the nut on the valve of fluidmaster than pull the fluidmaster out of tank stick the new fluidmaster in tighten nut srew the supply line back on turn on stop and let the tank fill up thats it if the fluidmaster dont shut off press the handle let some water out and adjust the fluidmaster screw Tip the water should stop a 1/4 inch below the flush valve tube in center of toilet

Getting black smoke from my hero honda passion plus bike

You have a sticking float/needle in the carburator. This stop fuel flow from the tank when the bowl is full.

Run the motorbikes engine to a normal running temperatue and thn set the air-fuel mixture by ...tightening all the way in taking care not to over tighten it as you may damage the tip of the screw,usually its thn 2 and half turns outwards or 3 , you will notice the change inthe idling of the bike's engine.
Its better to check the spark plug gap and also check the air filter, blow it with compressed air or if its tht dirty best would be to replace it.
Good luck.

http://www.motosindia.com/hero-honda-dawn.html#splendor This is a link to the engine specs of the hero honda splendor, hope this is of help, if you need any other info it should be in the bikes manual