While driving, I faced some problem in my bike. when
I raise the accelerator suddenly my bike's speed goes down. Anyone knows what is the problem?. I'm using
yamaha ss 125 cc bike.
If this is a carburated engine (not fuel injection) then it's likely that one or more of your carburettor jets is blocked or the diaphragm has a hole in it. Find a service manual for this motorcycle (there's probably a PDF of it somewhere on the internet) and follow the instructions for taking the top off the carb, you will need screwdrivers, small spanners or a socket set and a spray can of carb cleaner that comes with a straw. You'll probably have to take the fuel tank off to get the top off the carb. If you can't find carb cleaner, a spray can of WD40 will work.
Once you've taken the top off the carb (watch out for the spring and don't lose the little clip on the needle) you need check for pinholes in the diaphragm and clean the jets (all the little holes in the bottom of the carb including the one that the needle goes in and the one with a small pin coming out of it, nearest to the engine) by pressing the straw into the hole (with the other end connected to the spray can, in case you don't realise) and spraying the cleaner into it. Forcing the cleaner into the jets like that helps to blast all the goo out. If the diaphragm has a pinhole in it, you can use a tiny blob of glue (or instant gasket) to seal it but the blob must be as small as possible otherwise the diaphragm won't work properly and the engine still won't run properly.
Also, try Bradshaw's suggestion for adjusting the idle jet aka air screw.
Twisting the throttle opens the butterfly valve in the carburetor which allows it to suck in more air. With the increased air flow, the carb should also suck in more fuel. So, if enough fuel isn't supplied the engine can't handle the extra air. I don't know enough about your bike to offer a solution, but this is one possible cause.