Guys need ur help URGENTLY

guys i have a problem with my car.i own a 2006 cultus,bought it apl4 and now it has clocked to 53000km.the problem is that it used to get over on petrol.i got its carburettor cleaned and washed with the petrol,the problem was solved but now another problem is there.it doesnt switch off on petrol means to say that when i switch off the ignition switch the engine keeps on stalling.again went to my mechanic he again cleaned the carburettor,started the car and the thing gone worst then previous.the rpm was too much increased.upon doing setting of carburettor the car got stable(to some extent) but the problem was there.so my mechanic told me that the petrol is not good.i was having this thing in mind so before going to mechanic i topup my car with super of rs 500 from shell near sher paoo bridge.so came back home and decided to keep the engine on so that all the bad fuel can get burn.i also added 3 caps of stp in the tank but after keeping my car switched on for like 2 hours the petrol was not completely burnt and the problem was still there.so took the car to another mechanic and he told me to get the tampets adjust by the machinist(as they are fixed) and ur problem will be solved.i dont want to go for this solution as it needs opening of head gaskit and alot stuff plus i dont thnk ke tampets get loss so quickly.so cam home and shutt off the engine.again tried to start the car at 11 pm and it was not starting.the self sound was like of broken timming belt and the smell was horrible.so i need EXPERTS to help me out in this matter.im thinking to get the fuel tank serviced and it might solve the problem.wht u experts think.......?