walaikum asalam ,
cultus has a auto choke system , what is auto choke , auto choke system increase the rpm on start up so your engine gets stable , once your engine gets warm up your rpm goes normal , your problem lies some where there , check your auto choke it might not working properly .
second assumption your butterfly is faulty that rpm fault comes aswell
when your butterfly has a play in it , check your butterfly aswell .
hope that helps !!