Person from the Black country which is in the west midlands, very close to birmingham. The Black country dialect is very unique. Yam yam derives from 'you am' e.g 'yam coming with me ay ya?'
A couple of drug smoking munch seeking yam yam's in a car:
"Yam gewin the wrung way! Where am yo gewin kid?"
"I ay at all!"
"Ye yam, yo day tek that last turnin ya saft get!"
"Oh arr, soz I day realise"
"yo'll atta gew back im starvin I cor wait no longer kid!"