i had a problem like this cheak if the bottom pully and the cam pully line up with the 0 on the plastic cam cover when i refitted my head i lined every thing up and the cam belt wouldnt fit on the cam pully so i moved it down to fit which made it slack and then when i tightened the tensioner it pulled the bottom pully round and advanced the timeing and it ran like crap took me ages to figure it out