For the backfire check the vacuum hoses make sure there all connected and have no cracks. as far as the oil issue check the PCV valve make sure it's not plugged up and has suction also make sure the PCV hose is not cracked and fits good and who did the clutch was everything put back in place? Check on
www.mazdatruckin.com for the vacuum diagram much better then the one on the hood. Good luck!