Hi ..i have a 1994 chev silverado..the truck is running good other than the fact that it back fires once you run it past 70 or 80 km and you slow down ,you hear a loud back fire.i know for sure that it is rich in gas,like you can smell the gas once you start the truck it is not burning properly!so ,what causes the truck to burn fuel so poorly?also,it does not shift very smooth,and i know the tranny is good,it kinda kicks once it shifts to 2nd!please help?thanks.