What cause backfire when i deceleration or downshift? Its not my timing i've check already. Can it be my vacuum advance??? I got a 1962 chevy impala 283 v8 hei distrubutor, wt glass pack exhaust systems. which basicly mean straight pipe.

 

help to know what ya got

 

make model year and type of motor would be very helpful

 

Could be a clogged catalytic converter.

 

Insufficient back pressure,(usually an exhaust leak) will cause a backfire. If the exhaust is changed radically from stock and there's an absence of normal back-pressure, you could also get a backfire. Assuming you've got a high-mileage engine, worn valve-train components, such as a worn-out cam or very warped or leaky valves could cause a backfire. A bad exhaust, if unfixed, could ruin your cam and valve train by causing excessive backfiring. Is your crankshaft damper aligned properly?(They slip sometimes, leaving the timing mark out of sync) Check your distributor, be sure the rotor is aces, if it's loose or defective,-backfire! Are your engine specs true for the engine you're actually tuning?Be sure, because if you're setting the timing wrong, you're also tuning other specs wrong.

 

did ya ever think you practically have strait pipes your back pressure is down try gettimg new exhaust even when you go to a parts store and has the little box with the diff sounding exhaust reving you can clearly hear backfiring after it is reved with a glass pack its more or less a charcteristic of the muffler