what makes a 91 poniitac grand am 2.5 backfire from intake?

Question

i have change the fuel pump air mixture rebuilt the motor

Answer 1

Camshact timing off? - I have seen many engines with camshaft a tooth or two off, and engine still run (but poorly) depending on which way it is off! Sometimes if you put new timing chain or gears on,-- the marks on "new parts" are ambiguious, and might give you wrong impression on which mark is the right one! And of course it is not impossible to get timing belt on one tooth off (if your engine has one). Are you sure the ignition timing (on distributor) is right-- (if engine has one?)

Have wires in right order? If wires in wrong order (2 or more) it can be firing while intake vavlves open.

And then there is the fuel mixture, - if you aren't getting enough gas into engine it will backfire through cartb, - (and sometimes in the exhaust also!) Are you sure you have all the vaccuum hoses hooked up? If sucking vaccuum in through a big enough hole it will possible backfire (becasue fuel to air mixtur too lean), - but after a certian engine speed will be so neglible that you might not notice it!

One other possibility could be that the valves are not seating completely (if valves were ground). Your vlave lifters only have so much clearance (in which to adjust), if valves were ground, - it is possible that the valve stems weren't "dressed down" to compensate the extra amout valve sticks up when supposedly closes, - so that there is some space between valve and seat with cam on "low side of the lobe" - (lifters closed as far as they can). But the "machine shop man" that knows what he is doing will check these things out when reworking the head, - so it shouldn't happen , -- but I have seen several engines with this problem!!

Answer 2

Possible misinstalled timing chain or crossed plug wires

Answer 3

too much gas being sprayed from the tbi or carburator...or timing off. or firing order not right