306 backfiring.....?

My friend put a 306 into a 1993 mustang and it only has about 5 miles on it because it seems to be backfiring through intake manifold. He put the motor in like 4 years ago an switched the valve springs. he needs cash an wants to sell it to me, so before I jump into this and might be spending more money in parts than the car is worth, does anyone have an idea what this might be. It is running an starts right up an it seems as though all the power is there, someone said it might be a bent valve but wouldn't that affect the performance of it? It seems as though the 4 valve is staying open when it shouldn't.

Anybody who has any idea at all please let me know, I can get this car at a good price since it isn't running correctly and want to buy it if I can fix it cheap enough

Also, does anyone know anybody in the Easton/ Allentown are of Pa that specializes in mustangs or in fact Ford motors that could help me?