I just helped my friend replace his motor can't figure out what's wrong it is backfiring through the exhaust checked both o2 sensors and they are working probably pulling spark plugs tomorrow it's throwing no codes and have no clue what to do please help

Edit: it also idles perfectly

 

double/tripple check and make sure your spark plug wires go to the right place on both ends. very easy to get one off that allows the engine to idle

 

+1 with Craby, You should really hope that is it.

Back firing is timing, lean condition or a valve sticking. If the new O2s are showing good mixture that reduces the chances it is lean. Timing on a direct ignition system would be either a failing sensor or a stretched timing chain. (it could be some odd wiring issue) RPMs would make the timing chain issue worse. Last if the motor has been sitting you could be sticking a valve as it gets hot. This is about the only case I recommend an oil additive. Berrymans or Marvel.

Weak spark could also cause the issue but that should set a miss fire code unless it did not run long enough to set? As RPMs go up Cylinder pressure goes up and more voltage is required. A weak coil or a leaking wire or even a crack in the plug can cause a cylinder to dump raw gas in the exhaust as the spark fails.