have been working on a TPI swap in a 1971 Nova for about 3 months now and have finally got it to running decent. well very good considering it wasn't running period about 2 months ago. Anyways. the problem. the car starts up fine, idles, and revs; but here is the problem. when the idle slows down it develops a pop in the exhaust something like the timing is off and when it starts this you can not give it a good rev without it back firing. I have checked all that i know to check at the moment. the timing is set at 6 degrees is there a certain rpm it has to be set cause i set it at idle speed. I change the plugs, wires, cap, rotor, modle, everything put the reluctor. i have checked the TPS just to make sure i was getting the right readings and it is good. I also readjusted the valves to just make sure that they were not to tight caust i had a similiar problem before ant the valves were to tight, but it did not help it. I have not checked the fuel pressure, but previose test had showed it to be good. i do know that i have a few leaking injectors, but the fuel pressure does not drop below 40 psi overnight so i figure it would not be that big of a deal. any help would be apriceated. PS: the system uses a stock 730 ecm with vats deleted.

Still sounds like the timing is retarded or a valve is hanging open.
A.)Did you disconnect the Set Timing wire or EST connection before setting the timing to 6*
B.)By chance has your harmonic balancer slipped or have the wrong markings for your timing cover.

Have you checked for fault codes. My (maf) tpi swap project also has this issue sets a code 33, back fires when you try to rev it I haven't been able to pin point the fault yet.

You may have an issue with the map sensor.

i did unplug the est when i set the timeing. i have noticed that when i set the timing and plug the est back up. the timing jumps way advacnced, like completely off the timing tab. i have thought that maybe the timing tab could be wrong or the balancer is not right, but i dont really have a way to figure it out. Dont know if it makes a difference but i am using a MR. Gasket timing tab for i believe an 8 inch balancer which is what is on my engine. i also tryed just turning the distributor to see if got any better but it would just get worse which ever way i turned it. Also i have not checked the codes yet. i thought i may have had a bad map and changed it with one i had laying around. it could have been bad too but it still did the same thing.

That's normal. When you plug the EST connector back in, the ECM takes over control of the timing and it's typically pretty well advanced in the calibration.

Two things (other than engine mechanical problems) cause intake backfiring. Either too lean on fuel, or timing is way off.

If you have a dial back timing light, you can see how much advance the ECM is adding when you plug the EST connector back in.

Have you done anything to the engine that would require more fuel on throttle transitions (i.e., like a pump shot on a carburetor)? Sudden changes in throttle without the proper amount of fuel for the engine will result in intake backfiring. On my Miniram, I had to literally add about 30% additional fuel on the accelerator enrichment to add that much fuel during quick throttle transitions. Otherwise I was getting intake backfiring too.

Also, check your plug wires. One time I had faulty plug wire and I would intake backfiring under load during acceleration.