Backfiring, why?

[MonstaS4]

I'm not familiar with the sniper but could it be ignition interference? It's hard to tell from the video, but I assume you'll have an RPM value on the display. Is it fluctuating when you hear the pops? You'd also pick this up with a timing light I would have thought.

Make sure the crank angle sensor (or whatever the sniper uses for timing reference) wiring is well clear of the ignition leads. Spark is generated at 20,000+ volts and will easily induce electrical noise into a low voltage (~5v) signal cable used for timing reference if not correctly routed. I would think the sniper signal cable should also be shielded and earthed.

[Just puttering]

I have no idea what the sniper requires....
You stated 60 psi first then 75 psi later, what pressure does the sniper require in the manual?
If you have higher pressure, you will push more fuel through the injector than the computer expects. The computer has To wait untill the hego reaches 600 deg. To get a good reading. If you only have one hego, than the other head is just a calculated guess for afr.
When it first starts, it will run on the cold fuel table untill the hego starts, then the computer will start adjusting. When you start, it runs ok, then warms up and starts popping. When you turn it off then restart it when hot does it start popping immediately? If it always starts ok then popps when hot, i would look at the computer switching from base fuel when it heats up. Also, does it pop out one side only?
I know this info does not directly point to a fix. Just some details I found when fighting my FI issues!!

[FF33rod]

I have no idea what the sniper requires....
You stated 60 psi first then 75 psi later, what pressure does the sniper require in the manual?
If you have higher pressure, you will push more fuel through the injector than the computer expects. The computer has To wait untill the hego reaches 600 deg. To get a good reading. If you only have one hego, than the other head is just a calculated guess for afr.
When it first starts, it will run on the cold fuel table untill the hego starts, then the computer will start adjusting. When you start, it runs ok, then warms up and starts popping. When you turn it off then restart it when hot does it start popping immediately? If it always starts ok then popps when hot, i would look at the computer switching from base fuel when it heats up. Also, does it pop out one side only?
I know this info does not directly point to a fix. Just some details I found when fighting my FI issues!!

I originally stated 60psi as this was one of the very first starts, there was a lot going on and I actually didn't look at the gauge while it was running. When I stopped the engine, I looked over at the gauge and it was 60 at the time. The Sniper has a built in regulator so it's fine as long as you're providing it with that much or more. I've now established that when everything is running, the input PSI is around 75 for the RPMs I've seen so far.

Yes, there is only one O2 sensor. That's all the Sniper provides for. I'm not sure if it's popping out of one side only or not.

Thanks for the explanation on the base fuel map and the switchover. That may be something to look at. I'm wondering though that if that is the problem then why don't the setup instructions deal with this. We are not talking about a wild exotic combination of parts here nor are we talking NOS or forced induction. As such, this problem should be fairly commonplace and dealt with in the standard instructions.

At this moment I'm thinking that, in order of priority, the to do list is: a) verify that all injectors are working, b) try changing the idle timing significantly, c) if none of that works then try adjusting the fuel maps.

Still waiting to hear from Holley tech support. In the mean time, I'll try to get a data log - something new to learn.