All going well, then "boom"

[JSad]

I have a 91 ZR-1 with 53k miles on it that I bought a couple years ago. It was not well loved in the past. I have slowly been fixing things as needed. Mechanically so far I have repaired all the oil leaks, replaced the oil pressure switch, had the transmission rebuilt (bad syncros) including light weight flywheel and short shifter, added Heibeck chip, deleted the resonator, added performance mufflers, changed all fluids, fixed some electrical issues, have done a little body work and general detailing. Suspension was next project in line.

So, now the issue. the car has been running great. I parked it 3 weeks ago while I was refurbishing the top and doing some weather stripping. Today is a beautiful sunny Oklahoma day so I was going to drive it to work. When I started it, it "backfired", the noise coming from the engine bay, not out the exhaust it seemed. Then it died immediately. It would start but ran very rough and would not idle. Didn't see any smoke or obvious issues. I had to get to work so I didn't spend much time on it.

The simple solution is to send it to Heibeck and have a 510 package done but right now I need a slightly more affordable option, any suggestions on where I start looking for the problem?