360 Backfires Under Load

Okay. Gonna verify firing order first.

On the timing- I have a Summit balancer, I have the stock timing tab- the tab seemed go in only one spot. When I tdc'd using a plastic stop(possible error zone) the mark I found was around 3/4" towards the oil pan from the hole on my timing tab. I have an old skool timing light that connects to the plug wire(not inductive) It seems to work ok.

When I started to get it going I had the distributor 180 out for a round of crankings maybe six times over two days. I had prelubed it with a shaft and hi volume pump. Put the trans on and found the oil leak from the un-modified pan/hi vol pump clearance issue, changed to a stock pump, resealed and relubed with Rotella. Cranked it three times- saw a spark called it good and bought a new Street Avenger to replace the 1850. Next cranked it three times found the 180 out dizzy. Got it started, set the timing by ear and vacuum gauge took it the gas station. It idled well and cruised ok on minor throttle- backfire on pedal pressure- did not floor it.

Wanted to believe the single exhaust was the culprit I took it to the muffler shop. Same symptoms. Went over the timng found it to be around 15 before on the stock pointer and around five in my pencil mark. I dialed it down to 3 before and it seemed to drive better.

I wanted to start richening the mixture by backing out the idle mixture screws.