Mshimy

Hi everyone, I have a 72’ 240z that I bought 4 months ago, and have put on the road since early February and recently I’ve encountered a problem. Problem: Backfiring out of carbs under load in the med/high rpm range. Issue is absent with the choke on. Points, condenser, cap and leads replaced. Issue has got progressively worse after ~500 miles of driving since buying the car. N58 needles used with a slight ridge formed due to wear. Once the problem became severe the oil was found to be at the minimum level. Suspect valve clearances or needles to be potential cause, but it's probably a carburettor related issue. The car is a US import being driven in the UK I would appreciate any help possible, I'm desperate to get it back on the road Thanks a lot! Moe