I once had a problem a bit like this, and the fact that you say you have backfire when you swap the leads makes me wonder.... You may be 180 out.
These images are 140 distributor, but same principle. I'd check that on TDC the line on the body of the distributor match's up. The locator lugs on the bottom of the distributor is not central, and the offset can be changed by removing the spring, pushing the pin and rotating the lugs by 180 degrees.
In my case I had rebuilt the distributor and not paid enough attention on assembly. A long frustrating time later, I swapped this around and it fired instantly.
Also of you fit the gear in the block in the wrong way it will have the same effect. Given you have an unknown distributor, perhaps something is around the wrong way?
Hope it's as simple as 180 out firing on exhaust. If you have fuel and spark it must be something wacky, such simple engines.
Good luck with it