Backfiring; wrong timing?

When I was searching for what could cause the carb problems I saw that the valley gasket was deformed due to the backfiring of the engine so I took the carbs and inlet manifold off to see what the damage was;

 the valley gasket was just bend and could be reused for the moment. Having the backfire it made me thinking about the timing; did I made an error at the timing gears?

I dismantled the front cover and checked the timing but all was fine.

Three different books and three different drawings don't make it easier.

But in fact they are all the same and it's as good as impossible to get them wrong.

 So what about the cam shaft; did I had the right camshaft? I checked my bills and numbers and it seemed I had a camshaft for a 3.9 EFI but after doing some research on the internet I found out that this camshaft should even be better than the original 3.5 carb camshaft giving the engine more breathing.