New motor backfiring through carb

OP you need to learn to get on the firing stroke. There are only two ways to do so. If either valve cover is off, rotate the engine to TDC and examine the no1 or the no6 valves. Whichever pair is closed, that cylinder is ready to fire

The more popular method is pull out the no1 plug, crank the engine with your finger in the hole. As soon as you feel compression, look at the timing marks, and bring them up to where you want the timing, NOT TDC. Bring the marks up to 15BTC or wherever you want the timing.

Then look at the rotor. If it is not point at "just coming to" no1, adjust the dist/ plug wires as necessary. You can set timing on the starter. There is no reason you cannot static time these and start it right up.