The backfires and popping from the exhaust were down to timing, there was an electronic ignition in it that gave up so I went back to points, did static timing and that seems to have cured those woes. The backfire through the carb was down to me rushing and pure stupidity! Also, theres a new exhaust with gaskets after going onto it but now that its had a few heat cycles I'll tighten everything up again.Backfiring is often connected to air leaks in the exhaust system,
so it may be worth a good look round that.
Thanks for the input!