Doesn’t start after top end rebuild

[tom1104]

I’ve recently rebuilt my top end, bored 1mm oversized, ultrasonic cleaned carbs, all the correct jets and needle, no valve leaks, set gaps and timing. I have bright spark, fresh fuel, new battery. But for some reason the bike won’t start, when I crank it over the left carb backfires fuel every 5 or so turn overs. The bike was running previously before the rebuild but with low compression. I now have 150 on both cylinders. Also the ignition coils are new along with the condenser. I’ve been trying to get it to fire up for a few days now with no luck! Any ideas would be greatly appreciated
Thanks