I had virtually the same problem, is there any rust in the tank?

I had a dodgy fuel tap so check that its actually off when its turned to off. Main thing I had to change was the needle valves although they may look okay the most tiny amount of wear can render them useless. My tank was full of rust which had made its way into the carbs and furred up the needle valves wearing them down....replacements and a repaired tap and I was no longer using my expansion chambers as fuel tanks lol

Just some pointers from my experience.