Thats good.I have'nt played around with a Weber carb but i built 4MV Quadrajets for GM motors,they like 6 to 8 p.s.i.with a new needle & seat assy.Anyway your problem sounded like a fuel issue off the top of my head.Most of the time when motor goes lean under heavy load you'll have "lean backfire" or pop threw the carb and sometime exhaust,if its power enrichment circuit.
LOL as for the fuel pressure I forget we are deal'n with engines 1/3 c.i.d. size I used to play with.