I had the pumped 1.20.......MKII [IIRC] and it ran perfectly, once you got it running. The pump occasionally needed to have fuel drawn through it with a syringe to "wake it up". It could be that it would get gummed up. It never did fail to run and I figure that OS decided it was necessary to pump it so it would sell to the pattern crowd and to the sport guys who wanted to see 10 pounds worth of plane have unlimited vertical. These engines would purr right on through the worst aerobatics you could throw at them. They sold like hotcakes around here when they first came out....and $400 was a lot of money back then.
IIRC, some guys tried to run it with medium fuel tubing instead of large and had problems.