Normally needing to pump the throttle means the choke isn't coming on and you need to enrich the mixture. But the SU doesn't have an accelerator pump, so I don't think pumping the throttle will enrich anything.
Perhaps it's just letting more air in (when the throttle is down) to correct for a mixture that's too rich? (In that case, it would be the choke stuck on rather than off.) When it does start, do you get a lot of black tailpipe smoke? Another test for this would be to see if it starts with the accelerator held down (rather than pumped).
Jeff.