My Series had a replacement carb. Not sure if it’s origins but was probably a cheap one. It ran fine for ages. Then I had major running issues with it. Not sure what the problem was exactly but it was carb related. I stripped it down about a dozen times. Sometimes it would run ok then wouldn’t. Never did find the exact cause but something wasn’t right somewhere.
I then fitted an SU (HS6 maybe, would need to check) by way of an adapter. I never got the fuelling quite bang on (a little rich) but it ran very well and very reliably.