AFAIK the mk1 Fiesta 1.1 didn't have a needle jet or variable venturi - it had fixed jets which varied by engine capacity and application with the only adjustment being the idle mixture - a carb from a 1.3 Sierra is likely to be a bigger bore and give worse fuel consumption on the smaller engine.

Hello Do someone know how to adjust the needle jet on the carburetor. I’ve got a carb that is the same as the original VV, but it may have sat on a Sierra 1.3L. I could use an estimation of how many turns from the inside, so I know what to start from when adjusting. In addition, maybe I should set the mixture screw the same as my original old VV carb?

Apart from what RT said if your going to try it you could turn screw from where it is and adjust from that point turning clockwise will give weaker mixture and opposite to make richer, it is possible it will run rough as they dont always work on the fiesta engine

it was in my experience always better to fit new, though probably not worth it, but finding a carb to run it smoothly will be a bit of a challenge, but if it doesn`t anyway not much to lose.

if the carb is any good, to check, turn screw right in to close which usually the engine shuts down or as it tries to, open then adjust till engine runs as smooth as it can