I have the original dual HD8's and hisser (ASC) that came with the previous 1970 2.8 litre engine in my XJ6. The new engine is a 4.2 litre from '73 and comes with HS8 carbs and the probably wonky AED. I would really like to try the already cleaned up HD8's on the engine when I install it, but the needles in those are UVX and not UM as I am supposed to use with a 4.2 engine. It would work anyway, right? When I look at the needle curves at http://www.mintylamb.co.uk/suneedle/, they look fairly close, or am I missing something?

 

I talked to the local carb specialist, Förgasarteknik, and they confirmed that the difference between needles UM and UVX is minimal and would work fine. Just posting this in case others have the same question.