Stromberg mixture needle question

[Yellelan]

I have a '69 S4 DHC with all history from new. It had new B2AR needles fitted sometime in the mid seventies, with another set fitted some years later, which are still fitted. It's also had a few new diaphrams fitted. The needles are about the right setting with the shoulder flush to the bottom of the air valve slider. It has many times been decoked throughout the years, far more than I would expect for the mileage (about 90k), and it's running very rich which I suspect is why it's coked-up periodically. Speaking to Zenith technical, they say needle B1Y was originally fitted to 69-71 Elans, and B2AR was fitted to 71-72 Elans.

So I suspect I may have the wrong needles, but what's puzzling is that my B2AR needles measures a larger diameter at the top and the bottom than the B1Y specification which means if I fitted B1Y's it would run even more rich for the same height setting.
Does anyone have experience/first hand knowledge or information on needles for the 69 Elan, along with expected frequency of requiring a decoke?

[SENC]

Where the jets are set relative to the "deck" is also important, as that defines the interaction between needle and jet (and thus the space for fuel flow). If those were replaced, it is possible they weren't set properly. The orifice in the jet also wears over time - if that is the case with yours, jet replacement may be more useful than needle replacement.

The following thread includes several Lotus Service Bulletins (last few posts) on the topic, and may be useful to you. Pure speculation, but these may have driven the swap to B2ARs.

I'm assuming yours is a non-federal, non-emmissions Lotus, since you have adjustable needles. Either that or it has been "upgraded". My recollection is that the B1Y is leaner at idle, richer at high throttle.