Boxerworks.com

I haven't had one of those later Bing carbs in bits. I know they have the more conventional needle with a circlip that locates in grooves. The screw plug presumably bears on th circlip and holds the needle in place. Seems to me it should be possible to shim those needles if you desire a needle position inbetween two settings. That's a good thing given the 4 standard settings are quite widely spaced apart in terms of their effect on mixture. Or are they. Perhaps with this return to a more conventional needle location Bing had the good sense to make the grooves less than 1.8mm apart. If not I wonder why they made the change.

I liked the old twin Stromberg CV carbs that I had on a 72 Humber sceptre. They had infinitely adjustable needles just by sticking a long Allen key down through the dashpot. Bing could have done the same thing with those screw plugs.

It is not the distance between the grooves alone which decides how much difference in mixture there is between them. It is the combination of the distance between the grooves and the taper of the needle. So, a more gentle taper has smaller steps than the same distance grooves on a more severe taper. I do not know if they use the same distances on all grooves or change the grooves with each needle taper. I'm sure it varies among greatly different sized carbs, but I don't know if it varies within a single carb series.

That's a good point about needle taper. It's hard to tell from Bing part numbers like 46-241 or 46-242 what the difference in taper is if any. Maybe they are just part numbers and the code does not directly represent some physical dimension of the needle. I've seen completely different designations for Bing needles used in other applications like 11H2 and 6S2 where the numbers do mean something. In this type of designation I know from looking at charts that the 11 and 6 part of the code indicates needle taper with 11 being steeper than 6.

Whatever the differences are between airhead Bing needles it seems to me that changing one position has a big impact on the mixture and certainly bigger than one step change in the needle jet size. For that reason I would have liked finer adjustments of the needle.