This part two is about the selection of needle and spring. When you setting up SU carb for Land rover series, you will have thousand of options to decide which needle and which spring to use…
Why Needle and spring..? because the needle and spring will dictate the attitude of the car, The SU carb is very simple vacuum type carburettor, which can be considered as their strength, needle and spring selection to match with our engine is a very important step and it is very confusing…
Fortunately there are brave hearts that already convert their old Zenith carb, to the similarly old SU carburetor with varying degree of success… some are using dyno / rolling road to measure the result, some will just use what ever they got on the carburettor and hope for the best. Alternatively you can buy software to select the needle but it beyond the scope of the article
From what I have read, and research… they are few needles candidate to be a good match for land rover series here they are:
BGB needle: this needle used by ACR for their (very expensive) conversion, the BGB needle paired with black spring.
Since they do it to make money for their conversion, I believe they do a lot of (proper) test to come up with this pair. So… it should be the best pair for our land rover series correct..? Yes and may be No. The ACR is offering this conversion together with the performance head, performance camshaft and performance exhaust, so I think that work best on using their set up.
Unfortunately no one (not that I am aware of) is actually using SU HS6 with Black spring and BGB needle on the stock engine. Theoretically BGB needle will be too rich for stock engine.
BDM Needle: this is much leaner needle than BGB, but one land rover forum friend using BDM and red spring set up wich he can describe as working good.
Another candidates are “BBW”, “BDB” few forum lister using it but I am not sure what spring they use but what I have read they have some success using them…
What about my Set up..? well, since I have no money to do a proper research.. so I have to make do with what ever comes with the Carburetor… which is “BAK” with a green spring.
Just to give you the idea the difference for each of the needle candidate, below is chart showing the diameter profile for each needle, before that this is how to read the needle profile:
if we give them number from the top (as position number 1) and to the bottom ( position number 16), you will see the diameter for position number 1 and 2 is for idling, from 3-6 is cruising and from 6 above is revving…
Now lets see the needle profile of all our candidates, see below chart, (if it is too small just click it on the picture). Vertical axis is the diameter of the needle, the higher the smaller the diameter ( means richer fuel), the horisontal axis is the position on the needle
As above you can see, BGB that the ACR used, is the richest of all… BBW comes second so I think BBW can be used for slightly economical than BGB, and number three is my current set up “BAK” needle.
you will also see that the first in position 1-4 the diameter is almost the same for all needles, means that idling and some cruising is almost the same for all needle. the real difference will be after position 4 where some is significantly richer than the others.
Spring: other than needle there are few selection of springs but not as many as needle… Softer spring will easily lift the piston fast, providing more air flow, the stiffer spring will resist of the upward movement providing more fuel enrichment, the idea is to have top maximum carburetor piston lift when the engine is in the top /peak power :
|Code||Force in Oz.||@ Height|
|Blue||2 1/2||2 5/8″|
|Red||4 1/2||2 5/8″|
|Red & Green||11 1/4||3 7/8″|
As above mentioned, from the land rover forum I got confirmation that “BDM” (relatively leaner) needle with Red spring (relatively weak spring) is working sweet. using this as benchmark, my set up : “BAK” (richer than “BDM”) will work OK using green spring (stiffer than red).
I know I am over simplify things but my set up seems OK now… only rolling road will show whether this set up is optimal or not… I will not give you any suggestion on what set up to use, but you can use the above set up as starting point… I am not the expert, I just taking what I learn and share it
That’s it for now, until then… take care my aluminum junkies friends… :)