Modification from biased needle to fixed needle
Turbo Mini Forums

Hi Guys,

does anyone know of a modification to replaced the swing biased needle to a fixed setup?

i have tried using the search but i just ger zero results unless i search for one word like needle and then i get nothing on changing to a fixed style.

thanks guys

179hp on 12psi 1293cc of pure A series
14.0et @101 mph

Never done it myself. Can i ask why you'd want to?

You can convert a biased needle to a non biased needle so that it will fit in a non biased needle carb.

I've Not heard of converting a biased needle carb to a non biased needle, there are no benefits to do so?............

Exept if you like the hard adjustment work? How would you centralise the jet in the bridge then? Stromberg carbs have a fixed needle (some of them) but the jet can be adjusted to centre. From underneath.

you can do anything if you set your mind to it...
i rather blow it up winning than keep it together losing.

finish date set for project 1... march 2018

the older HS2 and HS4 carbs have a fixed needle.

The needles themselves are the same for the same jet size, but the biased needle has a collar that is pressed on. You just need to remove the collar to make it into a fixed needle. The jet is only adjusted up and down for idle mixture, it's central position is fixed. The needle itself is fixed in the piston which is a tollerence fit in the dashpot, which is a tollerence fit on the carb body, therefore no need to adjust the jet or needle alignment

The reason for having the biased needle is that it is supposed to give a better metering of fuel

Colin, I always thought that the idea of a swing needle was so that it did not need to be accurately centred on the jet ? (They will meter the same as any other surely, until they wear a bit off the side of the jet, as they do)

Earlier SU,s did have provision to centre the jet under a fixed needle and a right bloody fiddle it was too !

Yeh your right Dave. I've just looked at the assembly procedure of a HS and it does explain centering the jet, but it looks fairly simple to do so. Its been 20 years since i had a play with a HS carb though and 10 years since the last time I had a HIF appart lol.

The book does say though that the point of biasing the needle towards the engine was for improved fuel metering. I think this may have something to do with the needle shrouding the jet area directly behind it.

Edited by Sprocket on 4th Dec, 2013.

The reason a biased needle meters fuel more accurately is because the shape of the annular space between needle and jet will always be exactly the same shape, just varying in size as the piston and needle goes up/down. The shape is effecticely a "C" thick in the middle and thinning to nothing at the ends, just increasing uniformly in thickness at the needle rises.

A "centred" needle will be at best, if done very carefully, in the centre when it's fully in the jet. That gives an "O" of uniform thickness.
But no needle is truely straight, just tightening the grubscrew can put a slight lean on them. Thus when the needle rises the "O" doesn't just get thicker, it will most likely become lob-sided.
An "O" with the same ID and OD (ie, risen the same distance) will have a different coefficient of discharge if it is lob-sided compared to one that is perfectly symettrical.
Thus outwardly identical needles will flow different amounts of fuel depending on how straight they are and/or how well centred they are.
That's why Leyland /Burlen moved to biased needles, to ensure the discharge co-efficient was the same in every case (until they wear as David points out, but fuel is quite a good lubricant).

So reverting to a fixed needle/jet is a backward move.

Schrödinger's cat - so which one am I ???

Thank you Rod for a clear and concise explanation as I did not understand how the swing needle could be better - I do now.
In my experience both needle and jet do wear and its not a huge expense to replace them on any used carb one may pick up, just to eliminate another variable.

I have just had another thought.

If I have a modified needle ( I have, thanks to Robert) then it would seem to be important to replace it in exactly the same position in the piston if ever removed.My needle has a flat on it, which is the usual way to modify one, and, clearly, if the needle was rotated and then replaced, the annular gap would change in shape and, presumably,flow.

Does anyone think that this is significant ?

It might account for differences in tuning required after checking and replacing the same needle ?

David,

Without getting too technical, any orifice of identical cross sectional area, and the same pressure differential, will have a different CD (co-efficient of discharge) depending on its shape.
Round is best (IIRC), shapes with corners, worst.

So the theory with the SU is a repeatable thickness "C" is better than a random eccentric thickness "O".

If your needle is profiled with a flat along its length, I would just suggest making sure the flat is 180 degrees away from where the round bit is sprung against the jet.

That would be the most repeatable shaped orifice IMO.

If the needle is profiled around its whole diameter then its orientation will make little difference if it's in a biased jet scenario.

Schrödinger's cat - so which one am I ???

Burlen sell a kit to do the conversion.
ABF193 is the part number.

I bought a couple for use on the GT as it opened up the needle selection enormously being able to swap between the two types.

Metric is for people who can't do fractions.

Thank you Rod - that confirms my own thinking (without the technical understanding !)