mk1-forum.net

Hi Everyone,

Can anyone tell me if there are any advantages between the swing needle
type set up to the fixed needle type.
I have both sets of twin HS4s and wonder which would be the best
set up to fit to my 1275s

many thanks

The swing needle was made to ease (and make cheaper) the manufacturing process, as the jet doesn't need to be centralised.

The bias / angle of the needle makes it rub against the edge of the jet, and they both wear out quickly.
-When they said the swing needle was better for fuel delivery / atomisation they were trying to justify the change. -It is not better.

When I have had swing needle carbs, I remove the needle and file off the "tit" that makes it tilt. The needle then sits straight and can self-centralise, without rubbing against the side of the jet.
I find this reduces / stops the jet and needle wear.

Other than that, they work the same.

/Andrew

And they're generally different... ie if you are after a particular needle profile it will be either a fixed or sprung needle - you won't find the same needle in either.

Of course, you can turn one into the other... but that will involve (knowledgable) hand fettling...

Cheers, Ian

1071 S wrote: - you won't find the same needle in either.

Apart from "AAA", which is available in both...

And Fixed 100 thou "OA7" and Swing 100 thou "BCD" are identical as well.
There may be others...

Certainly in the 100 thou range, the swingers are mostly leaner.

1071 S wrote: Of course, you can turn one into the other... but that will involve (knowledgable) hand fettling...
Cheers, Ian

To convert a swing to a fixed is easy, especially if the needle is going into a
fixed type piston....
1) Take the needle with its collar attached and drop it between the jaws of a vice.
The jaws should be loose, there is no need to even "nip" them up.
2) Take a pin-punch and tap the needle out of the collar.
3) File off/remove the knurl. If you don't do this you might find that once you've
pushed the needle into the piston assembly that you have a major problem
getting it back out again


I've never tried going the other way, but if you've got the collar and a spring,
along with the correct piston assembly, it should be possible. If your piston
assembly was a swing type, I guess it would be quite easy to make a split
collar to take a fixed needle.

(The other) Ian

Why are the swing needles longer ?

So theres no possibility of it coming out the jet and jamming.

ianh1968 wrote:[
To convert a swing to a fixed is easy, especially if the needle is going into a
fixed type piston....
1) Take the needle with its collar attached and drop it between the jaws of a vice.
The jaws should be loose, there is no need to even "nip" them up.
2) Take a pin-punch and tap the needle out of the collar.
3) File off/remove the knurl. If you don't do this you might find that once you've
pushed the needle into the piston assembly that you have a major problem
getting it back out again

That's pretty much what I've done to convert.

Generally, if you go to the very small amount of trouble to centre the jet, the fixed needle is a better set up, especially if you have custom needle(s) fitted.

One small plus I will give the Swing Needle from my experience is it will give a smoother idle and slightly better fuel economy if the roads (or tracks) being traveled are very rough. Hardly a good reason to run with them.

almondgreen wrote:Why are the swing needles longer ?

Possibly the main reason is that many of the needles can be used
in different sized carbs. For example, the HS8 can take 90 thou
needles or 100 thou needles (???and 125 thou needles???)

A 90 thou needle in a 1.1/4" carb will only need to be about 9 index
points long, whereas a 90 thou needle in a 2" carb will need to be
15-16 index points long.

Making them all the same length simplifies manufacture and enables
the highest number of possible uses.

Ian

Thanks for all your comments, I think I will go with the fixed needle set
Any suggestions on what needles/springs I should go for before I get it put on a rolling road
my engine is, +60 lightened and balanced, sw507 cam, polished and ported head 10.25-1 c/r

Cheers

ianh1968 wrote:

1071 S wrote: - you won't find the same needle in either.

Apart from "AAA", which is available in both...
To convert a swing to a fixed is easy, especially if the needle is going into a
fixed type piston....
1) Take the needle..........I've never tried going the other way, ............

(The other) Ian

My point was that the car doesn't care whether the needle is fixed or floating.... the only thing that matters is the difference between the jet (say, 90 thou) and the diameter of the needle at any particular point of lift.... ie the area of the hole the fuel will flow through..

Both fixed and floating needles belong in the same set (for each jet size) but there are no common needle dimension sets... Well maybe there are some - just like there are needles that are so similar that they may as well be the same.

Some years ago I digitised the SU needle tables and loaded these into an Access data base. Using SQL queries you could search for needles with similar ( a bit richer here, a bit leaner there) characteristics to an existing or known needle.

Some were so similar that I could only assume that, when a new setup was required, SU (or whoever) would start with an existing spec and then modify it to optimise it for the new requirement. Then, having found the optimum spec they just measured it and gave it a new label. In the days before cheap computer power this was easier and cheaper then searching through the thousands of existing options.

Both types fit, so the trick is to find the right spec; whether this proves to be fixed or floating is irrelevant.

Cheers, Ian

1071 S wrote: Both types fit, so the trick is to find the right spec;
whether this proves to be fixed or floating is irrelevant.
Cheers, Ian

Ian, I agree with everything you say, especially this last bit...

You sound like the kind of techno-nerd that would like my spreadsheet.
I had thought of going down the database route myself, but went for the
spreadsheet concept in the end as this could be more easily used by
others without the need for an SQL server. People are scared enough
of the word "spreadsheet", never mind "database-server", which causes
many of them to run and hide...


(The other) Ian

"........You sound like the kind of techno-nerd that would like my spreadsheet....'

Hi Ian, not really technonerd I mean.

I did it purely as a practical exercise to improve my car. Of course it didn't occur to me that, having worked out the 'perfect" needle, I wouldn't actually be able to buy one. Seems logical now, knowing how many individual specs there are, that no one would actually keep them all in stock.

MS Access is not that common but SQL is no big deal. The power of it allows you to search the entire SU spec set for one specific needle. I linked Excel to Access to provide a display tool to translate dyno mixture graphs into needle options... Which, of course, turned out to be a complete waste of time.

Not sure how you'd do that with a spreadsheet.... noting of course that a spreadsheet is just a data base laid out in a particular way. The hardest part was actually digitising the SU handbook ... and prooof reading it

It all seems like a good excuse for a beer or 3...

Cheers, Ian