To date I have not been happy with the performance of the HS4's. Finally I have something really positive to report.

I bought myself another pair of ABD Floating needles and have reprofiled them to the same as No. 6 Fixed needles. I reckon I have the Floating needles within 0.5 thou of the No.6's at the first 9 stations. As soon as I installed them and started the car, I could tell it was going to be better.

With the ABD's I could only get a decent idle mixture when the jets were 18 flats down from level with the bridge. The colortune plug confirmed the same. With the reprofiled needles, the mixture screws are now set at 13 flats down.

I have been chaffing at the bit to go for a drive and between showers this morning I took the "S" for a run. I reckon it goes just about as well as with the old Hitachi's now. It is pulling really strong from 2000 to 5000rpm. With the normal ABD needles, it seemed to not run freely between 4000 and 5000rpm. The old Hitachi's used to rev really quickly from 3000 to 5000 rpm. I reckon I am close now. With a bit more tuning I reckon I can nail it with these HS4's.

The only thing it is missing is the induction roar of the Hitachi's, but the main thing is that I am finally happy with the SU's.

Regards Jon

Can you not buy No6 needles any more??

Cheers, Ian

Ian,
I am pretty sure that No.6 needles are still available. I was just too lazy to buy and fit the jet bearing kits for the fixed needles. I did consider trying to fit the No.6 fixed needle into the Floating Mount, but the top end of the fixed needle is about 2 thou smaller than the Floating needle and did not want to butcher it to make it work.

Sometime in the near future, I will probably buy a set of fixed jet bearing kits, just so I can compare the performance of the Floating needles and fixed type. I already have a set of No.6 and CP4 fixed needles.

Gives me something else to tinker with and any excuse to go for a run in the brick.

Regards Jon

Hi Jon, interesting discussion.... I think we need some adult supervision on this. Maybe someone will chime in??

Its been many years since my twin HS4s were setup.

My first question would be; “are all HS4s created equal”? Mine are ex Special Tuning dating from the 70s. I think they came with fixed needles.

However, many years later after a serious engine rebuild, (aging 1071 to overdone 1275) they needed a retune. After quite a few runs on the dyno we came up with a satisfactory setup. Looking in my little bag I find 5 sets of fixed and 3 (?) biased needles that didn’t make the cut.

I do not remember changing jets .. or buying any “jet bearing kits”.

So back to the question. Are jet bearing kits something that later HS4s need if you fixed neeedles? ...or did early HS4s have these as standard and so will accept fixed or biased needles? All the references Mr Google comes up with explain how to fit fixed needles in place of biased ones but nothing on the reverse? Midel and Burlens web sites will sell you “fixed” needles adaptors but not ones intended for biased needles. Do they assume everyone has these?

My memory tells me we just swapped between fixed and biased needles (using the appropriate adaptor) until reaching what seemed the best solution (maybe I just got tired of buying more needles). I’m sure these was no changing of jets.... And years ago I lost my little black book so have no idea what sort of needles are fitted now. (There doesn’t seem to be any sense in pulling things apart just to check.)

I was surprised you felt it less trouble to modify some existing needles than to just pop out the needle and its adaptor and stick in an alternative.

Cheers, Ian

I hope this is a helpful contribution to the conversation.
The piston for a swing needle has a larger hole in it to accept the needle holder and spring.


Fixed setup on RH; the other has an adaptor sleeve fitted to convert spring type back to fixed.
So it would not be easy to swap from one type to the other without an adaptor.


This reference from SU's book might help explain the need for a different jet bearing assembly due to whether the jet needs to be centred: especially last paragraph of 2.2.1

Hi John
Spoke to Adam (midel) today he is about to make a batch of SU needles and he will make some Number 6 needles
As floaters, i will take some of them off him.
Graham Russell

_________________
"It's better to be not informed than ill-informed"