schoolboy SU needle question

[dhenry]

Hello,

An easy one for somebody I'm sure. According to Vizard I should be looking at a No7 needle for my proposed engine mods in my single HS4. My current needle AAU has 16 stations where as the 7 only has 13. Does this mean the last station drops to zero or does the carb even use stations 14,15 and 16? A simple explanation would be grand. Thanks,

[andy1071]

The needles are different lengths, so they can't have the same number of stations

[dhenry]

Yes, I know they are different lengths. I was wondering why they were different lengths and why might one need to be longer or shorter. Does the shorter needle go to zero after station 13 because the engine needs to run that rich or is it that the needle wont even reach station 13?

[mk1]

the needle wont even reach station 13?

Correct

[andy1071]

Not all needles are originally designed for one carb size. Some for 1 1/4" some 1 1/2" etc.
Plus, it doesn't necessarily follow that the piston will lift to it's full height -though we want this on our engines to give maximum flow through the carb...

[surfblue63]

Isn't a No. 7 needle for a fixed needle (H type carb) and an AAU (HS type carb) a swing needle, hence the differant lengths.

There's a needle comparison at the following link

http://www.mintylamb.co.uk/suneedle/

[dhenry]

Isn't a No. 7 needle for a fixed needle (H type carb) and an AAU (HS type carb) a swing needle, hence the differant lengths.

There's a needle comparison at the following link

http://www.mintylamb.co.uk/suneedle/

Oh yes. I think I over looked that. So I need an equivalent "swinging needle" for my HS.

Further to my initial question. I am going to be running a 998cc with 8.7:1 CR. Kent 276 cam, 12g206 head (standard valves and polished ports) and a single HS4. I currently run a AAU needle. Some suggestions as to what needle to start off with before getting it tuned? Maybe an AAA or AAB? Cheers

[mk1]

Swing needles can be successfully run in non swing carbs, all you have to do is push them out of the little brass holder. Its very easy.

[andy1071]

And of course you can do the reverse and put 'normal' needles in the brass holder.


Whenever I had the swing needle set-up, I always filed off the pip that tilted the needle.
-It is said that biasing the needle in the jet improves the atomisation, and that's why SU did it.

But I believe the real reason was to make factory assembly quicker and cheaper.

The disadvantage with the swing needle is that you're guaranteed to wear the jet. By removing the pip, the wear risk is reduced (I would say cured, but at least every time the piston drops, the needle is centralised).

[surfblue63]

Swing needles can be successfully run in non swing carbs, all you have to do is push them out of the little brass holder. Its very easy.

Every day is a school day.