11 replies to this topic

[madaboutcherry]

i have a 998 fitted with an hs4 that has a fixed needle. in the new year i,m going to to treat myself to a stage 1 kit so i can overtake milkfloats etc. i assume that the kits come with a sprung needle. can i just swap them over

[jaydee]

Are you sure it is a fixed needle, and it is not just stuck? Usually Hs4 are with swinging needles.
Its a direct swap anyway, you just need the spring to fit a swingin needle.

[madaboutcherry]

it,s definately a fixed needle, it,s a 1968 mini.

[dklawson]

That sounds right. The early HS series were fixed needle while the later HS and HIF used swinging needle.

There are some computer programs that will allow you to compare needles. If you post here with what needle you get with your kit, someone here should be able to look up the comparable (or close) fixed needle.

[jaydee]

There you go, this is a very accurate comparing tool:
http://www.mintylamb.co.uk/suneedle/

[dklawson]

The "mintylamb" site above is great for giving a graphical comparison of needle profiles.

The link below is to a web page maintained by Paul Tegler.
http://www.teglerize...ledb/index.html

If you know a needle "name" key it in and it will tell you the diameter of the needle in the 1/8" steps listed in the SU manuals.
If alternatively you key in the diameter of a needle at perhaps 2 or three points, the program will return what needles match the two or three points you enter.
(Please note that Paul's web site includes both 0.100" and 0.090" needle sizes. You need to select one or the other before you start keying in needles/profiles).

Once you've identified a handful of needles you are interested in, use the mintylamb link above to compare their profiles. All good stuff.

[Asphalt]

Why not ask the seller of the kit if they can supplie a fixed needle instead?

And I'm about sure I've read somewhere that you can simply remove the collar of a swinging type needle and use it as a fixed one. Or you could just swap the carb's piston for a later one

[dklawson]

I believe you can snip the collar off a spring loaded needle but it seems a shame to do so since the spring loaded needles cost more to start with. Buying one and cutting the collar off is kind of like throwing away money.

Yes, you should be able to get the seller to supply the fixed needle for your carb and that would be the easiest option and most cost effective option.

The vacuum chamber and pistons on SUs are matched pairs. If you've only got a single carb fitted, swapping pistons is probably OK (to a point) but if you have dual carbs you need to be more selective so the pistons will rise and fall at the same rate. Visit YouTube and you'll find John Twist of University Motors has a video showing how to match pistons and vacuum chambers.

Of more significance when mixing carb components... the late HS4 carbs I've seen were designed to use different springs, had the ball bearing guide for the piston, and used jets that could not be moved/centered. It's probably OK to fit a spring loaded carb's piston and vacuum chamber to an early carb with a jet that can be centered. However, it probably wouldn't work as well to put a fixed needle piston and vacuum chamber on a later carb where the jet cannot be centered. The later combination can result in components that bind rather than work properly together.

[Asphalt]

However, it probably wouldn't work as well to put a fixed needle piston and vacuum chamber on a later carb where the jet cannot be centered. The later combination can result in components that bind rather than work properly together.

Just turn down the top of the jet bearing that pokes into the carb body by 1mm.
And hey presto, you have an adjustable jet for use with fixed needles.

Adaptors to fit a fixed needle into a late piston can be bought. They cost only a few quid.
This avoids messing with the pistons, as you pointed out.

I'm going that route. I can't see the point in swing type needles?
Except it keeps sales for jets up. And saves the little time for centering the jet!?

Cheers,
Jan

[jaydee]

Swinging biased needle will make throttle response a bit smoother, it wont do any wonders..

[dklawson]

Just turn down the top of the jet bearing that pokes into the carb body by 1mm.
And hey presto, you have an adjustable jet for use with fixed needles.

That's very clever indeed. I will keep that in mind!

[Ethel]

You can tap the needle out of the collar to make a fixed needle.