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Turning a tapered carburator needle

Turning a mixture throttle slide needle for an SU carburator

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JasonB17/10/2018 07:54:34
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14001 forum posts
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For a straight taper use a ctr at the head end, not held rigidly like in my photo. I purposely wanted a slight curve to give that conrod a "fish belly" also works well for classic columns.

You are less likely to get concave than convex, if there is any tendency for the work to deflect away from the tool it will end up convex. best to finish with some fine wet and dry backed up by some flat metal plate to get a fine finish that is nice and even along the length.

Michael Gilligan17/10/2018 08:02:44
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12026 forum posts
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Posted by JasonB on 17/10/2018 07:54:34:

...

You are less likely to get concave than convex, if there is any tendency for the work to deflect away from the tool it will end up convex.

.

Good point, Jason

MichaelG.

Martin King 217/10/2018 08:58:06
540 forum posts
171 photos

Hi, suggest that you contact https://burlen.co.uk/. they are the agents for SU and offer all sorts of specialist spares

Cheers, Martin

Jon Lawes17/10/2018 09:12:50
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223 forum posts

Burlen are very helpful, used to deal with them a bit when I worked with classic cars.

The local classic jaguar specialist uses sandpaper held in the chuck of an old pillar drill to gently remove some of the metal at the area which needs richening when rolling roading... I think that technique probably requires years of experience!

The needles usually have a three letter code which describes the taper, I think each letter describes the diameter at fully piston deflection, half deflection and idle, but only for the specific model of needle in use. Due to the different ways they are mounted (fully floating or spring biased for example) there are big differences in the needle spec.

Burlen will be a good bet here, they are not cheap but have never failed to help me out, especially with some of the more obscure older Astons.

Clive Foster17/10/2018 11:33:08
1505 forum posts
34 photos

If you do end up with slight concave or convex shape after turning a long V groove lap slid up and down with the needle rotating fairly slowly should straighten it out.

Clive.

Michael Gilligan17/10/2018 12:08:38
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Posted by Clive Foster on 17/10/2018 11:33:08:

If you do end up with slight concave or convex shape after turning a long V groove lap slid up and down with the needle rotating fairly slowly should straighten it out.

Clive.

.

A genuine question, if I may, Clive

(no, not just for the sake of debate)

What [aside from the obvious doubling of lap length] is the benefit of using a V groove lap ?

The reason I ask is that my approach would have been to use the diamond EZE-LAP

'Hone & Stone' https://www.fine-tools.com/ezelap-diasharpener.html

MichaelG.

Edited By Michael Gilligan on 17/10/2018 12:11:48

Clive Foster17/10/2018 12:38:14
1505 forum posts
34 photos

Michael

Was told that V groove lap is more stable and easier to use. So long as you keep even, light, pressure on a rotating workpiece it its pretty much impossible for the result to be other than straight. Especially if you can arrange things so the lap is longer than the taper so taper never runs out of the lap. This pretty much eliminates end tilt errors which have to be the major bane of folk who are not experienced lappers when trying to get things right right up to the edge.

Always assuming the V groove is straight of course.

I have a 4 piece set of the Eze-Laps with which I'm quite impressed. But I'd not back myself to bring a slightly concave or convex taper turned using Jasons method back to dead straight with them. Most likely just move the error around. I do OK with the little lapping I normally do but have never invested the hours of practice needed to get really good at it.

Clive.

Edited By Clive Foster on 17/10/2018 13:28:08

Michael Gilligan17/10/2018 12:58:49
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12026 forum posts
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Thanks, Clive

MichaelG.

Ian Lee17/10/2018 23:06:10
42 forum posts

Jason , am going by the measurement I have been given by John, the guy that wants them. Thaiguzzi, I think John may have spoken to Amal as I told him they still make carbs, if he has not spoken to them I will tell him. I must admit that I want to have a go at making them, some of the things I have already made for John are bonnet mounted pennant holders for two of the Queens cars in her collection, recently I made a worm and wheel for a windscreen wiper gearbox on a 1938 Vanden Plas bodied Bentley and reconditioned a 1927 Rolls crankshaft vibration damper which had been stood for nearley 60 years. I do like a bit of a challenge, I have found out that the numbering on the drawings for the needle seems to relate to the fuel mixture rate at different throttle openings rather that different or variating tapers. I do however like to find or aquire advice from people, The response I have had with helpful comments, ideas and advice from my request is really appreciated.

Ian

Ian Lee02/11/2018 23:17:27
42 forum posts

To everyone who commented thank you, I have managed to make these needles, I ended up removing the complete milling head from a small Clarkes milling machine I bought years ago from machine mart, I mounted it on my tool and cutter grinder, I then put the nickle silver rod in an ER32 collet and spun it at 2000 rpm against a 6 inch cup wheel, it worked a treat. I have just to put the circlip grove in them and we will try them out later this week, I will give an update about when they are intalled.

Emgee03/11/2018 00:31:36
929 forum posts
185 photos

Hi Ian

I used the the small size Dremel cut off wheels in a Quorn to cut the circlip grooves in a needle for a vintage motorbike carb, worked a treat.

Emgee

Michael Gilligan03/11/2018 00:51:35
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12026 forum posts
522 photos
Posted by Ian Lee on 02/11/2018 23:17:27:

To everyone who commented thank you, I have managed to make these needles, I ended up removing the complete milling head from a small Clarkes milling machine I bought years ago from machine mart, I mounted it on my tool and cutter grinder, I then put the nickle silver rod in an ER32 collet and spun it at 2000 rpm against a 6 inch cup wheel, it worked a treat. I have just to put the circlip grove in them and we will try them out later this week, I will give an update about when they are intalled.

.

Well done, Ian yes

MichaelG.

Georgineer03/11/2018 16:31:27
188 forum posts
10 photos

Thanks Ian for letting us know how it went - so often we never know the outcome of enquiries and suggestions. And, just for my own curiosity, how on earth does one break an SU needle?

George

Ian Lee03/11/2018 20:19:42
42 forum posts
Georgeineer, turns out that the original needles somewhere along the way got lost, The carbs for this Bentley were basically a one off and onle ever made for this car, the carbs were assembled from a variety of SU bits, the main jets were made by an engine tuner as were the needles, this is why it's not easy to get the thing to run. Thevidea is I make this pair of needles and hopefully it will run up through the rev range, then believe it or not John is going to one of those tune up spark plugs so we can see it burning.
Ian Lee20/11/2018 21:58:22
42 forum posts

Just an update, we have tried the needles and they work, however it is back to the drawing board as although the car actually runs it needs some choke so running a bit lean, I am now making some more needles with a different and slimmer taper, am trying to steps every 1/8th inch so will see if I can manage it on my Clarkson tool grinder.

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