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Centering The Needles


Darth Vader
Almost spelt it "needless"! Anyway, how do you center the needle in an SU? I cant find any info anywhere-our Bentley manual does not give any instructions about rebuilding carbs (I think it actually says "Take car to dealership"!) I've got the tool kit that Moss sells, if that's any help.

Dave Russell

Yoda - R.I.P
Jet centering is only needed on the older fixed needle type SUs. If you have newer spring loaded type needles, no need. The needle is flexibly mounted in the piston. There is a jet centering tool available but I don't think that it comes in the "kit".

If you don't have the tool, adjust the jet nut to be all of the way up (lean), loosen the large jet holding nut (large outer nut) & push the piston with needle all of the way down into the jet & tighten the jet holding nut. Check by making sure that the piston will drop freely to the bottom without the needle binding on the jet.

If you had the "centering tool" it could be inserted through the damper hole in the cover, (piston removed) & do the same procedure.

[ 03-17-2004: Message edited by: Dave Russell ]</p>


Jedi Knight
Country flag
Aloha William,

The Moss kit does contain a jet centering tool (atleast mine does, but it was purchased years ago). It is used on reassembling the older H type SU carbs. To use it, the needle is removed from the carb piston and the tool is put in its place. The tool looks dowel with peg on it, probably about a 1/2 inch long. The chamber and piston are then replaced in the carb body. After putting the jet assembly together and with the jet sealing nut loose, lower the carb piston so that the centering tool slides into the jet bearing and the the piston is resting on the bottom of carb throat. Now thighten the jet sealing nut. To verify the jet is centered, raise the piston and it should drop down with a "clunk" and the tool in the jet bearing. If it does, you now replace the needle and redo the "clunk test" and it should work fine. It is more difficult to describe than do, I've used it often over the years with good results.

Safety Fast,

Dave Russell

Yoda - R.I.P
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by MGTF1250Dave:
Aloha William,
The Moss kit does contain a jet centering tool (atleast mine does, but it was purchased years ago). To use it, the needle is removed from the carb piston and the tool is put in its place. The chamber and piston are then replaced in the carb body.
I made the guess that the present Moss kit doesn't include the centering tool because the kit sells for 19.95 & the centering tool alone goes for 17.95. The older kits may have "the tool".

In any event even SU does not advocate using the centering tool. Since the top step of the needle is only a couple of thousandths smaller than the jet the needle does a good job of centering the jet.

Also, as far as I know, you do NOT remove the needle & replace the piston. There is NO hole or connection from the damper chamber in the piston to the needle holder. The tool goes in place of the piston with the chamber in place.

As a matter of interest, the spring loaded needle is not favored by all folks & is sometimes replaced with the older solid needle. With the spring loading, the needle is always rubbing on & wearing the jet & needle. A properly centered jet with solid needle has very little wear.

[ 03-17-2004: Message edited by: Dave Russell ]</p>


Darth Vader
Our rebuild kit contained sprung needles (as did the carbs). I wondered about this, especially since we already reassembled one carb and the piston went up and down with no binding. Guess I got lucky on that one. Although, the big nut at the bottom of the carb (where the jet fits) ended up getting wound back up during reassembly. May wind it off a bit before reinstallation.
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