10 replies to this topic

[paulrockliffe]

I can't get my needle centred on one of my HS2s.



Is this here brass bit what is causing me grief? Can I move it to a more mutually satisfactory position, ie. the centre?

Thankingyou!

[liirge]

Might help you

[paulrockliffe]

ANyone have any ideas on this? Need to know if i need a new carb or not!

[miniboo]

are the needles not spring loaded so they are"floating" and self centre-ing?

[paulrockliffe]

Nope, not these ones, they're HS2s.

[dklawson]

There are several possibilities but let's start with... tell us how you've been trying to center the jet. I've never seen one that's all the way to one side.

[paulrockliffe]

I've been centring the jet the way I was told on a separate thread, I think it was you that told me how to do it actually? Do you know if the brass bit can be moved, or do I need to tyr harder?

I've been able to centre the jet so that the piston rises and falls properly, but as soon as the piston housing (damper?) is properly tightened onto the carb, the needle becomes tight again.

[dklawson]

Forgive my terminology if it doesn't jive with Haynes or Burlen. Let's go over jet centering again in brief. There's a "factory" method for centering and a shortcut most people take.

This is the shortcut most people take. Remove the dashpot from the vacuum chamber then remove the vacuum chamber complete with the piston and spring. (Paint mark one ear on the vacuum chamber before removal so you can put it back in the right place). Look down into the carb as shown in your picture. Run the jet adjustment nut up until the jet is flush with the bridge. There is a gland (or jam) nut just on the bottom side of the carb (just above the jet adjusting nut). Loosen this gland nut. Refit the vacuum chamber and piston. Push the piston down through the dashpot hole. Reach through the carb throat and lift the piston again and let it fall. Do this lifting and pushing down several times until the needle pushes the jet to the center. You want to be able to lift the piston through the carb throat and release it such that it falls to the bridge with a clunk. (Note that the dashpot is not in the piston when you're doing this). Once you get things free and centered, tighten the gland nut again and perform the test one last time to make sure things are still centered.

The official method involves more steps and it may be necessary in your case if with the jam nut backed off the jet still refuses to move. In the official method you're supposed to remove the jet tube from the float bowl, extract the jet from the bottom of the carb, then remove the spring that goes around the jet tube. You put it all back together without the spring and repeat the steps I listed above. Once you've centered everything and tightened the jam nut you remove the jet tube again and refit the spring. I have not removed the spring when I've centered jets but that's not to say that there aren't times when it's necessary.

[paulrockliffe]

Thankyou! I'll have a look at that when I've got the new needles (the ones currently fitted are setup for a different engine), didn't know there was a jam nut. Will I be able to play with the jam nut with the carbs on, or are they coming off again?

[dklawson]

You can center the jets with the carbs mounted, it's just a little harder to access the jam nut.

Since you didn't know there was a jam nut, let me take a second to describe what's going on when you make this adjustment. If you have a Haynes or SU manual, looking at the exploded drawings will help. In the meantime, look at the picture you posted.

The jet is the very center (bright brass) cylinder with the hole in it. It passes through another brass sleeve looking part (the darker bronze ring)... that's the bushing. The bushing is a tubular cylinder and a short distance down from the surface shown in your photo the bushing has a disk/flange. The disk is part of the bushing and it's like the brim of a hat. The jam nut pushes a plain washer up against the underside of this brim/disk to hold the bushing into the bottom of the carb body. As you can tell from your photo, the bushing is smaller than the hole through the carb body. The flange on the bushing is smaller than the counterbored hole in which it seats. So... if the jam nut is loose you can push the bushing anywhere inside the hole that goes through the carb body. When you tighten the jam nut it pushes the washer and flange on the bushing securely in place against the carb body so it won't move. Lifting and dropping the piston/needle with the jam nut loose allows the bushing to float to a position where it is centered in below the piston.

When you look at the pieces and the exploded drawings it will make sense and you'll develop an understanding for what you're trying to do. Since the needles you have aren't going to be used, why not practice centering the jet using them while they're installed?

[paulrockliffe]

Thanks again, I've a good idea whats going on now, as soon as you mentioned this jam nut I guess that would be what it does. I rebuilt the carbs myself (though I didn't touch the area that the jets sit in) so I know how most of the bits go together. Sounds likea relatively straightforward job now I know what's what.

Cheers!