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Thread: HD series Jet Centering - a Caution

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    HD series Jet Centering - a Caution

    Last year, I used the Moss jet centering tool to center my new needles and jets.
    JetCenterToolMoss.JPG

    These are my "new" needles; the jets are also worn:
    JetsWorn.jpg
    That's $120 + shipping wasted + cost of the tool.

    Learned my lesson. Centered my new jets & needles by observation.

    The jet bearing nut is 18mm - a socket works better than a combination wrench. It has to be first tightened by hand so as not to disturb the alignment. It's a trial and error process. When properly completed the piston will close with a clunk or clank.

    With the carb on the bench, remove the large spring inside the dome and replace the dome; remove the damper and oil. This way you can push the piston/needle assembly down more smartly than the spring does it. Then with the bottom assembly off the carb you can slide the jet into the bearing. When the jet seats observe if the piston is disturbed in any way. If so, that means there's friction between the needle and jet. Loosen the jet bearing nut and jiggle the jet. Tighten the nut by hand and push the jet in until it seats. If the piston doesn't move in any way the jet centering is good. Carefully and delicately tighten the nut such that the position of the jet bearing is not altered. Check again for friction between the fully seated needle and jet.

    When the jet bearing nut is tight, test by pushing the piston smartly through the dome with a phillips screwdriver - it should make a metallic click or clunk which indicates no friction on the needle.

    Addendum: make sure the jet diaphragm is in the same position every time you check it and when you tighten it up.
    Last edited by steveg; 11-17-2018 at 01:05 AM.
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    Re: HD series Jet Centering - a Caution

    I can't even tell how that thing is supposed to work. I've used the one in this kit--it's the little stubby thing in the far right of the photo--for years and it's always done the trick:

    https://mossmotors.com/tool-kit-su-carburetor

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    Re: HD series Jet Centering - a Caution

    You take out the piston, reattach the dome and slide it down through the inner tube. The narrower diameter goes through the jet bearing, as does the large end of the little pin thingy in the tool kit.
    Steve Gerow
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    Re: HD series Jet Centering - a Caution

    Quote Originally Posted by steveg View Post
    You take out the piston, reattach the dome and slide it down through the inner tube....
    Maybe that's the problem: you've removed the variances introduced by the piston. The little pin thingy goes into the piston in place of a jet needle, and allows for any offcenteredness--is that a word?--of the piston and the needle shank. These little kits are worth having; it's one way to balance the carbs without having to remove the air cleaners (PITA) to use a synchronizer--or a hose if you're old school--and, by comparing all methods they work pretty good. The little mixture adjustment wrench works where you can't get a spanner (on non-HD carbs).

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    Re: HD series Jet Centering - a Caution

    I just tried the little pin thing in my spare HD8. The small end was too large to fit in the jet needle receptacle, and the large end is .002" narrower than the Joe Curto jet.

    IMO the overall problem is with a centering tool one adds another layer of error.

    If either tool in one's possession is good enough to center the jet without rubbing, bravo - neither of mine will do the job without further centering by observation.
    Steve Gerow
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    Re: HD series Jet Centering - a Caution

    I think it's "offcentredness".

    Danny

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    Re: HD series Jet Centering - a Caution

    Steve,
    Sorry to see the tool did not work for you.

    I have both the full sized tool and the small insert from the balancing SU tool kit. I have found over the years both work brilliantly and result in the definitive “clack” of a well centred jet/needle. I have never had the wear problem Steve unfortunately had in his experience.

    The two tools have different applications depending on the state of your carbs. The long tool used by Steve requires the jet to be removed from the carb as the thin section goes through the full length on the jet sleeve. It does a great job of aligning the jet to needle when you are rebuilding the carbs. The smaller tool is great if your carbs are already built with the jet installed or perhaps you just replace the needle or for some reason want the “check” alignment. It is also good if you have never centred the jet and want to give it a go without removing the jet. You just remove the needle, install the tool and then crack the jet nut to allow alignment and then tighten followed by a check for the “clack”.

    Just my experience other may find the tools different.

    Cheers
    Tony

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    Re: HD series Jet Centering - a Caution

    As usual, Tony, thank you for the clarification! Evidently my technique needs refinement.

    Since my HD8 jets are centered and the carbs are mounted, will disassemble one of my old HD6s and revisit the tools. Will follow up later.
    Steve Gerow
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    Re: HD series Jet Centering - a Caution

    When the jet bearing nut is tight, test by pushing the piston smartly through the dome with a phillips screwdriver - it should make a metallic click or clunk which indicates no friction on the needle.
    Steve, IMO there is your problem - the "clack" should be heard as the piston 'free falls' by it's own gravity. You can push the piston all the way down and force a "clack" even if there is contact between the needle and jet but
    when they are aligned properly there will be no place the needle will touch the jet so there will be no friction to resist the free falling "clack" of the piston. Key here is 'free fall'.

    Dave



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    Re: HD series Jet Centering - a Caution

    Quote Originally Posted by red57 View Post
    Steve, IMO there is your problem - the "clack" should be heard as the piston 'free falls' by it's own gravity. You can push the piston all the way down and force a "clack" even if there is contact between the needle and jet but
    when they are aligned properly there will be no place the needle will touch the jet so there will be no friction to resist the free falling "clack" of the piston. Key here is 'free fall'.

    Dave


    You're right - "smartly" is the wrong word. I wasn't forcing it. I was holding the jet fully into the jet bearing with my finger while pushing the piston down with the screwdriver - while observing by touch if there was any friction between the needle and jet. The other way I checked it was, with the piston/needle fully seated on the bridge, I pushed the jet lightly into the jet bearing. Was the needle disturbed?

    It was more convenient to move the piston back and forth by hand rather than relying on the spring and gravity.


    I used this as a way of ensuring there was no friction and got the "clunk".
    Steve Gerow
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    Re: HD series Jet Centering - a Caution

    Steve, I figured you were careful and didn't mean to imply anything less but since it is so easy to affect the centering, I thought I'd mention it.

    I once rebuilt a set of HD6s and was very careful to be sure the jets were centered well but when I put them on the car, one had a slight bind that I finally concluded was caused by the slope of the carb - I had set the jet bearings with the carb body horizontal in my vise. There are manufacturing tolerances in everything - the bearing ID in the dashpot, the shaft OD on the piston, the jet OD, the jet bearing ID, plus wear of all the above - and, if you get unlucky and just he right combination of upper & lower limits on the individual parts, it can add up to looseness or tightness. In this case it was towards the loose side and the angle of the carb allowed the piston to run slightly off center. Now, no mater how sure I am on the bench, I always check for the gravity 'clack' after installation on the car.

    Dave

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    Re: HD series Jet Centering - a Caution

    Dave,
    Thanks for the tip!

    I'm a little bit lucky in that regard, as my carbs are horizontal.

    EngineComptCarbs.jpg
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    Re: HD series Jet Centering - a Caution

    This method hasn't failed me in forty-five (45) years. It's the pin (jet-centering device) that Bob alludes to in post #2









    Snug the jet-holding nut and make sure the piston still falls with a clack, tighten fully and test again.

    http://www.britishcarforum.com/bcf/image.php
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    Re: HD series Jet Centering - a Caution

    At least thirteen (13) of the first fourteen (14) pages of this album are devoted to the overhaul of a single set of HD8 carbs, if anyone is looking for some visual aids.

    http://spcarsplus.com/piwigo/index.php?/category/375

    If I had them to do over again, I think I'd see how well just putting new shafts work in the original bushes. While I was able to eventually get a good fit, they were more trouble than they should've been (even using the correct SU reamer).
    http://www.britishcarforum.com/bcf/image.php
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    Re: HD series Jet Centering - a Caution

    Last year, I followed the directions in SU CARBURETTERS TUNING TIPS & TECHNIQUES and rebushed my HD8s with new PTFE bushes - no ream. They were pretty OK, idle-wise.

    Since I had my carbs apart for the above-mentioned needle/jet problems, sent my carb bodies off to Tom Bryant to install and line bore his Delrin bushings - that should hold the carbs while I'm on this earth.
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    Re: HD series Jet Centering - a Caution

    Randy,
    Was this jet broken or did you cut it off? If the latter, why?

    screenshot.1531.jpg
    Last edited by steveg; 11-20-2018 at 08:20 PM.
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    Re: HD series Jet Centering - a Caution

    Quote Originally Posted by steveg View Post
    Randy,
    Was this jet broken or did you cut it off? If the latter, why?

    Attachment 56316
    Linky no worky. Can you just give me the image number?

    For example, the image number for this one is "image 1368" (sometimes shows as "IMG1368") http://spcarsplus.com/piwigo/picture...292/search/133

    Or, just copy & paste the whole address from your browser.

    I can say with certainty though, that I didn't cut a jet, maybe I was just showing a picture of how far the jet inserts into the piston (?) and the image got, well, cut off?
    http://www.britishcarforum.com/bcf/image.php
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    Re: HD series Jet Centering - a Caution

    Quote Originally Posted by steveg View Post
    Randy,
    Was this jet broken or did you cut it off? If the latter, why?

    screenshot.1531.jpg

    Steve, that jet was not cut. Look closely and you will see that the tube is a separate piece that appears to get soldered into the large part that is connected to the diaphragm. Possibly failed in place and came out of the carb that way?

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    Re: HD series Jet Centering - a Caution

    Steve, looking back at the pictures preceding the the one you're asking about, it looks like the "jet tube" (movable tube connected to diaphragm) had seized inside the jet bearing. I don't recall how much force was required to remove the jet assembly, but clearly I separated the pieces__it was not cut, but pulled apart.



    http://www.britishcarforum.com/bcf/image.php
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    Re: HD series Jet Centering - a Caution

    Interesting, Randy - thank you for assuaging my curiosity!
    Steve Gerow
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