HD-8 Jet centering

Well I’m about 6 months into my carburetor rebuild (nothing happens quickly at our house) and a bit puzzled by the method for jet centering via the factory service manual.

I’ve read in older discussions on this forum that using the little tool included with the SU kit works well, but read elswhere that it’s completely useless as the variation between parts is too great for a standard part to ever work. I’ll take this lists expertise over random internet information.

I have two specific questions though that I didn’t see answered. I got my rebuild kits from Joe Curto, which I believe is the best source for SU components… The kits of course included new jets and needles.

Following the factory service manual method (not using tool) I have a few anomolies.

  1. Upon initial assembly of everything, I found the piston to bind and not fall fully. I pulled the float bowl back off, and rotated the jet 90 degrees, and found that there was no more apparent binding; the piston fell all the way to the bridge.

  2. The factory manual says the sound of the piston hitting the bridge should be identical with the jet full-up or full down, but I notice a change in sound with the jet in the full-choke position. Is that acceptable?

My quesiton with centering the jet via the tool is as I understand this takes the jet itself out of the equation, but as I noted the orientation of the jet seemed to have a significant impact. The service manual also indicates it is critical to orient the jet in the same position, for final installation as when test fitting.

So summary of my questions

  1. if the piston falls freely and closes to the bridge, is that “good enough” or do I need to have identical sounds with full choke and choke off?

  2. If the tool works without the jet how do we account for the fact that the jet orientation appears to have an impact?


That’s easy: manufacturing tolerances!

Hi Brandon,
Here’s my 2 cents worth, the centering jet tool that came with the kit I have is worthless (assuming you have the same kit) and I use the needle in the piston to get it to where it should be. Secondly I think that if the drop test is the same for all the pistons with the jet in the full up position that should do it. When I did mine the first time back a few years ago I had a friend who had a real jet centering tool that worked but I’ve never seen another one and he has sadly, passed on.

I had read quite a few positive reviews of the tool on this site. Seems like results may vary!

I had one from Burlen. it worked well enough the first time I used it, then I… ummm… used it for something else. They look for all the world like a bit of scrap brass… this is a dangerous thing for a tool to do in a shop where clock parts are being made.

If you have access to a lathe it’s quite easy to make one.

But you can also use the needle to center the jet. Once you think it’s centered, take the bell/piston back off, then mark the upper portion of the needle with a Sharpie, and do another drop test. If the needle is scraping on the jet you’ll see the marks in the Sharpie ink.

If the Piston or bell are toast, or not fitted correctly then the needle position won’t be repeatable. Similarly, corrosion in the needle bore (where it is clamped) will cant the needle.

I use a homemade tool as well, to align the jet bearing concentric with the bore in the bell. I make sure that the bell is tightly attached to the carb body with all four screws. I have some difficulty tightening the jet locking nut without applying radial force against the bearing, which I’m afraid might alter it’s position from concentric. Usually, the result is good enough. But I’ve found that if I try to “freehand” the adjustment without the locating tool in the bearing, it’s hard to tighten the nut without moving the bearing.

If rotating the jet makes a difference, doesn’t that mean there’s slop between the jet and the bearing?

Fitting a new jet requires the jet guide to be loose .Fit the dashpot and
piston with the jet fully up, then, piston fully down, gently agitate the jet/jet guide to centralize to the needle, Note the jet orientation,
remove and tighten the jet guide, You may have to do this 2/3 times,
the jet guide tends to move when you tighten. If OK the piston will
move without moving the jet. If the piston moves say 1/2" without
moving the jet but max lift does the needle is bent.
The float chamber does not need to be fitted for this op.


The little tool that comes with the kit is trash. Mine wouldn’t even fit in the hole, probably the chrome plating was too thick. However this tool is perfect for the job…


It works on a different principal. It calibrates the jet carrier to the bell housing rather than to the piston. The piston is centered by the bell housing also so both things are calibrated to the same standard. It’s perfectly machined and led to 100% repeatable results. It’s also very easy to use. As an experiment I tried setting one using the book method and it felt fine. I then went in with this tool and it wouldn’t fit in. So I loosened it up and recentered it with the tool, then tried the factory test again, and it sounded the same as before, so the clunk method is not 100% perfect.

Never ever had a tool: this is how I did it, perfect alignment, every time.

On about a zillion SUs!

Erica loaned me her centering tool when I did my carbs and it worked so well I went out and bought my own. Yep, got it from Moss Motors.


I have rebuilt a number of sets of SUs.

I have found the insert centering tools to be of little use.

I use an 18mm ring spanner, the jet and the needle. Works well.

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Well I thought I’d update here. Last night I tried my center carb with the little mini-tool on hand. It sure was a snug fit into the needle hole, but I got it. Put everything together, tightened bearing nut, and it’s a big improvement.

  1. The orientation of the jet no longer seems to matter, piston falls freely with it rotated any way. Before it only fell freely with the jet in one particular orientation.
  2. With everything assembled and tightened the “clunk” is the same with jet all the way up, or full choke.

I do like that other tool, but seems like this method offers a decent way to do it for me. It does seem like there must be some variation in the quality of the little centering tools…