View Full Version : SU's again...Jet centering?

10-26-2012, 03:12 PM
A quote from BS Skinner's book: "The most common error in centering the jet is doing the whole bit with the jet slightly down from the lean position and so you are not on the fattest part of the needle and your setting is inaccurate." I have the Moss centering tool which works great. I think I have the jet as far as it will go.How can I be sure I have the jet cranked up all the way when the spring hides the threads?

10-26-2012, 05:15 PM
Turn the nut lean until it won't go any more, and make sure the head of the jet is against the nut.

If you want a little extra margin, you can remove the nut and move the jet up until it touches the bottom of the piston. If it is perfectly centered, the needle will not bind against the jet even with the piston resting on it.

I'm not sure what the "Moss centering tool" is, but if it really works well I would like to know more. I do know that the SU tool doesn't work very well for me; and I don't believe that anything that indexes only to the carb body will be accurate. The domes and pistons have some manufacturing tolerance. In fact, on my carbs, the domes have enough play on the mounting screws to affect the centering!

10-26-2012, 07:10 PM
Aloha Randall,

I beleive the jet centering tool referenced is the one described in the link:


Althought the description of it's use seems to be for a HS series carb it might work on the H series.

I use the centering tool from this SU tool kit for my H carbs. To use it, you remove the needle from the piston and replace it with the small turned bit (third item from the left). The piston is then dropped down on the bridge of carb and the upper jet bearing (and associated parts) can be lcoked down by the sealing nut. This should center the jet when installed.

10-26-2012, 07:21 PM
That is the SU tool kit (or a reproduction), and the one that I could never get to work adequately for me. Maybe my domes & pistons are just off by more than most or something, but I've had the same experience on several different carbs (including what ISTR were H4's on a buddy's MGA).

10-26-2012, 09:37 PM
<span style="font-size: 11pt"><span style="color: #003300">I've never had any luck with the SU kit centering tool, but the Moss (386-500) one shown in the link above has always worked perfect for me.
The screws on the domes on mine allowed a little slop that interfered with the setting but I found some replacement screws with a shoulder that solved that problem. Other H6s that I've done for friends didn't seem to have that slop on the screws/domes.(??)
Frank </span></span>

10-26-2012, 10:29 PM
Here is BS Skinner on the Domes: "The mounting flange surface on the dome often has nicks and burrs that should be removed.They cause the dome to sit improperly on the body,just a few thousandths of an inch cocked,and this distorts the dome.This leads to an otherwise unexplainable sticking of the piston.Get out your flat glass plate and a sheet of 400 grit wet-or-dry paper and a bowl of water.Smooth the face of the dome by rubbing gently on the paper on the glass surface.Go both with a circular motion and alternate back and forth. just a few strokes will produce a uniform grey satin surface. A jewelers file will likewise take off any burrs on the mounting face of the body."

10-27-2012, 01:43 AM
To sand a surface flat, always go in a figure eight on your sheet of paper. I was taught this by my father-in-law years ago and he was a pattern maker for Ford.


10-27-2012, 07:10 AM
The Moss centering tool positions the jet dead center with the damper off. Then fit the dome with damper and needle. The damper should drop down onto the bridge with a metallic clunk. Some domed tops have to be twisted very slightly left or right and the fixing screws re-tightened. Over tightening the dome screws can sometimes cause distortion and the jet moves off center.

Needles must be perfectly straight or they will bind or jamb up in the jet. New jets and needles can work wonders for precision tuning, as the hole in the jet wears oval causing over richness. Sometimes new jets and needles will cure running on.