SU Jet Bearing and Supply Tube Notes

May 2017, R. Kwas, updates on-going


Initial Mixture Setting and Adjustment
Considering the Jet Bearing


Recent questions caused me to look in some detail further into the Jet Supply Tube and associated components.  These components and their adjustment are what determines Fuel-Air Mixture, so are pretty critical.  This is a compilation of what I found, and was able to put together.  

What's going on here?  (Besides the fact that the brass Jet Tube is not brass colored, but silver, from a recent application of Graphite grease on my part!)


Jet Bearing Assembly, Exploded Detail:

Exploded assembly diagram showing Jet and Jet Bearing installation into carb body.  Jet Holding Screw(28) secures Jet Bearing(26) into carb body (with lateral clearance, allowing the ever-important Jet Centering), and Mixture Adjustment Nut(30) varies the depth of Jet itself into the Bearing allowing setting of initial mixture, as well as fine adjustment of mixture, as detailed below.

The observant reader will notice there is no provision present for holding the Jet preloaded into Bearing!  It is actually held there strictly by way of the return spring of the Choke Linkage (not shown in the diagram). The Adjustment Nut Spring(29) strictly preloads against the Mixture Adjustment Nut(30), and secures it in-place and from moving on its own!

Initial Mixture Setting and Adjustment:  When replacing a Jet with new, or just reassembling after a major service where the locations of these components may have been changed, initial set-up (for mixture) should start from:  Jet flush with Jet Bearing surface, then lowered 2 full turns (or "12 flats" of the Mixture Adjustment Nut(30)).  This will allow engine to start...then one can proceed with fine-tuning of the mixture.  According to my careful calculations, the 12 flats of adjustment are 0.0769" down from flush...detailed here:

Here are pictures of a Jet Bearing removed from Carb Housing and by itself for clarity, with Mixture Adjustment Nut(30) and Jet, but no Adjusting Nut Spring(29) or Jet Holding Screw(28) installed, to allow seeing the Jet Bearing threads.  

MAN is adjusted such that Jet is level/flush with Jet Bearing surface at venturi (Yellow).  Note number of Threads visible above JAN (Blue). MAN is adjusted two turns (12 flats CCW) such that Jet is below with Jet Bearing surface at venturi (Green).  Note number of Threads visible above MAN (Blue).  This is 0.0769" down according to calculations below.


Considering the Jet Bearing:

Threadcount of Jet Bearing over 1/2" is 13, so Threadpitch of MAN is 26. 

1/26 works out to 0.0385" per rotation of Jet (0.0063 per flat), or face will be 0.0769" down from level/flush with two turns/12 Flats. 

To better picture this, here is a little dimensional study to show approximate Metering Needle to Jet orifice clearance, at three key operating conditions:  Flush or Level with top of Tube, 12 Flats down, and Choked. It should be noted that this calculation does not take into account the amount Dashpot rises (typically around another 0.125", depending on Idle) as a result of ingested Air. 

Clearance at Flush is 0.001" =(0.100" - 0.099"), at 12 Flats down is approx 0.003" =(0.100" - 0.097"), and at Choked approx. 0.0092 =(0.100" - 0.908"). 

The point of this is to shown how literally a few thousands of an inch make all the difference between lean and "rich as hell", and how adjusting the mixture "one Flat" richer for winter operation is a change of mere ten-thousands of an inch!!! 


Link to full SW-EM Tech Article:  SW-EM SU carburetors


External material sources are attributed.  Otherwise, this article is Copyright © 2021.  Ronald Kwas.   The terms Volvo and SU are used for reference only.  I have no affiliation with either company other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here.  The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, or worshipped, at the readers discretion.  As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles, and future! 

You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source.  If you donít, youíre just a lazy, scum sucking plagiarist, and the Boston Globe wants you!  As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment. 


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