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SU Jets & Tuning

While doing a fairly major tune-up on my 120 OTS I again encountered the
irritant of having the (SU) carb jets stick in a raised position after I
have loosened the adjusting screw and am trying to richen the mixture. The
immediate temporary solution is to loosen the adjusting screw a couple more
turns than needed, remove the suction chamber and piston, and push the jet
down using a pencil sized wood dowel and something to give it a knock. With
the jet positioned on the rich side, the adjusting screw can then be
tightened and the jet pushed up to the approximate proper setting. I guess
the problem could be that the jet gland spring isn't strong enough to
overcome the friction of the gland washers or gum/varnish?. I'll probably
give in and remove the entire jet assembly to see if everything is as it
should be, but thought I'd ask if anyone else has encountered this and come
up with an enduring solution? It certainly would be nice to have the jet
follow the adjusting screw down without having to smack it each time. -
Thanks, Dick Cavicke, OTS S672776

The problem described by Dick seems to be different from jet alignment. He
does not have a problem with the pistom movement. The jet should be pushed
up with the adjustment screw and pushed down by its spring (gland spring)
when the adjustment screw is backed off. Something is apparently binding
the "jet and head unit" so that the spring force is not sufficient to push
it down and he has to manually push it down with the piston. The jet
alignment is done by centering the "bearing" for the jet on the needle
attached to the piston. Improper alignment of the bearing will result in
the needle touching the jet and interfering with the free motion of the
piston. That is not the problem described in this case. - Bruce Cunningham,
'53 XK120 OTS

Dick:  Sounds like the needle is not centered in the jet. While attending a
JANE SU carburettor workshop, Gary Hagopian supervised us on rebuilding
carbs. He emphasized theimportance of a test of the alignment. There is an
adjustment, but I do not recall right now what is was -- without a carb in
front of me! - Carl Hanson

Unless I'm completely missing the issue, perhaps the piston is binding on
the cylinder wall. If the jet is centered and no binding occurs, it's nice
to hear the metalic click when the piston/needle unit falls into the jet.
Since the piston/needle unit operates within a very close tolerance to the
cylinder wall (housing, dash pot, etc.), I would check to see if any dirt
is creating a binding.  I've noticed some walls with cuts and grooves where
the dirt or grit has cut into the aluminum.  While not a good practice, I
very gently remove the burr from the wall and also the piston/needle unit
with 500 grit wet-or-dry sandpaper. But these components started life with
a nice fit and one should be careful to not sand only enough to get a free
falling action. I hope this will help someone. - Bob Oates

I center my needles by assembling the jet loosely, then gradually tighten
the nut, making sure that the jet moves up and down without binding. Never
do it quickly, a little turn at a time and I have found it keeps the needle
centered. Question for anybody: I live at 3,000 feet above sea level...What
is the opinion on a different needle? - Barry Goldman

Carl and Bruce, Thanks for addressing my jet question. I don't believe it's
an alignment problem because the pistons and needles travel freely on both
carbs and the sticking problem is there even with the needles removed. The
whole thing is low on my priority list since the carbs are pretty well
adjusted for now. Next time around I'll remove the whole assembly and see
what's going on.  I may drop a note to Gary Hagopian in case the problem
and solution are well known to him. Incidentally, one of my reasons for
doing a tune-up was the fact that the engine was "running-on" after turning
off the ignition. Every time it did, it sounded terrible and the exhaust
smelled like a city bus. All indications are that the mixtures were
excessively lean despite getting what seemed to be a proper reaction to the
recommended mixture test beforehand. Richening the mixtures appears to have
cured that particular problem. Gary Summerfield memorial: (Win a
simultaneous subscription to all 12 Jaguar lists.) Does anyone wish to
tackle the questions: 1. Which carburetor's mixture is being tested when
the piston is raised?  Is it the opposite carb or the one whose piston is
being raised? (The 120 manual differs with later service instructions.) 2.
What is the correct amount to raise the piston, 120 book suggests 1/4",
others specify only 1/32"? (It may be academic since frequently it's hard
to tell how much the  piston has moved. Are they just trying indicate "move
it a little bit".) - Dick Cavicke

To all, Maybe the needle is bent. - Edward Blake

The carb not having the piston raised is the one being tested. Raising the
piston essentially turns off the Venturi in that piston shutting off the
gas supply. The engine should speed up momentarily because eliminating that
gas supply briefly leans out the mixture to those three pistons. When the
gas is completely flushed out of the manifold, those three pistons stop
firing and the engine should continue to run slowly but smoothly on the
remaining three as the firing order alternates with the other three
cylinders. There is a crossover tube in the middle of the intake manifold,
however, which allows some of the mixture from the working carb to get to
the other three cylinders. If the mixture is too rich on that working carb,
it can provide enough gas through the crossover tube to keep the other
three cylinders firing. If the mixture in the working carb is too lean, its
three cylinders will not run smoothly and the engine will either run very
roughly or die. Thus with proper adjustment of the mixture, the engine will
speed up slightly for a moment and then slow down and run smoothly on half
its cylinders. If you have a pretty slow idle normally, you will probably
have to set it up to about 1,100 or so to keep the engine from dying on
three cylinders even with proper mixture adjustments. It doesn't matter
much how high the piston is raised as long as it is raised enough to stop
the Venturi from sucking gas out of the jet. This doesn't take much because
with normal operation, the atomizing effect is just barely happening. Most
SU's have a little plunger under the piston just for this purpose. I have
always depressed this plunger all the way to adjust the mixture. It
probably moves the piston up about a quarter inch. The crossover pipe is
behind the butterfly valves so I don't think it makes any difference how
much you raise the piston. The 1/32" is probably sufficient to shut off the
gas from that Venturi. The Venturi, by the way, is named after G.B.
Venturi, an Italian physicist. My posthumous appology if I forgot to
capitalize the word anywhere above. - Bruce Cunningham

Barry G - re 3000 above sea level,  see the archives re this subject from a
few months ago.  I think we decided it made virtually no difference, but
check. - Regards, John Elmgreen

Concerning SU jets sticking, I had the same
problem a year back. The rear carb would not respond to tuning with the
back cylinders staying rich. Just by luck I pressed up on the jet tube and
felt it fall (fall up? I guess that makes sense) into place. The
enrichments mechanism drops the jets for cold starts and, if it does not
slide freely, it will stick in the rich position. After a few days of
starting the A cold with the bonnet up so that I could manually slide the
jet home, I gave up and pulled the jet assembly.  The last person inside
the carbs put the jet corks in dry, I believe.  After some judicious
fiddling, I was able to free the assembly. I did not have new corks around
so I re-used the old ones putting the better one on the bottom.  The rear
carb now works like it should with the exception of a slight weeping of
fuel.  It never drips it just gets a little wet once in a while.  A have a
new set of corks now and will probably replace them soon but the car runs
so well that I am hesitant to open the carb for fear that I may screw
something up. - Regards, Bill Eastman

Dick:  Isn't there a little push-rod on the right side of your H6 carb for
lifting the piston the right amount?  I thought that was what it is for. -
Carl Hanson

Dick C. verbalized a question which I had in my mind for a long time. The
manuals (most) say to lift the piston 1/32 of an inch, but the push rod
when fully engaged lifts it about 1/4 of an inch.  Bruce C. gave a very
nice answer to that question. - Dick White

It would appear that my carb jet (not needle) binding problem is not a
universal one. Bill Eastman came close to describing what I believe the
problem to be. That is that the jet seals are somehow restricting the
movement of the jet and the jet spring is not strong enough to overcome the
friction. Once again (belatedly) I've found that the manual actually
cautions: "Ensure that the jet is not sticking and is following the
movement of the adjusting screw." (It doesn't tell you how to fix it.) When
all else fails, read the directions. With apologies to anyone with trivia
sensitivity. I appreciated Bruce Cunningham's thorough explanation of what
takes place in the manifold when the carb piston is raised (using the
plunger/piston lifting pin). My curiosity has not been completely satisfied
because of the following specific anomaly: (Capital letter emphasis added
by me.) 1. The XK120 Operating, Maintenance and Service Handbook p48
specifies lifting the piston "approximately 1/4" on the FRONT carb and,
depending on what the engine does, adjust the REAR carb and vice versa. 2.
The XK140 OM&S Handbook p46 states: "lift the piston of the FRONT
carburetter by approximately 1/32" (.8 mm) when if:- (a) the engine speed
increases this indicates that the mixture of the FRONT carburetter is too
rich. (b) the engine speed immediately decreases, this indicates that the
mixture strength of the FRONT carburetter is too weak. (c) the engine
continues to run without change of speed then he mixture strength of the
FRONT carburetter is correct." "Repeat the operation at the REAR
carburetter to test its mixture strength and then recheck the front
carburetter since the two carburetters are interdependent." As Bruce C.
advised, apparently the amount one raises the piston doesn't really make
any difference and perhaps can be discounted. However, the fact that the
120 book says that when you raise the piston on one carb you're checking
mixture of the OPPOSITE carb versus the 140 book which says when the piston
is raised on one carb you are checking the mixture of that SAME carb.... is
a conflict that could lead to a lot of extraneous screw turning. Somehow
between the 120 and the 140, the process changed. Was there a change in the
manifold gas-path as well? Maybe with sufficient mixture tweaking, the
results are the same no matter which method is used. Still sleeping well.-
Dick Cavicke

And this gets stranger when you have a THREE S.U. setup.... - Jim Warren

The mixture adjustment (re adjusting for 3,000 ft above sea level) is used
to compensate for elevation (O2 density) change.  The needle profile will
affect the mixture throughout the demand curve. Various needle tapers are
(allegedly) available but probably only of concern for competition. -  Jim
Warren

Dear All, I have consulted a friend who for many years worked for SU in
their emissions lab. He has a set of SU set-up instructions which he will
mail to me. I will post them if anyone is still interested. Over the phone
he gave me the following outline which mostly conforms with what has
already been said. This is not XK specific but a general guide to twin or
triple SUs. 1) Remove damper and check that piston falls with a nice clunk
onto the jet nut. If it doesn't the reason is probably dirt or an off
centre jet. (Later SUs had a spring biased needle to ensure consistency but
not H6s or H8s). If the jet is off centre, adjust it by loosening the gland
nut, moving the jet tube and retightening the nut. Once you have a nice
freely falling piston replace the damper. 2) Get the engine to operating
temperature and ensure that the thermocarbs are off. 3) Unclamp the linkage
between the two carbs and adjust the air volume with the throttle stops to
be equal at idle. You do this by listening with a piece of hose at each
inlet. This is surprisingly accurate but if you want to be really flash you
can use a flow meter. 4) Lift the piston of one carb (there doesn't seem to
be a good reason to do either the front or the rear carb first) 1/32" Use
the lifting plunger or a penknife blade. The 1/32" isn't critical but the
piston only need to lift a gnat's whisker. 1/4" is way too much. What you
are doing as I think Bruce said is to reduce the Veturi effect of that
carb. You are trying to examine the effect of weakening the mixture.
Because the needle taper is slight a very small lift weakens rather than
richens but obviously if you continue to lift the richening effect of the
needle taper starts to interfere with your adjustment. 5) If the engine
speeds up when you lift the piston and stays speeded up then the mixture
was too rich (because weakening it slightly improved it). Conversley, if
the engine speed decreases immediately then you started with a mixture that
was too weak. The happy state that you are aiming for is a momentary
increase in revs and then a drop back. When you get this the mixture is a
tiny bit too rich but that's not a bad thing. For those interested in
emissions this will give a CO of about 8% provided all else on the engine
is A1. 6) Adjust the mixture on the carb whose piston you are lifting by
turning the mixture adjusting nut one flat at a time - up to weaken, down
to richen. It sometimes helps to blip the throttle open and let the engine
settle before repeating the lifting test. 7) When you have got the first
carb correct repeat the procedure with the other one. Then recheck the air
flow. If you have to adjust it (and you probably will) go round the
procedure again. Continue to do this until no further improvement can be
achieved. Finally retighten the spindle clamp between the two carbs. All of
this assumes that there are no induction leaks, the  float levels are
correct and that the engine is basically good. Competition :- What does SU
stand for and how did it arise? - Eric Capron

Dick, While it is a long shot, might your spring be "tired" and no longer
providing the required force. Springs do fatigue. Regards - Klaus Nielsen

Dick: You restored my confindence in my reading ability.The "Mk IV" owner's
and shop manuals both say to adjust either front or rear carburetors raise
the piston on the other one.  Everyone kept telling me that this was wrong.
Evidently, Jaguar must have changed their procedures when they changed
carburetors.  - William Kellner

Eric C, Would you please post those SU instructions when you get them.
Thanks - Tom McNicholas

Tom: Do you have the special tuning tool kit for SU carbs. If not, allow me
to send you one. - Bruce Baysinger

Fascinating - I had never heard of this 1/32" method before. I understand
how it works and it is probably more sensitive than the other method. When
I figure out how to precisely raise the piston that small a distance, I'm
going to try it. At idle, the piston is floating so maybe the thing to do
is measure the gap and make something small that is 1/32" larger to stick
in there and hold it at the higher level. It will have to be a very small
tool so as not to change the flow of air under the piston significantly.
Ideally it would be good to depress the little plunger 1/32" after it
contacts the piston but that would be tricky. Does anyone have some
practice with this techique? Speaking of three carbs, I remember doing it
on my Series I E-Type many years ago but I don't remember the details. I
think I lifted two pistons at once and adjusted the third to run on two
cylinders. - Bruce Cunningham

Bruce, Thanks!  I may have an SU tool set.  Don't think I ever knew how to
use it. Found my 1972 Uni Syn while going through boxes.  Last used on a
Volvo P544 B18D with worn throttle shafts. - Tom McNicholas

I am skeptical at this point about the 1/32" not being critical. I believe
the gas flow from the jet stops entirely when the vacuum (depression) is
reduced significantly by the lifting of the pistion. The tapered needle's
function is to allow more gas to flow out of the jet when the volume of air
through the Venturi increases under constant vacuum - otherwise the mixture
would get progressively leaner because the constant vacuum at the jet would
pull a constant flow of gas out with the increasing volume of air as rpm
increases. The slight leaning out of the mixture at 1/32" raising of the
pistion is due to the slightly decreased vacuum at the jet sucking less gas
out of the jet. With no vacuum (essentially what you have at fast idle with
the piston raised 1/4"), the taper of the needle won't negate the effect
because the fuel is sitting at the level in the float bowl which is well
below the top of the jet. So I'm assuming the 1/32" needs to be fairly
precise and using a knife blade to do it would block enough of the small
slot below the piston to raise the vacuum significantly compared to using a
very small tool in the slot. As I noted, I have never tried this technique
so this is all just theory for me. I'd like to hear from someone who has
done it both ways. My SU's are working so well right now that there's no
way I'm going to experiment with them. (If it ain't broke ...) - Bruce
Cunningham

Bruce Ignorance above all and mine especially ...what does the special SU
tool kit consist of and is it available from the usual sources? - Regards,
Klaus Nielsen

Klaus, It's two bent coat hangers that ride up and down in the dashpot with
the piston...well calibrated coathangers to be sure...and thanks for the
wiring diagrams. I'll need them in five years (my goal) - Jim Warren

Sometime in the past I came by a book entitled SU Jet Needle Data published
by The SU Carburetter CO. LTD.; Erdington, Birmingham, 24; 1950 Issue. The
book lists the needle diameters (to the closest ten-thousandth of an inch)
measured at 13 to 19 locations (every 1/8") along the length of each
needle. Each of the 75 pages covers up to seven needles with different
alphabetical or numerical designations. There is no text, just page after
page of the exact dimensions of a whole lot of needles. A prior owner of
the book added some needle designations and entered their dimensions by
hand. I strongly suspect the book was used for fine tuning the power curve
of race engines. If this data (about specific needles) would be of interest
to some of you, I would be willing to copy the appropriate page(s) and
attempt to post them or mail them. - Regards, Dick Cavicke

Hi all -- re:  1/32", 1/4", etc. -- on my '54 XK120 OTS, S674619 (sold in
'89) with H6 carbs and on my current original Mk IX, 792817 BW with HD6
carbs, I always used (and currently use) the lifting pins to set mixture --
no problems for 13 years with the 120, no problems now (almost 10 years to
date) with the IX -- for me at least, they DO work!  I suggest if the
lifting pins don't work, something is wrong in the carburretor(s) to cause
them to go wrong.  Hope this helps -- Larry Martz

To Dick White, I have now received the tuning and repair instructions for
the H series carbs. They are an SU publication and are in the form of 12
sheets of A4 with text and drawings. I'm not quite sure what to do with
them next. Any ideas? - Eric Capron

Here is a web site with great information on tuning SUs by using an exhaust
gas analyzer: http://www.team.net:80/www/morgan/tech/tuning.html - Dave and
Linda Freeman

I am on the digest plus I am a naturally slow thinker so these may no
longer be timely.  Nonetheless, I feel the need to add my two cents. I
believe that the two descriptions (low lift, high lift, which carb you are
adjusting)  may both be right.  When you lift the dashpot, you cause that
carb to go lean.  If you lift it too much, it stops delivering fuel
altogether.  The design of most multi SU manifolds allow for vacuum
equalization but they don't allow for much cross feeding.  In other words,
the back carbs can't feed the front cylinders.  So, when you lift the
dashpot, the way the engine runs immediately after that action is the
result of the leaning out of the carb whose dashpot you lifted.  So if the
engine speeds up immediately upon lifting, that carb is richer than
optimum.  However, once the manifold clears of fuel, the carb lifted
(unless the amount of lift is minuscule- say 1/32 inch) is dead so the
engine is running other carb.  In that case, you are checking the mixture
of the non-disabled carb and if it is lean, the engine will die.  Adjust
the off carb a flat or two above the lean run limit and you should be fine.
I expect that Jaguar updated their recommendations to account for tuning
3-carb systems which require the more precise 1/32 inch lift to lean out
and test a single carb vs. the typical disable and adjust the other carb
approach previously used. Personally, I use the dashpot lifting only to
make sure that I have them both about the same.  Then I tune by reading the
plugs.  The mixture under load is more important than the mixture at idle
anyway. Concerning needle selection  There is a computer program that, if
you tell it what needle you have, will show you what needles are richer or
leaner in the various zones.  It is on the team.net ftp server.  Surf to:
http://HOOSIER.UTAH.EDU/www/archives.html and click on Automotive FTP
directories.  Then click on sol/   to open that directory and then click on
suneedle.exe.  This will download the compressed executable.  Run it and it
will install haystack.exe (i.e. needle in a haystack) in the current
directory.  If you have any trouble drop me a line and I can try to e-mail
you the program. I hope this makes sense.  - Regards, Bill Eastman




If you have any questions or comments send e-mail to: dick white
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