[xk] Data from testing with a wide band O2 sensor

XK
1

I finally gave in and installed an O2 sensor in my 2 down
tubes (XK120 SE).

Fr the 10th time, motor=xk140MC , newly rebuilt with 9:1
pistons and new cams. I moved my stock xk120 intake/exhaust
system onto this motor.

You’ve already heard ad nauseam that I’ve been struggling
with a lean/rich issue between idle/low speed and high speed
(medium throttle). Plugs are black and quite sooty around
town but light and clean on the highway.

Anyway, I’ve only finished testing the front pipe/carb. Yes
I know there’s a crossover in the intake manifold, and I’m
ignoring that for the moment. The carbs were fitted with
‘‘RC’’ needles and then tuned per the manual. The ignition
system and all is is tuned up per the manual and all seems
to be running correctly other than a slight misfire at very
high rpm (>5000).

Results with RC needle:
IDLE: AF ratio is at 12:1. The pin lift test per the manual
yields the desired up and then down/rough rpm dynamics. Of
course, there’s the question of how much to lift the pin.
Some say a very small amount (1/32’’ to 1/16’’) and others say
lift it all the way up. I lifted it all the way. I then
leaned the idle by 1/4 turn, which brought the ratio up to
around 13.4:1. At this mixture ratio, lifting the pin all
the way up now causes the rpm to immediately fall and the
engine very nearly stalls. Stochiometric is 14.ish:1 . So my
idle is still rich but the pin lift test says ‘‘too lean.’’

LOW SPEED , low to medium throttle(under 2500rpm): AFR stays
pretty much between 10.5:1 and 11.5:1. Interesting to me is
that when I mash the pedal to the floor at 1500rpm, the
mixture goes about 0.5 richer but then leans back out to
about 11.5:1

HIGH SPEED, constant rpm (highway cruising at 3000-3500rpm).
Ratio stays around 11:1. Seems quite rich to me.

WOT (WIDE OPEN THROTTLE) from 1500rpm to 5000rpm. AFR starts
at around 11.5:1 but from 2500rpm to 5000rpm it leans out to
13:1.

I went home and swapped back to my original/old RF needles.
Checked idle and the ratio was still about 13.5:1.

Drove the same route and …
LOW SPEED , low to medium throttle(under 2500rpm): AFR stays
pretty much between 11.5:1 and 12.5:1. Interesting to me is
that when I mash the pedal to the floor at 1500rpm, the
mixture goes about 0.5 richer but then leans back out to
about 12.5:1. Doesn’t this seem a tad rich?

HIGH SPEED, constant rpm (highway cruising at 3000-3500rpm).
Ratio stays around 12:1 to 12.5:1. Seems a tad rich to me.
When I went to British Invasion with the RC needles, I was
getting around 15mpg (11:1 ratio). If I could get it to 14:1
I should see a 25% improvement in fuel consumption (creeping
up on Tom Brady and that fat old Mk IX that goes faster than
it should).

WOT (WIDE OPEN THROTTLE) from 1500rpm to 5krpm. AFR starts
at 12:1 and above 2500rpm leans out to 13.8:1.

I will look for a needle that’s leaner at low dash-pot
height and richer as the dashpot goes up. Anyway, I found
all of this pretty interesting…–
John
Boston, MA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

2

In reply to a message from John sent Sun 5 Oct 2014:

What I’d like to see is AFR around 13:1 for the WOT test and
never below about 12.5:1. So, I need a needle that is as
lean as the RF (or a tad leaner) for low speed/throttle, but
one that is about like the RC at higher speed/throttle.

The only other measurement I wish I had was dashpot height
so I knew what part of the needle needs to be different.

John–
John
Boston, MA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

3

In reply to a message from John sent Sun 5 Oct 2014:

Hi I am Wolfgang from Germany and you have to excuse my
poor english. I am driving a EV12 with a 6,0 L motor from
a '93 Limousine with 4 stromberg carbs. The car is driven
often on german highways. Most of the time with 70 - 80
mph, sometimes 100 up to 125 mph. Roundabout 5000 miles per
year, since 4 years.
The 4 carb needles are tailor-made by myself, after testing
the exhaust gases with a innovate lambda measuring-system.
The AFR is:
Roundabout 14 - 14,7 at idle. The idle is PERFECT.
Lean with roundabout 15,8 - 16,2 at part throttle and
steady speed between 40 and 125 mph.
Rich with AFR 13 - 14 at a speed of more than 130 mph.
The coulour of the plugs is bright gray to gray. Never
bright brown. The motor is healthy. The gas consumption
while cruising with 60 - 80 mph with a 2,88 rear axle and a
Getrag 5-speed with 0,81 is roundabout 13 L / for 100km,
or 21,4 mpg (if 1 gallon = 4,5 L ?). If a gallon has 3,8 l
this is 18 mpg. This a very low gas consumption for a V12
with roundabout 310 - 320 DIN-hp. A friend has a modern
Maserati Quattroporte with 390 hp. There is now difference
in the acceleration from 30 - 100 mph between his Masarati
and my EV12 cabrio. Therefore the jag is pretty strong.
Therefore I think the carbs of your car are much too rich.
I know, that in the USs the carbs are often set rich. I
have made the experience, and also red in technical books,
that at part throttle the carbs should be set to a Lambda
of 1,1 or AFR 16,2, bcause a low gas consumption.
Often you hear or read, that with such a lean mixture, the
motor will become too hot and burn the valves or the
pistons. This is wrong, because at part throttle, the
burning of a motor is still far away from beeing hot. You
can learn a lot about this, if you read instructions, how
to adjust the mixture of a small piston-motor plane by hand.
To adjust a perfect micture for your car, you need
knowlede, a simple innovate lambda measuring system for
roundabout 250 bucks and a book from Des Hammill about SU-
carbs. In this book is very clear explained, how to ‘‘shape’’
the needles of a SU-carb with sanding-paper. For SU-carbs
there are also available a lot of different needles.
Therefore it should be possible, just to find a
good ‘‘fitting’’ needle for your requirements. There is a
small program on CD-rom existing, whioh helps you, to find
the corrects needle of Su-carbs.
If somebody tells you, plugs should be middle-brown, tell
him, this common fairy tail is 50 years old. My car is a
strong runner, and has absolutely now problems with the
motor because of lean running at part (!) throttle. At full
throttle I have Lambda 0,9 for max power and inside cooling
of the cylinder. Also at idle, rich mixture helps for a
good idle.
Roger Bywater told me, that the 5,3 V12 HE-motor with
12,5:1 in the beginning was running with a AFR of 18:1.
Later Jaguar changed to AFR 14,7, because of emission-laws.
Regards Wolfgang Gatza–
The original message included these comments:

LOW SPEED , low to medium throttle(under 2500rpm): AFR stays

–
Wolfgang Gatza
Dreieich, Germany
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

4

In reply to a message from Wolfgang Gatza sent Mon 13 Oct 2014:

Wolfgang
Thanks for the reply.
I too used an Innovate sensor (MTX-L).
I agree that the mixture is far from ideal. My spark plugs
are very black when I drive in the city. I don’t like
polluting unnecessary and no reason to waste fuel if there
is a reasonably easy way to optimize the needle.

Anyway, I can measure my existing needles and calculate
the optimal diameter. What I now need is a reference that
can find me the closest needle with any missmatch to be on
the thick side. The I would cut or sand the needle as you
did to adjust any mismatch. My goal is to optimize power
and smoothness, followed by fuel consumption/ pollution.–
John
Boston, MA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

5

Hello John !
I don’t know, if there are thicker needles, you can sand “thinner”, that they will match your motor.
There is another way to reach the goal:
3. Buy 4 new needles and 4 new “Duesen” (i don’t know the correct wod in the moment) and mount them. Make another test ride and check the gauge.
2. If the mixture is still too fat, you can make a 45mm wide hole longline inside the two front silencers. This will weaken the mixcture ( I have testetd this and don’t tell rubbish). Make a testride again.

  1. Also you can mount a bigger air trumpet in front of the air filter box. This will weaken also the mixture.
  2. How are your air filters ? If dirty mount two new.
    I have make the hole into inside the 2 silencers and mounted bigger trumpets. Then I have measuered the mixture. The mixture was much too weak. The difference is big at full throttle and very small at iddle. Thenyou have to “reshape” the needles with a very small amount, mount them and make another test ride.
    If still too weak, sand them again plus another test ride. It is one full day of work (maybe more) , but at the end of the process, your car is running better, sounding a little bit more “sporty” and takes less gas. There is NO ather way to reach this goal. I don’t think, that you can find a needle for the Stromberg, which is fitting good.
    I have done this and I am very happy with the result.
    Regards Wolfgang Gatza
6

I saw once , and possibly still have somewhere a chart of all the SU needles and their diameters by increments.[ and letters]
The person who made the serious SU carbs for Midel and Burlen used to to cars on the road, he had as mall portable analyser and sat in the passengers seat while one drove his selected route which covered various types of driving , a long hill , a straight etc etc and when he got back , he’d pull the needles and comparing the chart would urn down needles st the appropriate points and do it again till he was happy… It worked very well , I didn’t think the needles chart wold be anything sectret but can’t find anything like it on line , or at least anything free.

7

That chart is in the Haynes manual for SU carbs. Haynes got it from SU Birmingham.

8

I ended up using an interactive needle selection tool via the SU Carbs website. Very helpful in picking needles with specific characteristics. Howeve, after a year with a needle that provided close to 14:1 mixture at idle and light throttle, I’ve gone back to the original needles. The originals are rich at idle and low speed and lean out at higher rom. However, the originals provide a much more stable and robust idle. I verified everything with an ultra wide base O2 sensor under real world driving conditions. I can’t explain why the engine likes to idle rich.