[xk-engine] triple Weber 45 DCOE - suitability and setup

XK-Engine
1

I know that there have been several discussions on this forum in
the past regarding the topic of weber installation. I would like to
ask anyone who has installed weber DCOE 45s 152s whether they are
experienceing any low rpm ‘lean-out’ on tip in/acceleration. This
apppears to be the most common complaint with these carbs and is a
problems that has bedeviled me and some of my collegues that have
installed these carbs. I would like to brain storm this problem
with anyone that is experencing the same problem with this setup.–
Anthony Strelzow
Vancouver, British Columbia, Canada
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2

In reply to a message from Anthony Strelzow sent Sun 16 Dec 2007:

Hi Anthony,

After installing many Weber setups on a number of cars including my
own 4.2 I’ve found one needs to rejet using a wideband air/fuel
meter or equivalent. Most ‘‘kits’’ are horribly misjetted. This
process is time consuming and expensive but it gets the job done
and is best done by someone that has considerable Weber experience
and several thousand dollars of jets in stock.

That being said, no matter how good your jetting is, if you open
all that carburetion up suddenly at idle you kill the velocity in
the ports. That lets the fuel drop out of suspension in the airflow
and you get a shot of air with liquid fuel on the port walls which
creates the stumble, bog, and stall. DCOEs are best for midrange
and high rpm. Don’t expect miracles with full throttle off idle.

Regards,
Rich Olson '67 OTS–
Richard Olson
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3

Anthony,
I’ve got a set on a 4.2L 9:1 sedan motor. Fitted with some old GTJ cams. No
problems with lean condition at tip off idle, maybe a little rich in fact.
Care to share your jet complement with us?

Bob Grossman-----Original Message-----
From: owner-xk-engine@jag-lovers.org [mailto:owner-xk-engine@jag-lovers.org]
On Behalf Of Anthony Strelzow
Sent: Saturday, December 15, 2007 5:00 PM
To: xk-engine@jag-lovers.org
Subject: [xk-engine] triple Weber 45 DCOE - suitability and setup

I know that there have been several discussions on this forum in

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4

In reply to a message from Anthony Strelzow sent Sun 16 Dec 2007:

I think Richard’s advice is the best Anthony, but to get you to a
reasonable point in advance of the rolling road, or whatever, try
contacting Paul Wigton on the E-type forum.

He has a lot of experience with Webers and could tell you what
parts to request from your supplier so you at least start off with
an approximately good set-up before you’ve bought a single extra
jet or emulsion tube. If you’ve already bought the kit ready-jetted
he may be able to tell you which unused parts to swap for credit
against replacement new parts if your carb supplier will cooperate.
It’d be nice to read how you get on.–
The original message included these comments:

the past regarding the topic of weber installation. I would like to
ask anyone who has installed weber DCOE 45s 152s whether they are
experienceing any low rpm ‘lean-out’ on tip in/acceleration. This
apppears to be the most common complaint with these carbs and is a
problems that has bedeviled me and some of my collegues that have
installed these carbs. I would like to brain storm this problem
with anyone that is experencing the same problem with this setup.

–
66 2+2, 73 OTS, 76 DD6 Coup�, 93 XJ12
Cambridge, United Kingdom
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5

In reply to a message from PeterCrespin sent Mon 17 Dec 2007:

Thank you to Richard, Bob and Peter for your replies and
suggestions. The weber setup on my 3.8 e-type with a mildly tweeked
cam (261deg at .436’’ lift) and 9.2:1 compression is as follows:
DCOE 45; 36mm main and 4.5mm aux venturi
55-F8 idle circuit jets
.50mm accel jets.
Main jet is 145 , emulsion= F11 and 210 air corrector.
I have the luxury (and curse)of two installed wide band Lamba
sensors (front and aft header branches)that allow me to monitor the
AFR (air to fuel ratio) at all rpms. Trouble is, it allows me to
see in detail what would usually escape most casual tuners.
Consequently, although I can tune to a reasonable idle and low
speed circuit performance with minimal spit back / lean stumble on
low speed circuit tip in, this setup is invariably too rich. AFR
readings are typically at 11.5:1 or less throughout the low speed
range of idle � 2500 (after which the main circuit starts to come
on stream. Most weber tuners use a richer than necessary idle
circuit jet setup to prevent the lean stumble I described before.
However, fuel economy and power output suffer. When the main
circuit (above rpm of @2500) is active �tip in� lean stumble is not
an issue primarily due to the strong �signal� formed by the high
velocity and mass of air drawn through the choke . Throttle changes
are followed by an immediately response to enrich the mixture on
demand. Also the emulsion tube well has a reservoir of fuel-air
mixture that is immediately available when needed. The slow speed
circuit lacks both of these features and there appear to be some
inherent design deficiencies peculiar to the series 152 of DCOE
45. Hence I certainly understand why so many others, like me, have
had a frustrating time getting these carbs to run well at low rpms.
The questions I have for those who have tried wrestling with these
carbs:
#1/- Has anyone noticed improvement after installing velocity
stacks (50mm or greater). The idea here being to gain greater
laminar flow and increased velocity past the transition hole
drillings.
#2/-Has anyone tried biasing the fuel and air drillings on the jets
towards larger sizes to gain more mixture volume in the transition
pot/reservoir. (example: 65 F2 jets where the ratio of air to fuel
drilling cross sectional area might be similar to a 40F9 jet).
#3/-Has anyone tried drilling a fourth transition hole to
increase/smooth the enrichment on throttle opening. I have in fact
done so but would like to hear from anyone else that has tried this
and hear their feedback. I am happy to share my experience doing
this modification (which proved very favourable) provided there is
any interest in hearing about it. It will take more space than I
have left on this post.
Looking forwards to hearing any feedback.
Cheers:–
The original message included these comments:

I think Richard’s advice is the best Anthony, but to get you to a
jet or emulsion tube. If you’ve already bought the kit ready-jetted

experienceing any low rpm ‘lean-out’ on tip in/acceleration. This
Cambridge, United Kingdom

–
Anthony Strelzow
Vancouver, British Columbia, Canada
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6

In reply to a message from Anthony Strelzow sent Wed 19 Dec 2007:

Anthony,

Sorry, or rather pleased, that I don’t have the problem but
have been up the Weber learniong curve over the last 4 or so
years.

#3 is an option that is commonly employed by a couple of
sources - I think the 45 DCO 9’s had 4 progression holes. It
seems to be an effective fix to the issue.

I am running Webers (152’s with three holes) on a 4.2
engine, 9.4:1 compression and a Rob Beere full race cam 310
duration (although don’t know how measured) and .445 lift,
1.8 inlet valves and 1.625 exhaust valves. So is probably
sucking more air than the 3.8 so the specs may be irrelevant.

Also it is in my repro D-Type and the exhaust headers are
over the frame and 2’’ OD. So the breathing is probably a bit
different on the output side which no doubt influences the
air flow on the input side. One recommendation was to use
3.5 auxiliary venturies (instead of 4.5’s). Rest is 38 mains
and using 65F8 idle jets, main and airs are 160 and 190 in
F2 emulsion tubes. Don’t have the benefit of lamda sensors
but CO levels at idle (if the gas analyser is correct) is
around 2 which is surprising based on everything I have
read. I have used color tunes to set the idle mixture, you
get to see how even 1/8th of a turn makes a significant
difference, and have also just set them all the same based
on what sounds best. One thing the color tunes tell you is
if one of the chokes is significantly different which I
think could be an issue if you are, as I read it either only
sampling one of three of the front and rear exhausts or
maybe it is that you are averaging the front three and the
rear three. Haven’t worked out how to read them under load
and trucking down the highway though. My experience is that
if too lean they spit back at steady throttle just before
the main circuit starts to kick in. So a blend of a bit of
technology and seat of the pants.

I have run with velocity stacks but not having the problem,
cannot comment. Don’t routinely use them as I don’t want to
drag small dogs, birds and stones into the engine - the
latter is real, but I really need shorter than stock to get
adequate clearance on the bonnet in one place.

I did get some 70F8’s made at one point but the individual
jet specs is way beyond me.

Regards

Keith

Tempero D-Type–
The original message included these comments:

#1/- Has anyone noticed improvement after installing velocity
stacks (50mm or greater). The idea here being to gain greater
laminar flow and increased velocity past the transition hole
drillings.
#2/-Has anyone tried biasing the fuel and air drillings on the jets
towards larger sizes to gain more mixture volume in the transition
pot/reservoir. (example: 65 F2 jets where the ratio of air to fuel
drilling cross sectional area might be similar to a 40F9 jet).
#3/-Has anyone tried drilling a fourth transition hole to
increase/smooth the enrichment on throttle opening. I have in fact
done so but would like to hear from anyone else that has tried this

–
Keith Bertenshaw
Rockaway, NJ, United States
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7

In reply to a message from Anthony Strelzow sent Wed 19 Dec 2007:

Hi Anthony,

Looking at your jetting it seems to me that most of what you’ve got
is in the ballpark. I might try an F16 emulsion tube. You have to
run some kind of velocity stack with Webers. The longest that will
give you 1’’ clearance with your air cleaner lid is probably best.
You might try a 55F9 idle jet and a smaller pump jet maybe a .45.
Look at your transition holes and see where your throttle plates
are. They should be before the first hole. If not, your tune is off
and the transition phase won’t work well. This is why a big box of
jets is required to get it right. One jet affects another and your
engine is unique. Check your float levels, your idle mixture sounds
rich for those jets. Most people use the settings for the old DCOE
9s which are way off for the DCOE 152s.

Regards,
Rich Olson–
Richard Olson
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8

In reply to a message from Richard Olson sent Thu 20 Dec 2007:

Thank you , Richard and Keith. Your comments are well taken and you
clearly have both been down this road before.
I have to tell you that during the restoration of this series 1 XKE
I had discussions with the restorer about my wish to keep the
original SUs in place. Having had to deal with Webers on an old
Alpha, Lotus Elan and early 911S I was wary about having to deal
with these again. Against my better judgment I relented and we went
ahead with the installation on the premise that since we had
reworked the engine so much even the SUs would have needed a lot of
sorting (needle re-profiling etc) to get the tune right.
What surprised me though, has been the relative poverty of
information regarding the Weber 152s�. The older DCOE 45 9s are
well documented and have been made to work reasonably well on jags
(I have personally seen several very successful installations). But
the new 152s�are a mystery. Stories abound about their having been
originally designed to work on smaller 4 cylinder engines (hence
the peculiar staggered 3 hole progression drilling pattern). But no
one seems to know for sure. Everyone agrees they are a problem on
jags with �lean stumble� on tip in at low rpms being the biggest
headache. But none of the usual sources for answers ( suppliers
such Pierce, Webcon, Weber North America, etc) have any specific
suggestions. Even my restorer, who claims to be THE leading expert
on Jaguars in North America (an outfit out of Austin Texas that
shall remain nameless) was at a complete loss as to how to tune
these carbs. His best effort resulted in a steady idle that evolved
into a spitting and hiccupping progression phase and an eventual
black soot belching upper end as one climbs the rpm band. (This was
after a restoration the cost of which could have purchased a new
Aston Martin with change left over! Anyone interested in details on
how to avoid pitfalls in a �full on� restoration are free to
contact me personally and I would be happy to recount the abysmal
and the sublime that can come from such efforts.)
So far the most productive �adjustment� I have made to improve the
off idle response has been drilling a fourth progression hole (I
have precise diagrams and specifications for those seriously
thinking of attempting same). The Idea came to me via the busy
people at the Alpha Romeo Forum who are way ahead of the curve with
these carbs due to their constant fiddling with all varieties of
things Italian. They have been drilling these holes for some time
and reported the same improvements that I have witnessed
afterwards. Unfortunalty, this modification does not cure the
problem entirely. Although AFR ratios are much improved though out
the lower rpm range and acceleration is less �ragged� and stronger
there still remains some lean stumble. This is particularly
aggravated under load.
------more on next post-----------------
Anthony Strelzow
Vancouver, British Columbia, Canada
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9

In reply to a message from Anthony Strelzow sent Sun 23 Dec 2007:

------------- from previous psot-----------
So far the most productive �adjustment� I have made to improve the
off idle response has been drilling a fourth progression hole (I
have precise diagrams and specifications for those seriously
thinking of attempting same). The Idea came to me via the busy
people at the Alpha Romeo Forum who are way ahead of the curve with
these carbs due to their constant fiddling with all varieties of
things Italian. They have been drilling these holes for some time
and reported the same improvements that I have witnessed
afterwards. Unfortunalty, this modification does not cure the
problem entirely. Although AFR ratios are much improved though out
the lower rpm range and acceleration is less �ragged� and stronger
there still remains some lean stumble. This is particularly
aggravated under load.
To try and gain improvements from this point I have been
concentrating on increasing the �signal� strength of the airflow as
seen at the progression ports and idle jets. Hence the idea of
velocity stacks. Also I have toyed with the idea of reducing the
main venturi to 34mm as a concession to upper end power to gain low
end tractability. Finally, and I have not seen any mention of the
veracity of this approach, I have thought of increasing the
saturation of the progression pot with mixture. I had thought that
by increasing, proportionately, the air and fuel drillings on the
idle jets, I would be able to have more mixture available at the
progression drillings when needed on throttle movement. The idea
would be to both enrichment the fuel flow (larger diameter
drilling) and lean out the air flow (larger diameter drilling) but
doing so proportionately to try and maintain air:fuel ratios in the
right �zone�. I am afraid, however, that this will mean drilling my
own custom jets to get the right mixture blend.
Any thoughts on these gambits? Is there anyone else out there who
has tried out any of these ideas? Also, I have exhaustively data
logged most of this development work (I have a realtime data logger
with which I am able to log manifold pressure (IMP), throttle
position (TPS), air:fuel ratio in the exhaust headers, exhaust gas
temperature (EGT), accelerometer etc. This information has been
very valuable in setup the carbs, ignition and ignition advance. If
there is anyone else who has a similar facility and would like to
see or exchange this information-please let me know.
Cheer:
:Tony:–
Anthony Strelzow
Vancouver, British Columbia, Canada
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10

In reply to a message from Anthony Strelzow sent Sun 23 Dec 2007:

Tony,

If I’m reading this thread correctly, you’re OK with your idle
quality which is governed by idle jet and screw adjustment. You
didn’t indicate where your throttle plate edges are with respect to
the progression holes so it’s unclear to me what’s going on with
the transition from idle to progression circuit. Assuming you’re
happy with the progression phase itself, is your problem with the
transition from progression to the main circuit? That’s governed by
auxillary venturi size and emulsion tube calibration ie. when the
main circuit becomes effective.

Decreasing the venturies to 34MM will increase your signal and
drivabilty albeit with a decrease in top end performance above
about 4500 rpm and may require a major rejetting to compensate.

Webers must have a velocity stack to provide the laminar flow that
doesn’t disturb the metering. Even if you run air cleaners (I hope
you are), they must be inside. Longer is better as long as you
don’t get closer than 1’’ of your lid.

Drilling a fourth progression hole is effective especially on a
small displacement engine such as BMWs and Alfas. These engines
have less of a signal to work with and it helps lengthen the
progression phase until the mains come in.

I find it hard to believe that you would need to custom drill your
own idle jets. There’s such a range of fuel/air bleed combinations
that you sould be able to dial it in.

Don’t forget your accelerator pumps. Those are usually
miscalibrated in the Jag installations I’ve worked on.

Finally, your A/F ratios are probably the most relevant data you’re
looking at. Don’t be fooled by lean spikes shown by O2 sensors that
are the result of over-rich misfires.

Regards,
Rich Olson '67 OTS–
The original message included these comments:

So far the most productive �adjustment� I have made to improve the
off idle response has been drilling a fourth progression hole (I
have precise diagrams and specifications for those seriously
thinking of attempting same). The Idea came to me via the busy
people at the Alpha Romeo Forum who are way ahead of the curve with
these carbs due to their constant fiddling with all varieties of
things Italian. They have been drilling these holes for some time
and reported the same improvements that I have witnessed
afterwards. Unfortunalty, this modification does not cure the
problem entirely. Although AFR ratios are much improved though out
the lower rpm range and acceleration is less �ragged� and stronger

–
Richard Olson
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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11

In reply to a message from Richard Olson sent Sun 23 Dec 2007:

Once again Richard Olson provides some sound advice.
Rich misfires show up lean with oxygen sensors due to all the
unburnt oxygen that goes down the pipe - a four gas analyser and
someone who can use it will help to decipher the exhaust gas brew.

A simple way to test the idle/progression mixture is to screw up
the idle speed until just before the main jet is coming in to play
(say 1800rpm) and then turn the idle mixture screw (about 1/4 turn)
from the point where it has been set for best idle. If the
idle/progression mixture is right turning the idle mixture screw
either way will see a drop in rpm from the theoretical 1800rpm,if
however an increase in rpm results from turning the idle mixture
screw in/down(leaner) then the idle/progression mixture is rich and
vice versa.

What about the accelerator pump bleed jet?What size is it running?
Fuel needed for acceleration could be being bled back into the
float chamber.

A problem I have heard about with SOME of these DCOE 45 152
Webers,which I think will not be causing the problems discussed
here,but ,for what it’s worth, is that the fuel passage in the
carburettor body to the main jet/emulsion tubes is smaller than on
earlier carburettors .
All that said these XK engines need a rich mixture to idle smoothly
whether running S.U.s,Webers or fuel injection.–
The original message included these comments:

Don’t forget your accelerator pumps. Those are usually
miscalibrated in the Jag installations I’ve worked on.
Finally, your A/F ratios are probably the most relevant data you’re
looking at. Don’t be fooled by lean spikes shown by O2 sensors that
are the result of over-rich misfires.

–
d r
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