[E-Type] SU Fuel level height measured below carb bridge at the jet?

E-Type
1

Can anyone tell me how far the properly set SU fuel level should
be below the actual carb bridge?

I have discovered that, when properly set at the float, my tri carb
fuel levels (looking down into the jets) vary from 2 mm below to
‘‘cannot see!’’

I will have to weigh the floats but figure that I can custom set
each float bowl lever until I get the right fuel level depth below
the carb bridge.

Thanks as always–
Dennis Vancouver Canada 69 Roadster
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

2

In reply to a message from Dennismo sent Thu 12 Feb 2015:

Dennis,

That’s not a parameter I look at, but based on how
inaccurate the mechanisms are, I’d expect the height to
vary from one stop to the next. ‘‘Cannot see’’ means that no
fuel is getting into the bowl! Heavy floats would have the
opposite effect, no?

Very wide variations can result from the forks being loose
on their pivots. They can be tightened up with pliers.

Jerry–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

3

Some thoughts…

Difference in floats (per your mention)

Difference in fuel level needle assemblies

Leakage at one or more of the carbs

Sticking float(s) or fuel valve

Might try looking down the jets with the ignition switched on to make
sure that if there is a leak, it’s being topped up by the pump during
inspection.On Thu, Feb 12, 2015 at 4:08 PM, Dennismo dennismaxmolnar@gmail.com wrote:

Can anyone tell me how far the properly set SU fuel level should
be below the actual carb bridge?

I have discovered that, when properly set at the float, my tri carb
fuel levels (looking down into the jets) vary from 2 mm below to
‘‘cannot see!’’

I will have to weigh the floats but figure that I can custom set
each float bowl lever until I get the right fuel level depth below
the carb bridge.

Thanks as always

Dennis Vancouver Canada 69 Roadster
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

–
Les…'68 S1.5 2+2

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

4

In reply to a message from Dennismo sent Thu 12 Feb 2015:

Hi Dennis,

Good question. I have a distance between 6-8 mm below bridge.
Approx. I have set the float level from 3rd carb (near bulkhead)
lower because fuel level was higher then other 2 carbs. I guess it
makes a difference the floatbowl from carb 3 is at the rear/left
side as the floatbowls from cabrs 1 and 2 are on the front/reight
side of the carbs. My car/engine is not prefectly level so.

Richard–
The original message included these comments:

Can anyone tell me how far the properly set SU fuel level should
be below the actual carb bridge?
I have discovered that, when properly set at the float, my tri carb
fuel levels (looking down into the jets) vary from 2 mm below to
‘‘cannot see!’’
I will have to weigh the floats but figure that I can custom set
each float bowl lever until I get the right fuel level depth below
the carb bridge.
Thanks as always

–
E-type Series II RHD 2+2 1969 http://www.richardbrand.nl
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

5

In reply to a message from kingrichard sent Fri 13 Feb 2015:

Richard,

FWIW the float bowls are all completely isolated, so the
angle of the car has no effect on their relative fuel
levels. If that were not true, the car would always stall
on hills!

Jerry–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

6

In reply to a message from mouton sent Fri 13 Feb 2015:

Hi Jerry,

I really love to see someone explain that too me.

Because my car is not level (rear to high), I always noticed
the rear carb fuel higher in the jet,then the other two for
some reason. So I guessed it is the fact the bowl is on hte
other side of carb.

Richard–
The original message included these comments:

FWIW the float bowls are all completely isolated, so the
angle of the car has no effect on their relative fuel
levels. If that were not true, the car would always stall
on hills!

–
E-type Series II RHD 2+2 1969 http://www.richardbrand.nl
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

7

In reply to a message from kingrichard sent Fri 13 Feb 2015:

Richard,

First, fuel can’t flow from one bowl to the others. Fuel
always comes in from the pressured fuel gallery, and is
limited by the needle and seat/float.

If the car is tilted, the level of the ‘‘center’’ of the
float is maintained the same by the needle and seat.

But your guess looks good to me. The level in the bowl is
one thing, but the height of the fuel in the jet is
affected by a fore/aft tilt the car might have. Rear high,
the rear carb would be a little higher than the others –
when it’s parked. But your rear won’t be more than an inch
or so higher than the front, will it? The angle will be
tiny, and have little effect.

Once you get on the road, acceleration and braking will
have the jet level dancing all around and it won’t be the
same as when static. That’s why you should not try to get
the jet levels identical but set them at 7/16’’ (SUs) all
around. If you adjust the rear one lower to equalize the
level, on the road the effect will be of having the level
too low – it will be leaner. Tuning to this will get you
into (some little) trouble.

Jerry–
The original message included these comments:

I really love to see someone explain that too me.
Because my car is not level (rear to high), I always noticed
the rear carb fuel higher in the jet,then the other two for
some reason. So I guessed it is the fact the bowl is on hte
other side of carb.

–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

8

“…The level in the bowl is
one thing, but the height of the fuel in the jet is
affected by a fore/aft tilt the car might have. Rear high,
the rear carb would be a little higher than the others –
when it’s parked. But your rear won’t be more than an inch
or so higher than the front, will it? The angle will be
tiny, and have little effect…”

Jerry, given that all carbs will be at the same tilt angle, then all
carbs would be effected to the same extent wouldn’t they? And the
difference in height (not tilt) of one carb over the others, given
that they are on the same plane, level or not, would not be an
issue…right?On Fri, Feb 13, 2015 at 4:53 PM, mouton jerry@moutons.org wrote:

In reply to a message from kingrichard sent Fri 13 Feb 2015:

Richard,

First, fuel can’t flow from one bowl to the others. Fuel
always comes in from the pressured fuel gallery, and is
limited by the needle and seat/float.

If the car is tilted, the level of the ‘‘center’’ of the
float is maintained the same by the needle and seat.

But your guess looks good to me. The level in the bowl is
one thing, but the height of the fuel in the jet is
affected by a fore/aft tilt the car might have. Rear high,
the rear carb would be a little higher than the others –
when it’s parked. But your rear won’t be more than an inch
or so higher than the front, will it? The angle will be
tiny, and have little effect.

Once you get on the road, acceleration and braking will
have the jet level dancing all around and it won’t be the
same as when static. That’s why you should not try to get
the jet levels identical but set them at 7/16’’ (SUs) all
around. If you adjust the rear one lower to equalize the
level, on the road the effect will be of having the level
too low – it will be leaner. Tuning to this will get you
into (some little) trouble.

Jerry

The original message included these comments:

I really love to see someone explain that too me.
Because my car is not level (rear to high), I always noticed
the rear carb fuel higher in the jet,then the other two for
some reason. So I guessed it is the fact the bowl is on hte
other side of carb.

–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

–
Les…'68 S1.5 2+2

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

9

In reply to a message from Les Halls sent Fri 13 Feb 2015:

I believe that’s correct. Tilt will create a small
difference between level in the bowl and in the jet of each
carb independently. Acceleration/deceleration will also.

But I’ve never heard these differences to be considered
problematic. I have heard some concern regarding tilt of
the front carb on Mark Xs and similar that have a ‘‘hisser’’
enrichment device between the front and center carbs. The
bowl of the hisser is fed from the bowl of the front carb,
and the distance between them is about 8 inches. But those
hissers have so many problems that underfueling/overfueling
on an incline is low on the list.–
The original message included these comments:

Jerry, given that all carbs will be at the same tilt angle, then all
carbs would be effected to the same extent wouldn’t they? And the
difference in height (not tilt) of one carb over the others, given
that they are on the same plane, level or not, would not be an
issue…right?

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

10

In reply to a message from Les Halls sent Fri 13 Feb 2015:

Les,

You Weber guys wouldn’t know ; -) but the SUs are
configured with two float bowls in front of the carbs, one
to the rear. I believe this setup is to try to balance out
tilt and acceleration effects. So, the front two carbs
will have a somewhat lower jet fuel level, the one in the
rear would have a somewhat higher level if the rear sits
high. Going uphill or accelerating, the opposite would
happen.

Right, because the float bowls are completely separate and
controlled by the needle and seat and float rather than
anything happening to the other carbs, difference in height
would make no difference. Angle of the top surface of the
fuel would, though, quite apart from height difference.

The top surface of fuel in the jet is collinear with the
top surface in the bowls; liquid does not care about the
shape of the container. So if the bowl leans, the surface
of the fuel takes an angle, and the level in the jet goes
up or down a lot more than the fuel at the edge of the bowl

(Actually the vacuum of the carb will always pull the fuel
out of the jet; the level I’m talking about is the level
that would be there if there were no vacuum. The higher
the level, the more fuel flows out at a given amount of
vacuum. The lower the level, the less flows = leaner
mixture)

And again, higher stance in the rear would result in such a
small angle at the carbs that I figure the effect would be
minimal because of that. Not so minimal in the case of
acceleration or hills, though. I have not calculated the
angles, but imagine holding a glass of water on the console
as the driver ran through the gears flat out then braked
hard. Spill, yes indeedy!

Jerry–
The original message included these comments:

‘’…The level in the bowl is
one thing, but the height of the fuel in the jet is
affected by a fore/aft tilt the car might have. Rear high,
the rear carb would be a little higher than the others –
when it’s parked. But your rear won’t be more than an inch
or so higher than the front, will it? The angle will be
tiny, and have little effect…‘’
Jerry, given that all carbs will be at the same tilt angle, then all
carbs would be effected to the same extent wouldn’t they? And the
difference in height (not tilt) of one carb over the others, given
that they are on the same plane, level or not, would not be an

–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

11

Ah now I get it. The front carbs are essentially draining the jets because the bowls are downstream and the rear being upstream will do the opposite.

Seems like a weird solution, why not just have different jet/needle/float settings for the rear carb?Sent from my iPhone

On Feb 13, 2015, at 8:27 PM, mouton jerry@moutons.org wrote:

In reply to a message from Les Halls sent Fri 13 Feb 2015:

Les,

You Weber guys wouldn’t know ; -) but the SUs are
configured with two float bowls in front of the carbs, one
to the rear. I believe this setup is to try to balance out
tilt and acceleration effects. So, the front two carbs
will have a somewhat lower jet fuel level, the one in the
rear would have a somewhat higher level if the rear sits
high. Going uphill or accelerating, the opposite would
happen.

Right, because the float bowls are completely separate and
controlled by the needle and seat and float rather than
anything happening to the other carbs, difference in height
would make no difference. Angle of the top surface of the
fuel would, though, quite apart from height difference.

The top surface of fuel in the jet is collinear with the
top surface in the bowls; liquid does not care about the
shape of the container. So if the bowl leans, the surface
of the fuel takes an angle, and the level in the jet goes
up or down a lot more than the fuel at the edge of the bowl

(Actually the vacuum of the carb will always pull the fuel
out of the jet; the level I’m talking about is the level
that would be there if there were no vacuum. The higher
the level, the more fuel flows out at a given amount of
vacuum. The lower the level, the less flows = leaner
mixture)

And again, higher stance in the rear would result in such a
small angle at the carbs that I figure the effect would be
minimal because of that. Not so minimal in the case of
acceleration or hills, though. I have not calculated the
angles, but imagine holding a glass of water on the console
as the driver ran through the gears flat out then braked
hard. Spill, yes indeedy!

Jerry

The original message included these comments:

‘’…The level in the bowl is
one thing, but the height of the fuel in the jet is
affected by a fore/aft tilt the car might have. Rear high,
the rear carb would be a little higher than the others –
when it’s parked. But your rear won’t be more than an inch
or so higher than the front, will it? The angle will be
tiny, and have little effect…‘’
Jerry, given that all carbs will be at the same tilt angle, then all
carbs would be effected to the same extent wouldn’t they? And the
difference in height (not tilt) of one carb over the others, given
that they are on the same plane, level or not, would not be an

–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

12

In reply to a message from Les Halls sent Sat 14 Feb 2015:

I still don’t get it.

Tilt/acceleration/deceleration indeed affect the rear carb
differently, but is the effect significant? On the XK150S
and the Mark X/420G, the center carb’s float bowl is to the
rear, so that the front carb is the one affected differently.

I suppose you would want to enrich when accelerating (float
bowl in front?) or on an upward incline (float bowl in
front?) so perhaps the E-type setup is the better design?
But then why not have all three float bowls in front?–
The original message included these comments:

Ah now I get it. The front carbs are essentially draining the jets because the bowls are downstream and the rear being upstream will do the opposite.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

13

In reply to a message from Les Halls sent Sat 14 Feb 2015:

Les,

Because the normal operation of the car is constant speed
and level, and then you want the float bowls to be set
identically.

Robert,

I don’t know how significant the effect of angle and
acceleration is, but I would bet it basically makes no
difference at ‘‘normal’’ road angles. Acceleration I believe
would make a difference because fuel would be pouring into
the front carbs if there were no vacuum. And I am only
guessing that one carb works the opposite because if there
is a significant effect while they would like the
additional enrichment at acceleration, they would prefer
that the car not die during hard braking.

We did get some information here in the past about how much
difference in fuel flow small differences in jet fuel
height would make, and the difference was very small as I
recall.

Jerry–
The original message included these comments:

Ah now I get it. The front carbs are essentially draining the jets because the bowls are downstream and the rear being upstream will do the opposite.
Seems like a weird solution, why not just have different jet/needle/float settings for the rear carb?

–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

14

In reply to a message from mouton sent Sat 14 Feb 2015:

Hi Jerry,

Thanks for your new replies. At speed and under load, I
reckonn the difference won’t be noticable, but it’s at idle
when there is. My rear carb always is flooded. The level
difference between rear and front (vertical height) is about
2 cm’s.

I read some lister not adjustigin float level by using the
well know 7/16 rod but measuring the fuel depth in the jet
and trying them to get even. This is what Des Hammill also
mentions in his book

Richard–
The original message included these comments:

We did get some information here in the past about how much
difference in fuel flow small differences in jet fuel
height would make, and the difference was very small as I
recall.
Jerry

–
E-type Series II RHD 2+2 1969 http://www.richardbrand.nl
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

15

Robert,
This is my view of the car stationary on a flat surface…my
view isn’t a reflection on whether or not the placements of the bowls
and their relationship with the jets would effect the running of the
car, only that given their configuration, I can now see why the fuel
in the rear carb (given that the float bowl is to the back of the carb
body/jet) would tend to allow fuel to flow INTO the jet, while the
fuel in the front two carbs (given that the float bowls are toward the
front) would tend to allow fuel to flow into the bowls and so AWAY
from the jets.

If my conclusion is wrong then I totally misunderstood Jerry’s
explanation of why the rear carb is configured differently to the
front two.On Sat, Feb 14, 2015 at 10:08 AM, Robert Wilkinson wilk@wustl.edu wrote:

In reply to a message from Les Halls sent Sat 14 Feb 2015:

I still don’t get it.

Tilt/acceleration/deceleration indeed affect the rear carb
differently, but is the effect significant? On the XK150S
and the Mark X/420G, the center carb’s float bowl is to the
rear, so that the front carb is the one affected differently.

I suppose you would want to enrich when accelerating (float
bowl in front?) or on an upward incline (float bowl in
front?) so perhaps the E-type setup is the better design?
But then why not have all three float bowls in front?

The original message included these comments:

Ah now I get it. The front carbs are essentially draining the jets because the bowls are downstream and the rear being upstream will do the opposite.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

–
Les…'68 S1.5 2+2

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

16

In reply to a message from kingrichard sent Sat 14 Feb 2015:

Richard,

Based on my own experience, I believe your issue is
something else. If your rear carburetor is always flooded,
the needle and seat there is leaking, or your float is
sinking. I’m assuming your car’s angle at rest has nothing
to do with it. Believe me, I have had to change needle and
seat several times before I got one that did not leak.

Good luck with the fuel level adjustment!

Jerry–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

17

In reply to a message from Les Halls sent Sat 14 Feb 2015:

Les,

With the car stationary on a flat surface: What about the
rear carb would make the fuel tend to flow into the jet?
What would make the fuel in the front carbs would cause the
fuel to flow away from the jets?

Given that you have a non-running car, the fuel in either
carb is basically a puddle, sitting still and with the
surface perpendicular to gravity. No flow in any direction.

If the car is running, the vacuum in the carbs is sucking
the fuel out of the jets, so in each carb the flow is
toward the jet, because it’s being sucked out from there.
Isn’t the flow unaffected by whether the bowl is in front
of or behind the jet in static and level conditions?

The mixture is basically determined by how much fuel gets
sucked out based on the area between the jet and needle,
and the vacuum in the carb. How much fuel gets sucked out
at idle depends on the vacuum, and to some extent the jet
fuel level.

The vacuum has to overcome gravity to pull the fuel up from
its static level. How much difference does the jet level
of the fuel make? I am of the opinion that it does not
make much difference, but of course the float bowl level is
a critical setting so it must make some. However, the
float bowl level not only affects the fuel level in the
jet, but also the rate at which fuel is admitted when the
level goes down. Which is the most important? I don’t know.

I have always had at least one SU-equipped car since about
1962, so I have tuned a few; beyond that I don’t know
much. I do have some engineering education though. Not an
expert, YMMV.

Jerry–
The original message included these comments:

This is my view of the car stationary on a flat surface…my
car, only that given their configuration, I can now see why the fuel
in the rear carb (given that the float bowl is to the back of the carb
body/jet) would tend to allow fuel to flow INTO the jet, while the
fuel in the front two carbs (given that the float bowls are toward the
front) would tend to allow fuel to flow into the bowls and so AWAY
from the jets.

–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

18

“…With the car stationary on a flat surface: What about the
rear carb would make the fuel tend to flow into the jet?
What would make the fuel in the front carbs would cause the
fuel to flow away from the jets?..”

You’re right Jerry, the config really comes into play under
acceleration and deceleration, as you pointed out. Not sure what I
was thinking when I stated that (not unusual for me, there’s many
times when I’m not sure what I’m thinking). I still think it’s a
weird idea, trying to solve a problem that apparently by your and
Roberts’ estimation and perhaps others, doen’t really exist??

So, under deceleration then…again, flat surface…the rear
carb will tend to go rich and the front two tend towards lean…why
doesn’t this cause a problem…maybe because with little or no
vacuum the needles are pretty much closed and so any additonal fuel
will be minimal…and perhaps there’s enough fuel/air mixture
“bandwidth” at the static setting to cope with these slightly
off-kilter conditions?

And please, this is no place to be modest about your experience with
these things,…:-)…and I’m not challenging you at all Jerry,
just trying to understand a phenomena that I haven’t encountered
before…and apparently I’m not alone in this, old chap.On Sat, Feb 14, 2015 at 1:31 PM, mouton jerry@moutons.org wrote:

In reply to a message from Les Halls sent Sat 14 Feb 2015:

Les,

With the car stationary on a flat surface: What about the
rear carb would make the fuel tend to flow into the jet?
What would make the fuel in the front carbs would cause the
fuel to flow away from the jets?

Given that you have a non-running car, the fuel in either
carb is basically a puddle, sitting still and with the
surface perpendicular to gravity. No flow in any direction.

If the car is running, the vacuum in the carbs is sucking
the fuel out of the jets, so in each carb the flow is
toward the jet, because it’s being sucked out from there.
Isn’t the flow unaffected by whether the bowl is in front
of or behind the jet in static and level conditions?

The mixture is basically determined by how much fuel gets
sucked out based on the area between the jet and needle,
and the vacuum in the carb. How much fuel gets sucked out
at idle depends on the vacuum, and to some extent the jet
fuel level.

The vacuum has to overcome gravity to pull the fuel up from
its static level. How much difference does the jet level
of the fuel make? I am of the opinion that it does not
make much difference, but of course the float bowl level is
a critical setting so it must make some. However, the
float bowl level not only affects the fuel level in the
jet, but also the rate at which fuel is admitted when the
level goes down. Which is the most important? I don’t know.

I have always had at least one SU-equipped car since about
1962, so I have tuned a few; beyond that I don’t know
much. I do have some engineering education though. Not an
expert, YMMV.

Jerry

The original message included these comments:

This is my view of the car stationary on a flat surface…my
car, only that given their configuration, I can now see why the fuel
in the rear carb (given that the float bowl is to the back of the carb
body/jet) would tend to allow fuel to flow INTO the jet, while the
fuel in the front two carbs (given that the float bowls are toward the
front) would tend to allow fuel to flow into the bowls and so AWAY
from the jets.

–
Jerry Mouton '64 FHC 889791 ‘MIK Jaguar’
Palo Alto, California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

–
Les…'68 S1.5 2+2

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

19

In reply to a message from Les Halls sent Sat 14 Feb 2015:

Got it, I think.

To paraphrase: Because the engine is sloped, a particular
fuel level in the bowl, transferred to the jet, appears at a
slightly different height relative to the bridge. So the
rear carb would require a different setting of the jet for
proper idle mixture than the other two. Looking down into
the jets of perfectly tuned, completely identical carbs,
with perfectly adjusted float heights, one would expect to
see the same fuel level relative to the top of the jet in
all three. But the jets themselves would vary in their
distance below the bridge depending on float bowl orientation.

My XJ6 engine isn’t sloped very much. If the E-type is, then
measuring the difference in height between front and rear
damper caps, and the distance between the cap centers should
(with the two numbers expressed as a fraction) allow one to
calculate the slope of the engine. To predict the expected
vertical disparity in the bridges, just measure the distance
between center of float bowl and jet, multiply by two, and
multiply by the fraction obtained above.
Wouldn’t be much difference on the XJ; don’t know about the
E-type.–
The original message included these comments:

This is my view of the car stationary on a flat surface…my
view isn’t a reflection on whether or not the placements of the bowls
and their relationship with the jets would effect the running of the
car, only that given their configuration, I can now see why the fuel
in the rear carb (given that the float bowl is to the back of the carb
body/jet) would tend to allow fuel to flow INTO the jet, while the
fuel in the front two carbs (given that the float bowls are toward the
front) would tend to allow fuel to flow into the bowls and so AWAY
from the jets.

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

20

In reply to a message from Les Halls sent Sat 14 Feb 2015:

No, I think you’re right Les, because the manifold slopes.
But that brings up another way to look at it. If this made
any difference, why wouldn’t Jaguar just rotate the position
of the mounting bolts slightly so that the carbs were level
even if the manifold and engine slopes slightly.

Maybe they did and I never noticed. More likely it is as we
suppose…the effect is theoretical but not worth correcting.

But as stated the effect of hills and
acceleration/deceleration are a different matter.–
The original message included these comments:

You’re right Jerry, the config really comes into play under
acceleration and deceleration, as you pointed out. Not sure what I

–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

Search the archives & forums - http://search.jag-lovers.org/
Subscription changes - http://www.jag-lovers.com/cgi-bin/majordomo
Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php