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

E-Type
21

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

Les,
Going rich on deceleration isn’t the concern,
deceleration, like idling is a waste of fuel anyway. The
big issue is what happens on accelleration, namely leaning
out.
Based on my experience, mostly with 2 carb saloons, but
the physics stays the same:
With the car level on a flat surface, all things being
otherwise equal and correctly adjusted, the fuel level in
the jet tube should be more or less the same, but it isn’t
all that critical an issue. AFAIK, a 7/16 float lever
setting equates to about 1/4’’ below the bridge in the carb
throat. Yes, there will be a difference because of the
mounting angle of the engine.

If your fuel level is set too high, the fuel _will_

leak out of the jets where the float bowl is FORWARD of the
carb if you park nose uphill, or out of the jets where the
floatbowl is BEHIND the carb if you park nose downhill.
Similarily the mixture on each carb will tend to
richen/lean when accellerating or driving up an incline or
decellerating or driving downhill.

Having said that, I thought that the rearmost cylinder

(s) on the XK tended to run hot… why would they mount the
float chambers so that the rear carb leans out while
accellerating? Wouldn’t it be better to mount the rear carb
bowl so that it would tend to go rich under WOT?

As the Saloons have a balance hole in the intake

manifold which connects both plenums together the issue
isn’t as pronounced as it would be on an isolated carb
setup like the E - a Saloon will run fairly happily with
only one carb delivering mixture, albeit not very
efficiently, nor for an extended period, but it will run
and even be driveable.

Andrew–
The original message included these comments:

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

–
1968 3.8S
Zurich, Switzerland
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22

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

What about the hill’s effect on the floats themselves. They
are constrained to move in a direction that isn’t vertical
unless the car is horizontal. So the force causing them to
close the float valve is no longer g, but a fraction of g
depending on how steep the hill is. I think that means that
the fuel level would rise a bit regardless of the direction
of the incline.

Again, probably an immeasurable effect.–
The original message included these comments:

If your fuel level is set too high, the fuel _will_

leak out of the jets where the float bowl is FORWARD of the
carb if you park nose uphill, or out of the jets where the
floatbowl is BEHIND the carb if you park nose downhill.

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

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23

What’s quite clear is that the action of the car in various normally
operating conditions (up down accelerate decelerate), has an effect on
the carbs/jet fuel level that is inconsistent across all three carbs
and so effects the mixture being taken in by the engine that is also,
inconsistent across all three carbs. We have to assume that Jaguar/SU
engineers of the period understood this quite well and engineered the
setup to produce an operating mechanism (reverse the config of one
float bowl) that minimzed any effect on overall performance. Damned
good those guys!!On Sat, Feb 14, 2015 at 4:43 PM, Robert Wilkinson wilk@wustl.edu wrote:

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

What about the hill’s effect on the floats themselves. They
are constrained to move in a direction that isn’t vertical
unless the car is horizontal. So the force causing them to
close the float valve is no longer g, but a fraction of g
depending on how steep the hill is. I think that means that
the fuel level would rise a bit regardless of the direction
of the incline.

Again, probably an immeasurable effect.

The original message included these comments:

If your fuel level is set too high, the fuel _will_

leak out of the jets where the float bowl is FORWARD of the
carb if you park nose uphill, or out of the jets where the
floatbowl is BEHIND the carb if you park nose downhill.

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

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–
Les…'68 S1.5 2+2

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24

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

Within reason I don’t think the angle affects the closing
force on the jet… at 90 degrees, sure, you’re probably
going to observe a bit of leakage, due to the valve not
closing.

What definitely does affect the fuel level is the point at
which it can leave the enclosed system… Once the fuel can
start flowing out of the jet doesn’t that point determine
the level of the fuel?

If you leave the tap running in the bathtub what determines
the maximum fill level of the tub? (other than your
concern about having to re tile the damned floor).

Aside from that, the arms of the fork are fore/aft of the
motion c/l of the float, so as long as the fork arms
haven’t too much slop in them they will tend to compensate.

From memory the total arm width is about 1/3 of the float
diameter.

Actually, it is most probably an ‘‘angels on a pinhead’’
question, the point is,

a) what fuel level is high enough that
a1) the vacuum can suck the fuel out and
a2) the volume in the float chamber can flatten out
transient supply/demand hysterisis (This is a function of
the radio between the float chamber and jet annulus areas
and the reaction time of the pump to switch from deadheaded
to stroking methinks)

b) what level is low enough that fuel won’t leak out of the
jet annulus when parked on an incline.

Now all we have to do is define ‘‘incline’’ - this will
probably bring differing inputs from prarie gophers vs
mountain goats.

Andrew–
The original message included these comments:

What about the hill’s effect on the floats themselves. They
are constrained to move in a direction that isn’t vertical
unless the car is horizontal. So the force causing them to

–
1968 3.8S
Zurich, Switzerland
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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25

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

It can’t/couldn’t be all that much of a concern, otherwise
Rolls Royce would have come up with toroidal shaped float
chambers centered on the jet/needle centerline, or better
yet, gymballed toroids!

Andrew–
The original message included these comments:

inconsistent across all three carbs. We have to assume that Jaguar/SU
engineers of the period understood this quite well and engineered the
setup to produce an operating mechanism (reverse the config of one
float bowl) that minimzed any effect on overall performance. Damned
good those guys!!

–
1968 3.8S
Zurich, Switzerland
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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26

Haha! You’re right Andrew!Sent from my iPhone

On Feb 14, 2015, at 5:36 PM, JagWaugh andrew.waugh@bluewin.ch wrote:

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

It can’t/couldn’t be all that much of a concern, otherwise
Rolls Royce would have come up with toroidal shaped float
chambers centered on the jet/needle centerline, or better
yet, gymballed toroids!

Andrew

The original message included these comments:

inconsistent across all three carbs. We have to assume that Jaguar/SU
engineers of the period understood this quite well and engineered the
setup to produce an operating mechanism (reverse the config of one
float bowl) that minimzed any effect on overall performance. Damned
good those guys!!

–
1968 3.8S
Zurich, Switzerland
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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27

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

HaHaHa. That’s giving them a lot of credit.

But perhaps you’re right. Most SU HD applications are twin
carb, so according to your logic I presume there should be
one in each orientation. I just tried to google up some
photos. Both bowls are in front on the SS and the 120. Looks
like they are opposite on the XJ6. They would have to be
opposite on cars fitted with starting carbs, to make
room–so that proves nothing. So maybe they figured it out,
but definitely not initially.

OTOH, the HIF carbs eliminated this issue…maybe that was a
rationale for their introduction.–
The original message included these comments:

inconsistent across all three carbs. We have to assume that Jaguar/SU
engineers of the period understood this quite well and engineered the
setup to produce an operating mechanism (reverse the config of one
float bowl) that minimzed any effect on overall performance. Damned
good those guys!!

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

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28

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

That and emissions/fuel leaks. Around 1967 Amal the bike
carb brand switched from the monobloc carb (float bowl
alongside slide and needle jet) to the concentric version
which like the HIF and Stranglebergs has the fuel bowl
below and around the jets (with a roughly toroidal float).

The concentric was more compact and could be fitted at
increased angles without leaning or enriching the mixture.
Lateral G was only an issue on sidecar outfits which could
have monoblocs with a float bowl each side.–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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29

“…But perhaps you’re right. Most SU HD applications are twin
carb, so according to your logic I presume there should be
one in each orientation…”

Don’t know Robert, it probably depends on how many cylinders and angle
of engine etc etc. And with only two carbs maybe it’s because they
are both close to the center of the engine and so any effect on the
carbs would be almost exactly the same and all cylinders would respond
accordingly…who the hell knows, right.

While we were wading through this it occured to me that this problem
goes away with carbs like the webers due to the location of their
float chabers…then Pete chips in with more or less the same
thoughts re Amal carbs. I guess finally they got the msg. However, I
don’t recall anyone posting about interminable problems with over-rich
or overlean cyliners 1 and 2 on the E, so this is probably all a storm
in a teacup and its value is more theoretical than anything else.On Sat, Feb 14, 2015 at 5:48 PM, Robert Wilkinson wilk@wustl.edu wrote:

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

HaHaHa. That’s giving them a lot of credit.

But perhaps you’re right. Most SU HD applications are twin
carb, so according to your logic I presume there should be
one in each orientation. I just tried to google up some
photos. Both bowls are in front on the SS and the 120. Looks
like they are opposite on the XJ6. They would have to be
opposite on cars fitted with starting carbs, to make
room–so that proves nothing. So maybe they figured it out,
but definitely not initially.

OTOH, the HIF carbs eliminated this issue…maybe that was a
rationale for their introduction.

The original message included these comments:

inconsistent across all three carbs. We have to assume that Jaguar/SU
engineers of the period understood this quite well and engineered the
setup to produce an operating mechanism (reverse the config of one
float bowl) that minimzed any effect on overall performance. Damned
good those guys!!

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

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–
Les…'68 S1.5 2+2

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