My twin ZS carbs do not have the bypass valves installed (they are
blocked off by a plate). The carb insulation blocks are notched/cut-
out for air flow from the rear of the carb into the manifold. Does
anyone know if these air ‘‘vents’’ are needed for carbs without
bypass valves? I have several sets of gaskets for these blocks,
some are solid (that block off these vents) and some are notched to
allow air flow around the carb body. Any help out there? I’ve just
rebuilt the engine and noticed when reinstalling the carbs these
notches. The car doesn’t want to idle well (unless the choke is
operated) but at high speed, runs great. The problem is related to
the carbs.
Jim
'68 2+2–
Jim Tison
Cumming, Georgia, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Jim Tison sent Wed 29 Dec 2004:
Jim,
The bypass valve is a bit wierd. The gasket that normally
goes under the valve has a ‘‘leak path’’ that goes between the
two holes and, for the XKE, needs to be there. One of the
holes this leak path goes to is the hole your are concerned
about. You may want to confer with Joe Cuerto about this,
but I beleive that if the leak path is not maintained, you
idle will be screwed up. So, my belief is that you need the
gasket with the leak path under the block off and you need
the insulator and gaskets with the cut outs.
Why did you blank off the bypass valve? If the diaphragm in
it is intact, it can’t do much harm. Personally I would put
it back on and adjust it down if you want to.
Another item that can ruin you idle is the really bad damper
diaphragms that are on the market lately. They apparenetly
are not made of a material that can handle the gas fumes or
heat or both and tend to crack and tear very quickly and
thus screw up your idle.
Richard Liggitt–
'70 E Roadster 1R11998, '98 XK8 Roadster, '02 Mini Cooper
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Jim Tison sent Wed 29 Dec 2004:
ZJim,
I just reviewed this and I need to correct myself. I thought
we were talking about a later car. On a '68 the rear bypass
valve is missing (blanked off). The front one should be
there and connecting to the manifold through an ‘‘elbow’’ hose
directly from the bypass. Since the bypass valve on the
front carb does not vent into the manifold through the carb
body and the rear carb does not have a valve, you do not
need the gaskets with the cutouts, although they wouldn’t
hurt you. This, of course, is assuming you have a ‘‘stock’’
setup. However if the diaphragm in the front bypass valve is
bad, the car will idle badly. Best thing to do in that case
is plug up both ends of the elbow hose (or repair the valve)
and see what happens.
Richard Liggitt–
The original message included these comments:
My twin ZS carbs do not have the bypass valves installed (they are
blocked off by a plate). The carb insulation blocks are notched/cut-
out for air flow from the rear of the carb into the manifold. Does
anyone know if these air ‘‘vents’’ are needed for carbs without
bypass valves? I have several sets of gaskets for these blocks,
some are solid (that block off these vents) and some are notched to
allow air flow around the carb body. Any help out there? I’ve just
rebuilt the engine and noticed when reinstalling the carbs these
notches. The car doesn’t want to idle well (unless the choke is
operated) but at high speed, runs great. The problem is related to
the carbs.
–
'70 E Roadster 1R11998, '98 XK8 Roadster, '02 Mini Cooper
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Richard Liggitt sent Thu 30 Dec 2004:
Thanks again Richard,
That is interesting. I had a mechanic state that my carbs
weren’t ‘‘matched’’, but he never explained. Both carbs are blanked
off (front and rear). I’ve sent Joe Curto an email asking basically
the same questions but haven’t heard anything back yet. I have the
Haynes carb manual, but it doesn’t mention much about bypass
valves. Where could I find more info regarding the setup you
mentioned - one carb blanked off, one connected to the manifold?
This could be my problem. The car never has idled correctly.
Jim–
Jim Tison
Cumming, Georgia, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Although I’m not familiar with your 6 cyl. engine set-up, I would do as (I
think) one other lister suggested. What I mean is that, if it were mine, I
would install two by-pass valves and adjust them so they are basically
permanently closed.
If you can find the externally adjustable type then you have the option of
adjusting one or the other (or both) to open when exceptionally high
manifold vacuum is present, i.e. when backing off the throttle at high rpm.
and road speed. That’s when they come into operation and eliminate
backfiring and popping through the carbs.
To sum up, at idle they would be closed and at high engine speed, but if you
still had problems, you have the option of adjusting without a strip down.
There is an extensive write-up in Jaguar
Publication E123B/3, “Supplementary
Information for 4.2 Litre E-type and 2+2
cars”, Section Q “Exhaust Emission Control”,
page QY.s.1 and onwards. Page QY.s.8
addresses the function of the bypass valve.
The way it reads, there is a bypass valve on
each carburetor. But that may apply to the
later Ser 2 only (1969 and on). The drawings
show a cap on the intake manifold where the
earlier versions (1968) had the heating duct to
the exhaust heat chamber, so that points to
the later setup.
My early 1968 has only one bypass valve which
is situated on the front carburetor. Another point is
that the earlier units had no water jacket on the
primary intake manifold. Only on the secondary one.
The later ones had an additional primary water jacket,
probably to offset the loss of the exhaust heating
chamber and associated duct.
A lot of things were tried out in a very short time
to try to meet Federal standards and some never made
it into the Service Manuals.