Zenith UK Carburettor Information

Explanation of Terms Used in the Charts

(Listings are for the 1931 - 1946 period taken from an original Zenith dealer booklet)

Abbreviation Explanation
Econ Abbreviation for "Economy Jet"
S.R. Abbreviation for "Slow Running Jet"
C.S. Abbreviation for "Contract Sheet", Zenith internal use only, always to be quoted in correspondence with Zenith
Comp Abbreviation for "Compensation jet".
This jet cannot be confused with the "main jet" as the external diameter of the two jets vary, the main jet being
smaller than the compensating jet in diameter. The jets have the same "flow" calculations and are stamped with the
size numbers. The sizes normally follow in stages of "5" , although "half-size" jets are sometimes used. The greater
the number stamped on the jet, the larger is the drilling of the orifice.
Vent Abbreviation for "Ventilation to the capacity well".
In V type carburettors the ventilation is in the screw over the capacity well. Numbers are stamped on the head of
this screw when the size of the hole differs from the standard diameter. In certain carburettors where the capacity well
is cored to the required size and has no loose tube fitted, the top of the well is open and no screw is incorporated.
In 'U' type carburettors the ventilation to the compensating jet is by means of holes drilled in the head of the slow
running jet carrier, the size being stamped on a flat of the hexagon head.
The letters 'P.T. and F.T. indicate Part Throttle and Full Throttle ventilation respectively, where the carburettor has an
economy device of the diaphragm type as in the VIG carburettor or the spring blade type as on the VEM carburettor.

"C" Tube Abbreviation for "Capacity Tube"
This is a separate loose fitting in certain types, but in others the capacity well is cored the correct diameter to give the
necessary reserve of fuel for acceleration purposes.
N & S Abbreviation for "Needle & Seating"
The size being stamped on the hexagon of the seating.
Choke Abbreviation for "Choke Tube"
This may be cast the correct size in the barrel of the carburattor or may be a loose detachable fitting. The size
shown on the choke tube indicates the smallest bore of the restricted part in millimetres. Therefore, if the number is
difficult to decipher, a pair of internal callipers will enable the size of the tube or cast restriction in the barrel to be
checked.
Normally, the screw retaining the choke tube does not protrude beyond the inner bore of the tube. In some cases,
however, a long or extended screw is used to assist distribution of the fuel.
In 'U' type carburettors no bar was fitted across the choke tube as standard, but it has been used on a few occasions
to help distribution.

Start Device On cerrtain 'V' type carburettors a type of starting device was used that employed a spring loaded air valve. The
spigotted end of this valve was fitted with a loose sleeve, the diameter of which is specified on the setting list. The
sleeve was not marked, but the size is the outside diameter of the sleeve in millimetres.
Certain 'U' type carburettors has a starting valve employing a starting jet only, which screwed into the top face of the
lower half. Ensure that this jet is screwed into the side to coincide with the position of the starting valve on the upper
half of the carburettor. In certain instruments the face of the lower part was tapped to accomodate the starting jet on
either side, a blank starting jet being fitted in the side not used, although such blank jet is not strictly necessary.

Prog Abbreviation for "Progression Jet"

• Main • Petrol Menu •

Abbreviation	Explanation
Econ	Abbreviation for "Economy Jet"
S.R.	Abbreviation for "Slow Running Jet"
C.S.	Abbreviation for "Contract Sheet", Zenith internal use only, always to be quoted in correspondence with Zenith
Comp	Abbreviation for "Compensation jet". This jet cannot be confused with the "main jet" as the external diameter of the two jets vary, the main jet being smaller than the compensating jet in diameter. The jets have the same "flow" calculations and are stamped with the size numbers. The sizes normally follow in stages of "5" , although "half-size" jets are sometimes used. The greater the number stamped on the jet, the larger is the drilling of the orifice.
Vent	Abbreviation for "Ventilation to the capacity well". In V type carburettors the ventilation is in the screw over the capacity well. Numbers are stamped on the head of this screw when the size of the hole differs from the standard diameter. In certain carburettors where the capacity well is cored to the required size and has no loose tube fitted, the top of the well is open and no screw is incorporated. In 'U' type carburettors the ventilation to the compensating jet is by means of holes drilled in the head of the slow running jet carrier, the size being stamped on a flat of the hexagon head. The letters 'P.T. and F.T. indicate Part Throttle and Full Throttle ventilation respectively, where the carburettor has an economy device of the diaphragm type as in the VIG carburettor or the spring blade type as on the VEM carburettor.
"C" Tube	Abbreviation for "Capacity Tube" This is a separate loose fitting in certain types, but in others the capacity well is cored the correct diameter to give the necessary reserve of fuel for acceleration purposes.
N & S	Abbreviation for "Needle & Seating" The size being stamped on the hexagon of the seating.
Choke	Abbreviation for "Choke Tube" This may be cast the correct size in the barrel of the carburattor or may be a loose detachable fitting. The size shown on the choke tube indicates the smallest bore of the restricted part in millimetres. Therefore, if the number is difficult to decipher, a pair of internal callipers will enable the size of the tube or cast restriction in the barrel to be checked. Normally, the screw retaining the choke tube does not protrude beyond the inner bore of the tube. In some cases, however, a long or extended screw is used to assist distribution of the fuel. In 'U' type carburettors no bar was fitted across the choke tube as standard, but it has been used on a few occasions to help distribution.
Start Device	On cerrtain 'V' type carburettors a type of starting device was used that employed a spring loaded air valve. The spigotted end of this valve was fitted with a loose sleeve, the diameter of which is specified on the setting list. The sleeve was not marked, but the size is the outside diameter of the sleeve in millimetres. Certain 'U' type carburettors has a starting valve employing a starting jet only, which screwed into the top face of the lower half. Ensure that this jet is screwed into the side to coincide with the position of the starting valve on the upper half of the carburettor. In certain instruments the face of the lower part was tapped to accomodate the starting jet on either side, a blank starting jet being fitted in the side not used, although such blank jet is not strictly necessary.
Prog	Abbreviation for "Progression Jet"