CARBURETORS: ZENITH
Idling Well Sizes
Curb. size Well
1" 40-45
iii" -I5-50
11" 511-55
1 f" .5.5-60
2" 60-70
Selecting the Trial Setting
When other data is not available you can refer to the charts' on
preceding page and select the sizes of chokes and jets for any
engine.
How to use charts: To determine the carburetor setting for any
given engine first locate the figure at the top of the chart which
corresponds with the "stroke" of the engine.
Then locate the figure at the side of the chart which corresponds
to the "bore" of the engine.
Follow the stroke column downwards to the point where it meets
the lines above and below the figure representing the "bore." This
will give you the proper choke size.
By referring to the table in the lower left hand corner of the
chart you will find the proper jet sizes to be used with each choke
size.
Example: Engine 4 cyl. 3i' bore, 5" stroke; choke size 19.
The setting of carburetor should then be 19 choke, 18 main jet,
20 compensating jet. (For the older models the main jet and compensating
jet sizes will be multiplied by 5; for example, the main jet would
be ISX5=90 and the compensating jet 2JX5=100.)
Example: Engine 6 cyl. 31" bore, 5" stroke; choke size
21; carburetor setting would be 21 choke, 20 main jet, 22 compensating
Jet. (i"))r the older models the main jet and compensating
jet sizes would be multiplied by 5.)
Note: Owing to the different designs of engines, manifolds, heating
devices, etc., it is impossible to construct charts which are absolutely
correct, but the preliminary setting charts will give you a setting
which will function satisfactor.ly in the majority of cases. In
the few- exceptional cases a change of one size on either the main
or compensating jet will usually overcome any trouble.
Trying Out the Setting
Note that these instructions apply to working out a new setting
or to the correction of one already installed.
First: Be certain that all connections are O.K. and the fuel is
turned on.
Second: Back off the lever stop screw all the way and then screw
it back in until you know the butter-fly is opened a little bit.
Then screw the idling adjusting needle valve all the way in to
its seat and then back it off about a turn and a half.
Third: Now turn on the switch, close the choker and start engine.
When it starts gradually return choker to its open position. Let
it run a few minutes until it is well warmed up, then back off
the lever stop screw until the engine runs at the desired idling
speed, at the same time screwing in or out on the idling adjusting
needle valve, until the engine is running evenly.
If the idling valve has to be opened more than three complete
turns from its seat put in an idling jet one size smaller.
If you have to screw it in to within less than a turn from its
seat put iu a jet one size larger.
Fourth: Now by quid: opening of the throttle the engine should
speed up with no hesitation with spark fully advanced. If it spits
back or hesitates, put in a larger compensating jet. It should "spit-back" a
little with the spark fully retarded on quick opening of the throttle.
If it does not, put in a smaller compensator.
Fifth: Now take it on the road and try it on a good grade. .If
it doesn't have power and seems sluggish, try smaller jets.
If it lacks power but "spits" or "coughs," try
larger jets.
If changing of jets more than two sizes up or down from what you
started with does not give you desired results, change the choke
tube according to the following indications:
(a) If you have to use larger jets to get acceleration (quick
opening of throttle) or power use a smaller choke.
(b) If you get good acceleration but poor power with smaller jets
put in a larger choke.
Note: If with a given carburetor adjustment you can quickly accelerate
the car or truck with-out spitting hack and climb a good hill without "loading
-up" and sluggish action your mixture is pretty near right
and the economy will take care of itself.
ZENITH CARBURETOR; SV MODELS
Zenith model "SV" carburetors have three main parts:
the bowl, which contains the float mechanism and the jets; the
barrel which contains the two venturis, and the throttle butterfly;
and an air in-take, containing a strangling device and temperature
regulator.
Adjustment': Like all Zenith carburetors, "SV" models
have a fixed adjustment.
Applies to all Zenith carburetors except those with double choke
or venturi, as models "Sr' and "5V." On the "ST" and "SV"'
double venturi carburetors, increase choke 2 sizes, leave main
jet the same, and reduce compensator jet 2 sizes.
The adjustments here also apply to the "ST" model. The
difference between the "ST" and "SV" models
is that the "ST" is a smaller size and the compensating
jet is in the bottom, whereas on the "S\ it is on the side
(see I, Fig. 19). The "Si"' is made in 1' and 1f" size.
The "SV" is made in
1) 2", 1)f" and 2". a For engine bore and stroke
and make and size carburetor of different engines see pages 1053,
1035, 966, 996.
The venturi, main jet (G), compensating jet (I) and idling jet
(P), whose proper sizes constitute the setting, never vary in size,
and once right, stay right (Figs. 17, 18, 19).
Operation: Fuel enters the carburetor through the union body (D)
and is filtered by the screen (D 1) (see Figs. 17, 1S).
It enters the bowl through the needle valve seat (S), and when
it reaches the proper level, raises the float (F) in the bowl,
which causes the needle valve (Gl) to stop the flow of fuel.
Idling device: When the butterfly throttle valve is nearly closed,
and the engine is "turned over," there is a very strong
suction at the edge of the butterfly, where the idling hole is
located. Tinder this condition little or no gasoline is supplied
by the main jet (G) or the cap jet above it.
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