Carburetor & Venturi Sizes
Size of carburetor: The carburetor capacity required depends upon
the piston displacement (piston area X stroke X number of cylinders)
and upon the service for which the engine is to be used. The
capacity of the carburetor is gauged by the throat diameter of
the large venturi.
To find the size carburetor needed for a given engine, find the
cubic inches piston displacement from tables on page 1047; table
below will then show the carburetor size and venturi size recommended.
(For bore and stroke of different size engines see pages 1058,
966, 996; for make and size of carburetor see pages 1055, 966,
996, of Dyke's Auto Encyclopedia.)
Example: Given a four-cylinder engine of 33/t" bore, 5" stroke,
installed in a pleasure car. The piston displacement, according
to table on page 1047, is 221 cubic inches, and from the table
below, we find this to require a 12" venturi, in a 1%" size
type "0" or "OS" carburetor.
If the engine were used on a truck, it would probably be better
to use a 33" venturi, in a type "M" or "MB" carburetor.
The tables on this page are computed in the case of the Model "O" carburetor
for maximum torque and volumetric efficiency at 1,700 r.p.m. and
for Models "M" and "MB" carburetors at 1,200
r.p.m.
For faster and slower speeds, a corresponding addition or reduction
should be made in piston displacement value before selecting the
venturi size.
Larger venturi sizes may be used on engines which have proper application
of exhaust heat on the intake manifold.
For fewer than four cylinders on one carburetor, use the same venturi
size as required for four cylinders of the same bore and stroke.
Determination of Mixture Range
In determining the mixture range, the engine should be well warmed
up and in good condition, so that it fires on all cylinders without
missing at any speed.
Care should be taken that the fuel used approximates that which
would be used in later service.
Care should also be taken that the ignition system is in good condition,
that all cylinders have compression and that there are no manifold
leaks.
The most satisfactory method is to find the actual mixture requirements
of the engine in terms of the gasoline needle adjustment setting
required for different speeds and loads, then modify the variables
in the carburetor so that these individual requirements will all
be met by one position of the adjustment needle.
One side of the high-speed needle slot should be marked for definite
identification with a file scratch or punch mark and also the setting
noted in terms of number of turns or notches from a seating position.
The first step is to find the needle setting required for best
power at wide-open throttle, low speed. It is recommended that
the standard carburetor discharge jets and accelerating wells be
used at the start. (See also pages 1245 to 1249.)
The car should be tried on a grade that will re-quire the full
power of the engine at low speed, and different adjustments tried
until the least needle opening that will give maximum pull is ascertained.
If no hill is available, try opening the throttle from idling at
5 miles per hour and find the adjust-
Table of Carburetor and Venturi Sizes for 4- and 8-Cylinder I Table
of Carburetor and Venturi Sizes for 6- and 12-Cylinder
Engines According to Piston Displacements Engines According to
Piston Displacements
PISTON DISPLACEMENTS VENTURI CARBURETOR PISTON DISPLACEMENTS VENTURI
CARBURETOR '
CUBIC INCHES SIZE SIZE CUBIC INCHES SIZE SIZE
(Tables don't scan well; use the image below
to read these values)
Type M Type 0 Type M & 0 Type M. & O Type M Type 0 Type
M & 0 Type Al & 0
117 to 147 80 to 101 146 to 164 too to 113 j"
147 to 163 101 to 112 2" 164 to 181 113 to 124 tom"
163 to 178 112 to 122 *114;," %" 181 to 798 124 to 136
178 to 195 122 to 133 tti," 198 to 217 136 to 149 ttfr"
195 to 212 133 to 145 217 to 237 149 to 162
212 to. 231 145 to 158 0254,2,, 237 to 258 162 to 177 :sic
231 to 250 158 to 171 tail' 1" 258 to 278 177 to 191 'sat"
250 to 268 171 to 184 :74," 278 to 300 191 to 206 'In" lv
268 to 290 184 to 198 it %" 300 to 322 206 to 221 4 %"
290 co 310 198 to 213 t9~" 322 to 345 221 to 237
310 to 332 213 to 228 11%," 345 to 370 237 to 254 '64t"
332 to 354 228 to 243 %" 370 to 396 254 to 271 ''.,W 1%11
354 to 378 243 to 259 y1•' 396 to 421 271 to 289
378 to 403 259 to 275 1 421 to 448 289 to 307 1 Ii"
403 to 427 275 to 292 1 6 " 13k" 448 to 475 307 to 326
1 %6"
427 to 478 292 to 328 1 /" 475 to 532 326 to 365 1 %" lt/Z
tt
478 to 533 328 to 365 rl i4" 532 to 594 365 to 407 '11 '46"
533 to 592 365 to 405 1 14" 594 to 658 407 to 459 1 84"
592 to 652 405 to 446 1 N" 13$" 658 to 726 459 to 498
I si3"
652 to 715 446 to 488 1 726 to 796 498 to 545 1 84" .1% "
715 to 780 488 to 535 "1 74" 196 to 870 545 to 597 111
9rf6'
780 to 850 535 to 584 1 j4" 870 to 950 597 to 650 1 %"
850 to 920 584 to 632 1 t6" 2" 950 to 1030 650 to 750
1 96" .2 n
920 to 1000 x1 1030 to 1110 p1 84"
1000 to 1080 104" 1110 to 1200 It%6"
1080 to 1160 1 %" 21% " 1200 to 1290 1 3/4" 2% CI
1160 to 1240 81174" 1290 to 1380 ittl "
1240 to 1330 1 %" 1380 to 1480 1
1330 to 1520 2" 2~" 1480 to 1680 2' 21k "
1520 to 1710 at2 /" 1680 to 1890 d2 84"
11710 to 1920 2 %" 3 1890 to 2120 2 t4" 3 tl
1920 co 2140 2 2120 to 2380 2 /"
*Standard venturi size for this size carburetor.
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