Mixture Proportion Needed from Carburetor
Effects of manifold condition: Irregular flow of liquid in the intake manifold exists to greater or less extent in all engines, particularly at wide open throttle and low speed.
To get smooth engine operation under such conditions, the mixture delivered at the carburetor must be such that the cylinder getting the least liquid fuel from the intake manifold still gets a rich enough mixture for a good firing charge: iii other words, the poorer the distribution, the richer the mixture required from the carburetor.
Since the manifold is more nearly dry at partly closed throttle than at wide open throttle, a leaner mixture may be used at closed throttle.
And since the distribution is usually better at high speeds than low speeds, a leaner mixture can also be used at high speeds. All these differences are greater in winter than iii summer.
Fig. 13. Actual size of liquid accelerating charges necessary for best response to sudden opening of throttle at low speed, with 3'4" bore, 4 2' stroke, six cylinder engine.
The most important effect of change of manifold conditions on mixture requirements, however, is during quick opening of the throttle or acceleration.
At closed throttle running, as shown on Figure 10, the manifold is relatively dry, but the moment the throttle is opened, a wet condition ensues and the fuel discharged from the carburetor, instead of going to the engine along with the air as a vapor, goes much more slowly as a stream along the walls, so that if a steady uniform mixture feed is maintained at the carburetor, the engine will falter and miss for several revolutions until the stream of fuel gets to the cylinders.
It has been found that this hesitation can be avoided if at the moment of throttle opening an extra supply of gasoline be delivered into the air stream, the amount of this extra supply needed de-pending upon the conditions of vaporization after the throttle is open and upon the engine speed.
To insure good response of a 34"x4)'" engine, in passing from condition of Fig. 10 to that of Fig. 11, requires an extra charge equal to about five regular fuel charges. But with a hot spot as in Fig. 12, orwith a manifold very hot as in summer weather, a much smaller accelerating charge is necessary.
The amount of accelerating charge necessary also varies with the volatility of the fuel, much less being needed with "high test" or "aviation grade" gasoline.
In determining the carburetor specification for any given engine, an effort should be made to select an average amount of accelerating charge, enough to give fair response to the throttle in the colder months and yet not too much for good warm weather performance.
If the accelerating capacity be made large, to favor the cold weather operation, there will in warm weather almost certainly be a stumble or hesitation when the throttle is opened from idle, due to the more complete vaporizing of the over-size fuel charge.
Such a hesitation or "flat spot" (momentary lack of power) will be more noticeable with a warm engine than a cold one, and with good gasoline than with poor. Though this action may be objectionable in warm weather, it is an indication that the carburetor setting will be unusually satisfactory in cold weather, particularly as regards the response to full throttle opening at low speeds.
Effect of mixture conditions on ignition: Practically all of the ignition systems in use today when in good condition will give a spark in the cylinder regardless of fuel mixture conditions. But when some part of the ignition is not in proper shape, as for instance, a cracked or partly fouled spark plug, weak coil or improper breaker gap, mixture conditions have considerable influence on whether or not the spark will jump.
The spark can julep much more easily in the low compression which goes with part throttle opening than in the full compression which the engine has at full throttle.
Also, with a cold mixture charge, the spark can jump more easily with a rich mixture setting than on a lean one.
At low speed idle with a very low compression in the cylinders and a slow opening of the breaker gap, the spark may not form at all (the electrical tension leaking across), if the spark plug terminals are too close together, so that an engine usually idles best with a spark plug gap of about .025" to .032". But if the gap is too wide the engine cannot fire cold at full open throttle and low speed (see also pages 1299, 1301, 1302).
With hot With unheated spot mani- intake manifold fold nor- engine partially
mally warm. warm.
Steady load compared with acceleration: In all carburetor work, it is necessary to keep a clear and definite distinction between what happens under steady speed and load, and what happens under sudden change of speed and load.
As shown in the preceding chapter, the mixture requirements may be very different under these two sets of conditions and this must he taken into ac-count before any satisfactory understanding of carburetor action can be gained.
The following explanation of the Stromberg carburetor action is, therefore, limited to the action and duty of the various parts under steady running conditions.
In the detailed description of each model, reference will be made to the means provided for accommodating mixture proportion to the engine requirements when the throttle position is changed.
Action of plain jet: It is generally believed that a simple plain fuel jet in a carburetor air opening of fixed size tends to deliver a continuously richer mixture as the engine suction and air flow increases, but this is not accurately true.
If a simple plain jet be tried in a carburetor with fixed air entrances of the size commonly used, it will be found that an adjustment can be reached which will give smooth operation and apparently a uniform mixture from perhaps 16 m.p.h. speed up to the maximum; but as the throttle is closed to slow down the engine below the 16 m.p.h., the mixture will get lean so rapidly that the engine will slow down, ceasing to fire.
To make the engine fire at 10 m.p.h., the fuel jet will have to be increased almost 50 per cent in capacity, with a corresponding increase in fuel delivery from 16 m.p.h. on up. It will also be found

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Carburetor Manuals: Stromberg