Assuming that the Carburetor is properly adjusted so as to give flexibility of running to the motor, allowing it to run in a smooth manner and not showing any tendencies of richness such as "loading up" under a hard pull, foul smelling exhaust, etc., and that it is not leaking, we may safely assume that it is not the source of trouble. The function of the Carburetor is to measure the fuel and the air according to the demands of the engine.
Any engine requires a certain volume of explosive mixture to develop its normal power, this volume being measured by the size of the cylinders, their number, and the number of times they must be filled in order to propel the car a certain distance. Assuming that the engine cylinders have perfect compression; that the ignition is strong and provides the spark at the proper instant; that all bearings are free and lubricated properly; that the brakes are not dragging; that all the tires are pumped up to their proper pressure; that due consideration is paid to the driving of the car etc., we may say that the car will run on a minimum quantity of fuel.
BUT:-
If the compression is weak, if the spark is weak or late, if the brakes are dragging etc., even a small amount, we must use more fuel to make up for the wastage at the weak point.
It is essential to go over the entire engine and car before attempting any Carburetor change, unless, of course, the action has been such as to point clearly to Carburetor trouble.
We therefore recommend that suggestions for adjustments relating to the power and flexibility of the car be followed, and it may safely be assumed that the economy - as far as the Carburetor is concerned - will take care of itself.
In making adjustments, other than for idling, keep on the lean side of the mixture, using the smallest jets that will give satisfactory power, speed and acceleration, when the engine is well warmed up. Do not sacrifice power in attempting to gain economy - the adjustment giving maximum power and flexibility is the most economical in the long run.
A good indication of a proper mixture is when the engine will "spit back" when cold, but will give no evidence of leanness after running for several minutes to bring it up to its normal working temperature.
Under hill climbing conditions the Compensating Jet is exerting its maximum influence on the mixture. Therefore, if the power of the car is low, if missing and a tendency to stall occur, increase the size of the Compensating Jet.
If irregular running of the engine occurs, the exhaust having a "rolling" or "surging" note, and the engine gradually slows down, it is a sign that the mixture is too rich. Therefore put in a smaller Compensating Jet.
A Choke Tube too large will cause irregular running under the above conditions and it will be impossible to secure any combination of jets that will result in smooth powerful running. The velocity of the air past the jets will be too low, causing a "loading" condition to exist even though the jet sizes are comparatively low. This is due to the fact that raw fuel, not sufficiently atomised is drawn into the cylinders causing slow and incomplete combustion. In this case try a smaller Choke Tube, correspondingly reducing the size of the jets.