Maxwell Carburetor 101

Adjusting the Carburetor

from Tim Simonsma

First Things First

Most automobile carburetors have two separate fuel delivery systems, one to supply the correct fuel-to-air mixture (approximately 1 part fuel to 14.7 parts air by weight) at an idle and one to supply fuel at high speed. The Maxwell, however, uses only one fuel delivery control system, the needle valve (A). The idle is controlled by the air supply. Since the needle valve is the only fuel control, we must adjust the needle valve first assuring an adequate supply of fuel for normal operation of the automobile and then adjust the air supply to control the idle.

First, set the carburetor so that we can get the engine running. The float (D) fuel level should be set approximately ¾ - 1 inch below the top of the fuel bowl. By unscrewing the top of the carburetor cup (fuel bowl) this measurement can be made. Typically the top of the carburetor needle valve orifice (H) is 5/8 of an inch below the top of the fuel bowl, it is imperative that the fuel level be maintained below this level, as fuel will run out this orifice. After insuring the fuel level is correct put the carburetor cup cover (fuel bowl top) back on. The adjustment for the stock carburetor needle valve should be ½ turn open. (If you are using one of Vern Campbell’s tapered carburetor needle valves this adjustment should be 1 full turn open.)

Adjust the carburetor gas valve (top cylinder) with an opening of 1/16 inch air gap and maintaining the ability to close this completely.

Adjust the carburetor air valve (lower cylinder) (this is the quadrant adjustment on top [bottom on 1906] of the carburetor) to 3/8 - 1/2 inch opening.

These adjustments are fat and designed strictly to get the engine started.

1^st Time Starting the Engine

With the gas turned on, the ignition switch on, and spark lever retarded, press down the float valve (C) until gas drips from the bottom of the carburetor. Cranking the engine should produce an engine start.

Occasionally an engine is hard or unable to start and you will have to manually prime the engine. This is done by removing both spark plugs and placing gas or starting fluid directly in the spark plug holes (about a teaspoon or so should do it). Re-install the spark plugs and crank the engine.

The engine will run on only the fuel that has flooded the carburetor for about 5 – 10 seconds.   As we have not adjusted the idle portion of this carburetor, it is important to open the throttle to medium speed and keep the engine running. Also, running the engine at a medium speed with the spark fully advanced will allow the engine to warm up and make these next adjustments possible.

Adjusting the Needle Valve

With the engine warm and running at a moderately high rpm, turn the carburetor needle valve clock-wise closing the fuel off until the engine stumbles or loses power. Then open the carburetor needle valve ¼ turn. This is a preliminary setting. To fine adjust the mixture, we need a road test. The engine will not idle at this point so you will have to keep the engine at a medium speed while this test is done. Take the Maxwell out on flat level ground or preferably a good moderate hill that the Maxwell can climb in high gear. If the Maxwell does not climb the hill, stop and open the carburetor needle valve ¼ turn and repeat the road test until it climbs easily. Now that the Maxwell will climb the hill, stop and close the carburetor needle valve 1/8 of a turn and repeat the road test. Each time the Maxwell climbs the hill stop, and close this valve 1/8 of a turn until a loss of power and the engine begins to stumble. At this point open the carburetor needle valve 1/8 of a turn and the car should climb the hill without stumbling. We have now set the carburetor needle valve adjustment on this carburetor and it should not be moved from this position. This is the leanest possible setting that can be obtained from the carburetor needle valve. Do not move this needle valve again.

Setting the Idle

At this point the engine runs well at high rpm but may or may not idle. Allow the engine to come down to an idle. If the engine has a tendency to quit, loosen the screw holding the quadrant on top of the carburetor and close the carburetor air valve (lower cylinder) down in small increments (this would be turning the quadrant clockwise) until the engine does not die. With the engine at an idle, slowly turn the quadrant in the counterclockwise direction in small increments allowing the engine time to adjust for excess fuel in the carburetor air valve (lower cylinder). Turn the quadrant counterclockwise until the engine will not idle and then turning it clockwise until the engine will idle. We now have the quadrant adjusted in the leanest and most efficient setting possible. After the engine has idled for a few minutes, reach under the carburetor with your hand and feel the small hole on the bottom of the carburetor (carburetor lower cap). It should be dry to moist and definitely not dripping gas. If gas is dripping from the hole on the bottom of the carburetor you need to turn the quadrant in a counterclockwise direction (reducing the supply of fuel). This adjustment is more easily made with the engine at a high idle (about 450 –500 rpm). By turning the quadrant we are creating a vacuum in the center of the carburetor literally sucking the gas out of the pre-adjusted carburetor needle valve orifice. You should think of this quadrant adjustment as a gas valve, (for the idle circuit only), supplying fuel to the lower portion of the carburetor so the fuel can be sucked up through the center stem of the carburetor and exiting out the carburetor gas valve ( top cylinder) into the engine manifold. Turning the quadrant clockwise increases the amount of fuel and turning the quadrant counterclockwise reduces the amount of fuel.

We are now ready to adjust the engine idle. This should be done by turning or removing portions of the idle stop post on the top of the carburetor. I prefer to cut the top ¼ inch of the post in half and rotate the post as an eccentric cam. The ideal speed is owner’s own preference, (I set mine at about 300-350 rpm.) It should be noted that as you reduce the engine rpm you have slowed the air moving into through the carburetor and you might need to close down the quadrant adjustment slightly.