The model DC carburetor is a plain tube carburetor; that is, its main fuel-air mixture passage consists of a venturi section and discharge nozzle. A fixed jet and adjustable high-speed needle valve of limited size meter the high-speed fuel supply as it is fed into the venturi section of the throttle barrel. The low-speed needle valve meters the low-speed fuel-air mixture supply as it is fed into the throttle barrel near the throttle disc. There are no moving parts except the throttle shaft and disc and the bowl float mechanism.


A properly adjusted carburetor requires little readjustment. It should not be necessary to change the adjustment of the low-speed needle more than 1/8 turn and the high-speed needle more than 1/4 turn, richer or leaner, to obtain correct mixture for a change in weather conditions.

Before attempting to correct faulty engine performance through carburetor adjustment, eliminate other possible causes for poor engine performance such as bad spark plugs, improper spark timing, misadjusted tappets, dirty air cleaner, or leaky carburetor and manifold connections.

The air-fuel mixture for low engine speed is regulated by the low-speed needle. The fuel supply for high engine speed is regulated by a combination fixed jet and adjustable needle. The fixed jet dominates the regulation of high-speed fuel supply. The highspeed needle provides a means of supplementing, to a limited degree, the fuel supplied by the fixed jet, when it is found that slightly enriching the mixture improves engine performance.

Both the high-speed needle (1) and low-speed needle (2) turn inward (clockwise) to make mixture leaner at the respective speeds for which they adjust. Backing them out (counterclockwise) makes mixture richer.

A carburetor may be adjusted as follows:

1. Make sure carburetor control wire is adjusted so throttle lever (3) fully closes and opens with handlebar grip movement.

2. Turn both the high- and low-speed needle (1 and 2) all the way in (clockwise). Do not close off either needle too tightly or damage to needle and seat may result.

3. Turn low-speed needle (2) (counterclockwise) about 1-1/2 turns. With needle in this position, engine will start, but low-speed mixture will probably be too rich.

4. Start the engine and after it has reached operating temperature and the choke has been moved to the open position, correct the adjustment of low-speed needle. Turn low-speed needle (2) in (clockwise) 1/8 turn at a time until mixture becomes so lean that engine misses and is inclined to stop; then, back needle out (counterclockwise) 1/8 turn, or until engine hits regularly with spark advanced and throttle closed and engine running at idle speed. Starting and all around carburetion will be better with low-speed adjustment slightly rich, rather than too lean.

5. Adjust throttle lever stop screw (4) as necessary, to make engine idle at proper speed with throttle fully closed. Turn screw clockwise to make engine idle faster and counterclockwise to make engine idle slower. Do not idle an engine at the slowest possible speed because an extremely slow idling adjustment causes hard starting. Changing the idle speed with throttle stop screw is likely to change the low-speed mixture slightly. It will, therefore, be necessary to again check and correct low-speed needle adjustment by the same procedure followed in making the initial adjustment.

6. Check high-speed adjustment, after low-speed adjustments have been completed. Run motorcycle or Servi-Car on the road at various speeds between 20 miles per hour and maximum speed. Have spark fully advanced. Best all-around engine performance can usually be found with the high-speed needle (1) set from 3/4 to 1-1/4 turns open.

0 0

Post a comment