Adjusting Carburetor

(To Identity Items, Refer to Illus. 108)

Before attempting to correct faulty engine performance, attention should be given other items which have a direct bearing on, and can affect carburetor adjustment as well as engine performance.

A. See that throttle and spark controls are correctly adjusted.

B. Remove drain screw from bowl of carburetor. Drain and flush bowl to eliminate dirt and water.

C. Drain and flush gasoline strainer.

D. Check air cleaner to be sure passage of air through cleaner is not restricted by an excessive accumulation of dirt in filter element.

E. Check manifold packing nuts and carburetor mounting screws for tightness.

F. See that spark plugs are clean and correctly adjusted. If condition of spark plugs is questionable, install new ones.

G. Check adjustment of valve tappets.

H. Check compression of both cylinders by operating starter pedal slowly.

I. Check condition and adjustment of circuit breaker contact points.

J. Check for poor or loose connections in wiring, particularly at battery terminals, switch terminals and circuit breaker condenser.

K. Check battery to be sure it is not nearly discharged.

L. Check for intermittent short circuit due to damaged wiring insulation.

M. Check gasoline tank cap to be sure air vent is not plugged.

A carburetor once properly adjusted requires little, if any, readjusting. At the most, it should not be necessary to adjust the needles more than one or two notches richer or leaner to correct mixture for a change in weather conditions.

Both needles (2) and (3) turn down (to right) to make mixture 'leaner" at the respective speeds for which they adjust. Backing them out (to left) makes mixture "richer." Both needles are held in whatever positions they may be turned to, by a spring and ball plunger which drops into notches in the needle adjusting screw.

A carburetor that is badly out of adjustment may be readjusted as follows: First, make sure carburetor control wire is adjusted so throttle fully closes and opens with handlebar grip movement. Turn both the low and high speed needles all the way down (to right). Then, back up (to left) low speed needle (3) about 5 turns, and high speed needle (2) about 2 turns. With needles in these positions, engine will start, but mixture will probably be too rich. Start engine and after choke lever has been moved to "open" position and engine is normally hot, correct the adjustment of both needles.

Adjust for low speed first. Turn low speed needle down (to right) one notch at a time until mixture becomes so lean that engine misses and is inclined to stop; then back needle up five to ten notches, or until engine hits regularly with spark advanced and throttle closed or as nearly closed as it can be set and still have engine run at idling speed. Next, adjust throttle lever stop screw (6) as may be necessary to make engine idle at proper speed with throttle fully closed. Turning screw to right makes engine idle faster. Turning screw to left makes engine idle slower. Don't idle an engine at the slowest possible speed, because an extremely slow idling adjustment causes hard starting. Since changing idling speed with throttle stop screw is likely to change low speed mixture to some extent, it will be necessary to again check and correct low speed needle adjustment by the same procedure followed in making the initial adjustment.

Starting and all-around carburetion will be better with low speed adjustment slightly rich rather than as lean as it can be made.

After low speed adjustments have been completed, run motorcycle on the road to check high speed adjustment. Run at various speeds between 20 miles per hour and wide open. Have spark fully advanced. If adjustment is too lean engine will sluff and backfire ¿rough carburetor on quick throttle opening to accelerate from low speed. If adjustment is too rich engine will be sluggish and will lope and miss at intermediate and high speeds. Adjusting somewhat over-rich is preferable to adjusting too lean, because a lean mixture causes overheating.

Best all around engine performance is usually found with high speed needle set as follows: O.H.V. Engine carburetor—model M-35 about IVs tons open, model M-25 about IV* turns open, model M-75 about IV* turns open; Side Valve Engine carburetor—model M-51 and M-51L about 1V2 turns open.;, ,

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