No change should be made in the carbureter adjustments until after an inspection has been made to determine if the trouble is in some other unit. It should be noted that the gasoline lines are clear, that there is gasoline in the vacuum tank, that there are no leaks at connections between carbureter and engine, that the ignition system is in proper condition, and that there is even compression in all cylinders.
If it is necessary to test adjustment or to make a readjustment proceed as follows:
Set air screw so that the end is flush with the end of ratchet set spring. Loosen packing nut on needle adjustment. Turn gasoline adjustment to the right very carefully so as not to injure the needle point, until the valve is closed gently against its seat. Then turn to left approximately one complete turn which will bring notch in the disc handle directly below the guide post above it. Tighten packing nut to hold needle firmly as set.
The notch in disc handle of needle is put in handle after the needle has been carefully calibrated by a flow-meter at the factory, therefore the notch in handle should register with guide post above it. This setting of needle valve is absolutely essential to get the best results. The object in directing that needle be first turned to the right until closed is to insure against two or more turns open, as from closed position to notch (usually about one turn) is the normal setting. This being true it is not necessary to turn needle in to the right firmly but merely far enough to be sure that when turning back to the left, to the notch registering with guide post, that the needle is not more than once around or one turn from its seat.
Set stove heat and damper heat as previously instructed above. Pull out choker to closed position and start engine in usual manner. As soon as engine has fired release choker three-fourths of way in. Run until engine has warmed up then push choker all the way in, remembering to never use choker longer than necessary, as when not needed it has a tendency to foul up engine and ruin the lubricating oil in the crank case.
Next, set air screw for good idle by either turning to the right a little or backing out to the left as the needs of the engine require, remembering that first of all, the needle must be set as described. With the needle so set and the engine warmed up, the adjustment of the air screw for proper idling is easily accomplished by using a little care. If the air screw is turned in too tight, the motor will roll. If the air screw is not tight enough, the motor will hesitate and perhaps stop entirely. To make a nice clean adjustment for idle, first having set needle as described, turn air screw in quarter of a turn at a time until engine, does roll; then turn back to the left until engine hesitates, indicating that mixture has too much air and is too lean; next turn air screw in to the right three or four notches at a time until engine runs smoothly. This accomplished (and it is very easy to do by proceeding as directed above) the proper adjustment for the entire range of the engine will have been attained, thus insuring the best economy and power.
MODEL "S" MARVEL CARBURETER
If the engine idles too fast with throttle closed, the latter may be adjusted by means of the throttle lever adjusting screw.