See Figure 3F-12. On the Model MD carburetor,fuel enters carburetor at inlet connection (A) flowing past inlet needle and seat (C) into the fuel bowl. Fuel flows from bowl past main nozzle adjusting screw (T) into main nozzle orifice (W) and into nozzle sump (Z).

Idle and slow speeds: Fuel reaching its level in the carburetor passes main adjusting screw (T) through main nozzle orifice (W) and into idle tube (L). High manifold vacuum at throttle disc (G) draws this fuel upward past idle tube outlet orifice (M) where it mixes with air from channel (P) adjusted to requirements by idle mixture adjustment screw (O) through channel (J) and into air stream at idle discharge ports (H) where it mixes with additional air passing the slightly opened throttle disc (G).

High speeds and full power: When engine is pulling a load throttle disc (G) has opened further reducing suction and minimizing fuel discharge at (H) and increasing air flow to a high velocity through venturi

(R). This air draws fuel from main nozzle (Y) supplied from bowl, past main nozzle adjusting screw (T) through orifice (W). As engine speed or load increases air is automatically bled into the main nozzle through tube (U) which causes a proper proportion of fuel drawn from sump (Z) in relation to adjustment to be metered at that speed range.


A carburetor once properly adjusted requires little if any readjustment. Before attempting to correct faulty engine performance through carburetor adjustment, eliminate all other possible causes for engine trouble. Such as bad spark plugs, incorrect spark timing, misadjusted tappets, dirty air cleaner, or leaky carburetor and manifold connections.

Idle mixture adjustment screw (1), turns to the right to enrich mixture for the idle speed range. Backing it out (turning left) makes mixture leaner.

Main nozzle adjusting screw turns to the right to lean mixture for the high speed range. Backing it out (turning left) makes mixture richer.

0 0

Post a comment