MOTORCRAFT Model 2150 2-Barrel: Description
Motorcraft 2150 carburetors consist of a float system and 5 fuel metering systems: choke system, acceleration system, main metering system and power enrichment system. In addition to these systems, some carburetors may be equipped with some or all of the following systems: altitude compensation, high speed pullover system, throttle positioners and/or variable high speed bleed system. The float system maintains a preset level of fuel in the fuel bowl. The fuel bowl is internally vented to the air cleaner on all models. Some models are also externally vented to the canister. Some carburetors have a filler block in the fuel bowl on the left side.
All electric choke systems with choke pulldown diaphragms are used with the 2150 carburetor. The electric choke used with V6 engines gets electrical power from the battery; the choke cap and oil pressure switch are connected in series. The electric choke used with V8 engines gets electrical power from the alternator stator; choke cap wire is connected directly to alternator stator terminal. The choke system used on V8 engines also uses thermactor hot air assist. Choke cap on both V6 and V8 systems include ceramic heater discs (2 on V6 engines and 1 on V8 engines). On V6 engines, the first disc is always energized when engine is running to unwrap the spring, which opens the choke plate at a calibrated rate. The second disc is energized when the temperature in engine compartment is 54-74°F (12-23°C) to further open the choke plate. On V8 engines, hot air from the manifold is applied to the choke housing to open the choke (serves same function as first disc on V6 engines). At temperatures of 54-74°F (12-23°C), a temperature sensing switch energizes the ceramic heater disc to open the choke further (same as second disc on V6 engines). See Fig 1.
Choke pulldown diaphragms are of 3 types: modulated choke pulldown, positive choke closure (PCC) and modulated positive choke closure (MPCC). The modulated choke pulldown assembly includes an additional spring which allows choke plate to move under certain cold engine starts. The positive choke closure also has an additional spring, but it is on the outside of the pulldown link. This spring adds more force to hold choke plate closed during engine starts. The modulated positive choke closure is a combination of the other 2 pulldown assemblies. The positive choke closure is used during engine starts and the modulated choke control operates during engine warm-up. All choke pulldown assemblies use manifold vacuum to provide a leaner air/fuel mixture after engine starts. The choke is opened by a link connected to the pulldown diaphragm. See Fig 2.
The idle system provides the correct air/fuel mixture at curb idle speeds. The amount of mixture entering the engine is controlled by the tapered idle mixture adjusting screws. The adjusting screws extend into the curb idle discharge ports and change the metering area of the ports. The adjusting screws are preset at the factory and 2-piece tamper-proof caps are installed to prevent adjustment of the idle mixture screw setting. The acceleration system is an external diaphragm type pump which provides additional fuel necessary to increase engine speed from idle to full throttle. As the idle system phases out, the acceleration system provides fuel until the main metering system can provide the necessary air/fuel mixture.
The main metering system provides the necessary air/fuel mixture for speeds above curb idle. The main metering system consists of a main venturi, booster venturi and main fuel discharge port in each bore. Fuel from the fuel bowl is drawn up through the main well bleeds by the vacuum created by air flowing through the venturi. Air is added to the fuel from the anti-siphon bleeds and high speed bleeds through holes in the main well tubes. This air/fuel mixture is discharged through the main discharge nozzles in the booster venturi and mixed with the air flowing through the main and booster Venturis. The amount of air/fuel mixture admitted to the engine is controlled by throttle plate position. The enrichment system consists of an enrichment valve diaphragm which adds extra fuel to the air/fuel mixture during periods of increased load or high speed operation.
The altitude compensation system provides additional air (added between venturi booster and throttle plate) to lean the air/fuel mixture at higher altitudes. The high speed pullover system is an air bleed in the air horn with a calibrated hole which adds extra fuel for high speed operation. Various throttle positioners are used to increase idle speed during A/C operation and to permit slow return of throttle plates to idle position during deceleration.
The variable high speed bleed system provides a direct link between air/fuel mixture and throttle plate position. A cam on throttle shaft engages a lift rod as the throttle opens. The lift rod raises metering rods in the high speed bleed holes to decrease air flow through the high speed bleed holes. This allows better high speed operation and low speed response.