Rochester Monojet Carburetor Technical Help

Used from the mid 60's to 1979 on most GM vehicles and on some GM trucks up until 1986

The 1MV monojet carburetor is a single down draft unit using a triple venturi in conjunction with the plain tube nozzle. The main venturi is 1 7/32" in diameter and the throttle bore is 1 7/16".

Fuel flow through the main metering system is controlled by a main well air bleed and a fixed orifice jet. A venturi velocity power enrichment system is used to provide good performance during moderate to heavy acceleration and at higher engine speeds.

An exhaust gas recirculation, E.G.R. system controls oxides of nitrogen emissions. The E.G.R. valve is operated by a vacuum signal taken from the carburetor throttle body.

A vacuum supply tube installed in the carburetor throttle body connects by a passage to timed vertical ports located in the bore of the throttle body and float bowl. The ports provide a vacuum signal to the E.G.R. valve in the off idle and part throttle operation of the carburetor. The E.G.R. valve, mounted on the intake manifold, circulates a metered amount of exhaust gases to the combustion mixtures to lower peak combustion temperatures, thereby reducing oxides of nitrogen during these ranges of engine operation.

The E.G.R. system is not in operation during the engine idle. A pleated internal paper fuel inlet filter is mounted in the float bowl behind the fuel inlet nut to give maximum filtration of incoming fuel.

The carburetor has an aluminum throttle body, a thick throttle body to bowl insulator gasket, and a internally balanced venting through a vent hole in the air horn which leads into the bore beneath the air cleaner. The float bowl is externally vented under extreme conditions through a pressure relief valve system.

The carburetor part number is stamped on vertical section of float bowl, next to fuel inlet. An idle stop solenoid is used to control idle. The solenoid is electrically controlled through the ignition switch. When the ignition switch is turned off, the solenoid is deenergized, allowing the carburetor throttle valve to close further, preventing the engine from running after the ignition switch is turned off. On manual transmission models, the solenoid also deenergizes when the clutch is disengaged.

Rochester Monojet CFM

The Rochester Monojet (often referred to as the 1MV) is a single-barrel carburetor that was commonly used on many GM vehicles in the 1960s and 1970s, particularly on six-cylinder engines and some smaller V8s.

When it comes to its CFM (cubic feet per minute) rating, the Rochester Monojet typically falls in the range of 170-190 CFM, depending on the specific application and configuration. However, it's essential to note that CFM ratings can vary based on the specific engine requirements and tuning.

Rochester Monojet Identification

Some Rochester Monojet carburetors may have the number stamped on a white round riveted piece of metal on the inlet side of the carburetor.

What Does The Bowl Vent Do?

The float bowl vent on a Rochester Monojet carburetor serves several crucial functions:

Atmospheric Pressure Equilibrium: The primary purpose of the vent is to maintain atmospheric pressure inside the float bowl. This is essential for accurate and consistent fuel metering. If the float bowl were sealed, changes in fuel volume (due to consumption by the engine) would create a vacuum or pressure inside the bowl, which could interfere with the proper flow of fuel through the carburetor's jets and circuits.
Fuel Vapor Escape: On hot days, especially after shutting off the engine, fuel within the carburetor can vaporize. The vent allows these vapors to escape, preventing excessive pressure buildup within the float bowl.
Overflow: In case the float or needle and seat malfunction and allow too much fuel into the float bowl, the vent provides an exit route for the excess fuel. This can help prevent flooding, although in some cases, it might also mean fuel spilling out of the carburetor.
Anti-Percolation: Some carburetors have vents equipped with a bimetallic coil valve. When the engine is turned off and the engine bay temperature rises, this valve closes the vent. This helps prevent fuel from boiling (percolating) in the carburetor, which can lead to hard starts and rough idling when the engine is restarted.

However, it's worth noting:

The vent should remain unobstructed and clean to function correctly. If it's clogged or obstructed, it can lead to fuel delivery problems and erratic engine behavior.
On some vehicles, especially newer models (relative to the age of the Monojet), the float bowl vent might be routed to an evaporative emissions system to capture and recycle fuel vapors, helping reduce emissions.

Always ensure that the venting system is in good working order when servicing or diagnosing issues with a Rochester Monojet or any other carburetor. Proper ventilation is crucial for optimal carburetor function.

1980 Monojet 1ME Exploded View & Parts List
1981-85 Monojet 1ME Exploded View & Parts List

Float Level Adjustment
Monojet Float Level

Fast Idle Cam Adjustment

Vacuum Break Adjustment

Choke Coil Lever Adjustment
Quadrajet Adjustment

Trouble Shooting the Monojet M MV Carburetor

If your Monojet float is black Nitrophyl we recommend you replace it at rebuild time. Nitrophyl tends to absorb fuel over time.Watch a video about rebuilding the Rochester Monojet carburetor.

Rep

lace your Monojet with NEW carburetor

Rochester Monojet Technical

Rochester Monojet CFM

What Does The Bowl Vent Do?