Rochester Quadrajet Technical Help

GENERAL DESCRIPTION OF THE ROCHESTER QUADRAJET CARBURETOR

There are three basic carburetor models, the 4M, 4M, and 4MV. Except for the variation in choke mechanism, these three models are basically the same.

4M = manual choke operation

4MC = automatic choke carburetor with the choke housing and coil mounted at the side of the float bowl

4MV = automatic choke model designed for use with a manifold mounted thermostatic choke coil

The Quadrajet is a four-barrel, two-stage cabruretor of down-draft design. Its simplicity in contruction makes it easy to service, yet its versatility and principles of operation make it adaptable from small to very large engines, without major casting changes.

The Quadrajet carburetor has two stages in operation. The primary (fuel inlet) side has small 1 3/8" bores with a triple venturi set-up equipped with plain tube nozzles. Operation is similar to most carburetors using the venturi principle. The triple venturi stack up, plus the small primary bores, result in more stable and finer fuel control during idle and part throttle operation. During off-idle and part throttle operation, fuel metering is accomplished with tapered metering rods, operating in specially designed jets positioned by a manifold vacuum responsive piston.

The secondary side of the Quadrajet has two large (2 1/4") bores. These, added to the primary, give enough air capacity to meet most engine requirements. The air valve is used in the secondary side for metering control and supplements the primary bores to meet air and fuel mixture requirements of the engine.

Quadrajet Parts

The secondary air valve mechanically operates tapered metering rods with move in orifice plates, thereby controlling fuel flow from the secondary nozzles in direct proportion to air flowing through the secondary bores.

The float bowl is centrally located to avoid problems of fuel spillage causing engine turn cutout and delayed fuel flow to the carburetor bores. The float bowl reservoir is small in design to reduce fuel evaporation loss during engine 'shutdown' hot.

The float system has a single pontoon float and fuel valve for simplification and ease in servicing. An integral fuel filter, if used, located in the float bowl ahed of the float needle valve, is easily removed for cleaning or replacemnet.

The throttle body is aluminum to reduce overall weight and improve heat conduction to prevent icing. A heat insulator gasket is used between the throttl ebody and bowl to prevent fuel percolation in the float bowl.

Early Quadrajet applications use a shim between the throttle body and flange gasket. The shim is used to protect the carburetor aluminum throttle body from exhaust gases flowing through the heat cross-over passage in the intake manifold.

IDENTIFICATION

Buy Your Quadrajet Parts Here.

Cleaning - Soak the parts in a bucket of carburetor cleaner (any parts store type), overnight. Flush with hot water. Run thin wire down all of the small passage ways. Ethanol will leave deposits that cleaner will not remove.

Polish the accelerator pump well so that the pump cup glides smoothly. Any binding will cause the cup to fold over and tear.

Free Rochester Quadrajet Manuals
Rochester Quadrajet Carburetor Manual

Electronic Version

Beginning in 1972 Quadrajets used a bowl vent connected to the EGR canister.

Quadrajet 4 MC, 4 MV Carburetor Trouble Shooting
E4MC, E4ME Adjustment Procedures

Metering Rod Sizes

Primary Jet Sizes

I have a Rochester Quadrajet Marine Carburetor #17059286. It appeared to be flooding at idle and the fuel was leaking excessively from the throttle body linkages (on both left & right side). I assumed the carb needed to be rebuilt and the leaking was normal (from a drain in the carb) when flooded. I rebuilt it using a kit I ordered from you. I reinstalled the carb and I'm having the same issue of fuel leaking from the throttle body linkages. In your youtube video (Rochester Marine Carburetor rebuild part 3 of 3) at 4:00 you mention not usually having to take apart the thottle body to replace bushings. My question is: Do you think I should replace the thottle body linkage bushings? And if so, do you carry the kit? I think this is what I need to do. However, I read a post online that said the fuel at the thottlebody should already be atomized, and should therefore not leak out the linkage,and that would indicate that the carb is flooding due to the float not being adjusted properly. I set the float to 3/16 like the instructions stated. Any thoughts on this would be greatly appreciated.

Your problem is not bushings. The carburetor is simply puking out too much fuel. The bushings are replaced only if you have excessive free play in the throttle shaft. Flooding fuel will leak out anywhere it can and onto the throttle valves and out the shaft is the 1st place fuel likes to go.

Too much fuel boils down to getting too much in the float bowl. Did you test the (brass) float for leaks? Immerse the float in hot water and if there are any leaks, it will bubble. If you have a Nitrophyl float (black plastic kind of thing), replace it. They absorb fuel over time.

Did you put the fiber washer where the seat screws into the float bowl.

Many times after rebuilding a carburetor and after re-starting the engine, crud from the tank and the fuel lines will rush up to the carburetor and into the needle & seat causing it to stick open. Take the carburetor apart and wash out the needle & seat and try again.

Manually move the float up and down and watch for any kind of rubbing, or catching. Also watch the needle to make sure it is move straight up and down and not getting caught up.

Too much fuel pump pressure could be holding the needle open. Any chance you replaced the pump with a new one? New pumps are always suspect because they come from China and they don't seem to care about specifications very much yet.

These are just a few ideas, but bottom line is that too much fuel is getting into the float bowl.

Dis Assembly of the Accelerator Pump Lever

Remove the accelerator pump arm by using a small drift punch and beat the roll pin towards the carburetor air horn. Hold a thin blade screw driver, or something similar between the roll pin and the air horn. This will make moving the pin back in place easier. To remove the pin completely (our kits include a new one), bend up the end nearest the air horn and pull out with a needle nose.

Some applications from 1967 to the late 1970s used a hot idle compensator that was located in the rear of the float bowl (our feature car had the place in the casting, but was not equipped with one). It was used to compensate for an overly rich mixture from excessive fuel vaporization in high-heat conditions. A bimetallic spring that responded to temperature would then open a port and introduce fresh air into the idle circuit to lean the air/fuel ratio.