Rochester 4GC

Hello, I'm new to this forum and hope someone can help me with my problem.

I'm in the middle of restoring a Range Rover Classic. The car has a 3.5 litre V8 (215ci). As you probably know Landrover bought the rights to build the aluminium V8 from GM. I wasn't satisfied with the twin Zenith/Stromberg carburetors, orginally mounted on the Range Rover. That's why I bought the Rochester 4GC. This kind of carburetor was also installed on the original Buick 215 ci V8, correct me if I'm wrong.

I bought the carburetor from a member of the local Landrover club. The carburetor should have been mounted on a Buick 215 ci, but I'm not quite sure because the ID-tag is missing. Also there is no cast carb number stamped on the side. The only thing I can tell about the carburetor is that it has wedge type floats with dimples.

The Rochester 4GC has been completely rebuild using a carburetor overhaul kit (#DAT 125) which I bought from the guys from classic buicks.

CONNECTION PROBLEMS

I have some questions concerning the connection of the carburetor.

Choke system:

As you know the Rochester 4GC has an automatic choke. The choke system is composed of a thermostatic coil, vacuum piston, offset choke valve and fast idle cam. Its operation is controlled by a combination of intake manifold vacuum the offset choke valve atmos*pheric temperature, and exhaust manifold heat.

After dismantling the choke housing assembly I discovered that the choke baffle plate was damaged. Can I still get this kind of baffle plate?

As the engine warms up, hot air from the exhaust manifold is drawn into the thermostatic coil housing. The hose that connects the exhaust manifold to the carburetor choke housing assembly is missing. Can you tell me how the choke housing is connected to the exhaust manifold?

Is it also possible to get a manual choke for the Rochester 4GC?

Vacuum connections:

The most difficult part is the connection of the vacuum ports. In picture 1a you can see the carburetor mounted on the intake manifold. The red arrow in picture 1b shows a constant vacuum port which is situated at the secondary side of the carburetor. I presume this vacuum port is connected to the brake system? Is this correct? As my car is a 4x4 I will need another constant vacuum port for the Diff lock. I can drill a hole in the intake manifold and connect the hose for the diff lock to this vacuum port.

After inspecting the throttle body assembly I discovered another vacuum port which is located at the primary side or pump plunger side of the carburetor. You can see this vacuum port in picture 2 indicated by a red arrow. I presume this is the idle speed air adjustment port. Should this be blanked or must it be kept open?

Timed vacuum port:

This item is beginning to give me nightmares.

In my car (Range Rover) the distributor has a vacuum connection. This vacuum connection on the distributor is connected to a timed vacuum port which is located on the original Zenith/Stromberg carburetor.

When I studied the Rochester 4GC I could not find a timed vacuum port. Can you tell me where (on the Rochester 4GC) I must connect the hose of the vacuum connection of the distributor. In stead of using the original distributor I will get a Mallory dual point or unilite distributor but these also have vacuum connections. So the problem remains....


I know this this is quite a lot of information. But I hope you can help me sort things out.

Yves

 

Got a photo of the damaged baffle? I'm thinking this is a stamped thin-gauge sheet steel piece that fits inside the choke housing. If so, should be easy enough to fabricate one out of a small piece of sheet steel or brass.

The exhaust manifold may have a couple of ports in it--one to allow filtered air into it, and another for the heated air to be drawn out. So you need two connections, one from a source of filtered air (look at the carb air horn--lots of newer carbs had a port suitable for this at the rear. Maybe yours does, too.) and then the heated air is drawn into the choke housing. Metal tubes were used, along with a short piece of high-temperature silicone hose. As for me, I'd happily garbage-can the hot air choke coil, the hot air tubes, etc. and convert to electric choke. You might find that an electric choke coil from a Quadrajet may work. If you do this, be sure to omit the choke-coil gasket. It's not used with electric chokes, and will disrupt the ground circuit. There are manual choke conversion kits available. They are all made in Communist China and are hatefully low quality.

Yup, the vacuum port at the rear is suitable for power brakes. I don't know anything about diff locks--do they take a large vaccum source? Perhaps you could Tee into the brake port? If not, you're right--it takes little effort to drill 'n' tap the manifold.

I can't help you with the second port. (other than to ask: What is an "idle speed air adjustment port"?)

This carb may not have a timed vacuum port. You may have to run the vacuum advance from manifold vacuum. If you were clever enough, MAYBE you could drill a passage for timed vacuum, but I'd consider that more work than it's worth.

Why do you need a different distributor?

 

The baffle plate is indeed a thin gauge plate. So this should be easy to copy.

I would be more than happy to change the original choke and convert to an electric choke or manual choke. Where can I get these?

This carb doesn't have a timed port. So I think I will use manifold vacuum and adjust the distributor. I don't really need another distributor, but I want a Mallory distributor which is an upgrade and gives more spark.