How to Rebuild a Rochester 4GC Four-Barrel Carburetor
This article appeared in The British V8 Newsletter - Volume XIII, Issue 2 - May 2005by: Greg Myer
Four Webers sitting on top of my Buick 215 in my MGB-GT V8. Wow, what a sight! Fire it up and listen.... there's a nice burble at idle with no "missing". Blip the throttle and there's instantaneous response! Slip into first gear and slide the clutch out. Off like a shot! This is the place where I usually wake up from my dream.
Real world: My 215 isn't together yet. I'll be using the stock Rochester 4GC that came on the Buick when it left the plant in 1962. Many of us do that, at least to get the car running. We have so many areas to spend money on, that if the carburetor can be rebuilt on a shoestring, we'll spend the budget somewhere else. After all, we know the carburetor fits and was intended for this motor. Good starting point.
Sometimes finding the right intake manifold at the swap meet to work with a Holley
and keep things under the hood isn't easy. The good news is that the Rochester 4GC
is not a bad choice. Even though the carburetor is 40 years old, it can be made to
run very well indeed.
As good as the Weber set up? NO! Keep dreaming! The Webers are infinitely adjustable; from idle to an RPM range beyond the Buick's capabilities. The 4GC can provide excellent street performance however. You will need to rebuild it first. Yes, you will. Unless it's coming off a running engine, it will have dried up gaskets and accelerator pump and old gas left in it. It needs cleaning!
Kits are available. While I was working on Larry's Ford powered MGB, he ordered a kit from D&D for me. He also got me the Crower Cam 50232 that many British V8'ers like to use. What a pal!
Cam changes, higher compression, headers, lighter car, and 5-speed transmissions all affect the fuel requirements of any given motor. Can the 4GC handle it? Yes! Actually, it's quite versatile, as we will see.
First off, after you have the rebuild kit, you'll need 2 cans of carb cleaner. There are many brands out there. Spend a little extra here. The cheaper ones just don't seem to do the job. Also, when you pick them up, make sure they have the small applicator tubes taped to them. These are important as they'll help you clean the various passages in the carburetor. There are a bunch of these small passages and they play major roles in how the carburetor functions. Every one of these needs to be thoroughly cleaned for the carburetor to work right.
When you disassemble the carburetor, note how things are put together. If you've never done one before, you'll be amazed when it's time to reassemble it how many ways it's possible to do it wrong. Taking pictures first isn't a bad idea. Linkages especially! Then: follow the instructions in the kit. This article will only hit a few high points and a few modifications you may want to try. Unscrew the air-horn, or top of the carburetor. Lift it off slowly so as to avoid damaging the floats. The accelerator pump will come up with it. Set it aside.
The main body of the carburetor is now open to you. Remove the 3 screws holding the primary venturi cluster. It may require a bit of prying. Go easy, one side than the other. The 2 clusters appear interchangeable to the novice. In fact they can be screwed into the wrong location. No good! The primaries have the accelerator pump nozzles and the secondaries don't. I use an old cupcake baking tin to keep the parts sorted from each other. Everyone has one of these. Just snoop around in the kitchen, it's there. (I prefer to do this when I have the house to myself.)
Or, pick one up at Wal-Mart. While you're there in the kitchen wares, there are lots of inexpensive tins and pans that prove useful in the shop. I keep the primary side parts on one end and the secondary parts on the other. The jets are a fine example here. They will screw into the other end, but really foul things up. The primary venturis are smaller, and therefore require smaller jets. Don't mix them up. If you do, look at them before installing. The hole in the secondary jet is larger. Get them where they belong!
Don't just turn the carburetor over at this point. There's a check valve in the bottom of the accelerator pump well. It's larger than the one on the pump discharge passage. There are also small springs in at least 2 locations. If you turn the carburetor over without thought to these and they bounce across the floor and out of sight: panic! It's the natural reaction. It won't help. The valves look like ball bearings and new ones come in the rebuild kit. There are no springs in it however, so don't lose them. The base plate is held on by 3 small screws and one large one in the center. The base plate has the idle set screws in it. Before removing them, turn them in counting the turns and lightly seat them. They should be very close to the same side to side. If they are way out, or off the carburetor, when you re-assemble it lightly seat them and back out 1 1/2 turns. This will be close enough to start and run and tune things up.
As good as the Weber set up? NO! Keep dreaming! The Webers are infinitely adjustable; from idle to an RPM range beyond the Buick's capabilities. The 4GC can provide excellent street performance however. You will need to rebuild it first. Yes, you will. Unless it's coming off a running engine, it will have dried up gaskets and accelerator pump and old gas left in it. It needs cleaning!
Kits are available. While I was working on Larry's Ford powered MGB, he ordered a kit from D&D for me. He also got me the Crower Cam 50232 that many British V8'ers like to use. What a pal!
Cam changes, higher compression, headers, lighter car, and 5-speed transmissions all affect the fuel requirements of any given motor. Can the 4GC handle it? Yes! Actually, it's quite versatile, as we will see.
First off, after you have the rebuild kit, you'll need 2 cans of carb cleaner. There are many brands out there. Spend a little extra here. The cheaper ones just don't seem to do the job. Also, when you pick them up, make sure they have the small applicator tubes taped to them. These are important as they'll help you clean the various passages in the carburetor. There are a bunch of these small passages and they play major roles in how the carburetor functions. Every one of these needs to be thoroughly cleaned for the carburetor to work right.
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When you disassemble the carburetor, note how things are put together. If you've never done one before, you'll be amazed when it's time to reassemble it how many ways it's possible to do it wrong. Taking pictures first isn't a bad idea. Linkages especially! Then: follow the instructions in the kit. This article will only hit a few high points and a few modifications you may want to try. Unscrew the air-horn, or top of the carburetor. Lift it off slowly so as to avoid damaging the floats. The accelerator pump will come up with it. Set it aside.
The main body of the carburetor is now open to you. Remove the 3 screws holding the primary venturi cluster. It may require a bit of prying. Go easy, one side than the other. The 2 clusters appear interchangeable to the novice. In fact they can be screwed into the wrong location. No good! The primaries have the accelerator pump nozzles and the secondaries don't. I use an old cupcake baking tin to keep the parts sorted from each other. Everyone has one of these. Just snoop around in the kitchen, it's there. (I prefer to do this when I have the house to myself.)
Or, pick one up at Wal-Mart. While you're there in the kitchen wares, there are lots of inexpensive tins and pans that prove useful in the shop. I keep the primary side parts on one end and the secondary parts on the other. The jets are a fine example here. They will screw into the other end, but really foul things up. The primary venturis are smaller, and therefore require smaller jets. Don't mix them up. If you do, look at them before installing. The hole in the secondary jet is larger. Get them where they belong!
Don't just turn the carburetor over at this point. There's a check valve in the bottom of the accelerator pump well. It's larger than the one on the pump discharge passage. There are also small springs in at least 2 locations. If you turn the carburetor over without thought to these and they bounce across the floor and out of sight: panic! It's the natural reaction. It won't help. The valves look like ball bearings and new ones come in the rebuild kit. There are no springs in it however, so don't lose them. The base plate is held on by 3 small screws and one large one in the center. The base plate has the idle set screws in it. Before removing them, turn them in counting the turns and lightly seat them. They should be very close to the same side to side. If they are way out, or off the carburetor, when you re-assemble it lightly seat them and back out 1 1/2 turns. This will be close enough to start and run and tune things up.
Clean everything! It can't be too clean. Use that tube on the spray can and point
it down the various passages (wear eye protection when you do this as it
may shoot back).
Make sure all the passages are open. There are 4 circuits in carburetor s like these: Idle, off idle, accelerator and main (jets). Each is important. The idle and off idle use the same feed passage, but the idle discharges below the throttle plate, while the off idle discharges just above it. This passage can affect the fuel metering well above idle.
With the main body inverted, the auxiliary throttle valve assembly in the secondaries is visible. The whole unit pulls out.
This is an air valve that uses the engine's need for air to push against the spring pressure to allow more air flow into the engine: a kind of vacuum secondary arrangement. If we leave it out do we have mechanical secondaries? Yes. I've done this and was surprised at the difference on a Chevy 283 I had years ago. The body of this valve assembly forms part of the venturi however. Some turbulence may result inside the carburetor so it would be better to leave the body in without the plates and shaft. The 283 has more pull through the venturis, and no problems resulted. Either way quicker response is the result.
Vacuum secondaries are designed to prevent too much air being available to the motor before it needs it. This is ideal for heavy cars, or cars with automatics and high rear gear ratios. Mild camshafts work well with vacuum secondaries, too. But we've changed all that. We're not going to get more CFM with this modification; we'll just get the air sooner when we crack the throttle.
Maybe you've heard some stories where this type of thing produced problems. I've experienced it myself on a Rochester Quadrajet. The Q-jet has small primaries and huge secondaries, with air doors on top. These doors are the mechanism that slowly opens to keep the engine from bogging. Also, they are attached to metering rods which fit in the secondary jets. Removing the doors means you must remove the rods. This gives you a transition from the very small primaries to all the carburetor at once. It's like flushing the toilet....but with gas, not water. Even in a car with low gears there's a noticeable bog; then hang on because she'll go like gangbusters! The fuel consumption suffers in this set up too.
The 4GC does not use the auxiliary throttle valve to meter any fuel. There are no vacuum feeds or lines to worry over either. Just remove the plates and shaft, and you're good to go.
Next, look at the base plate with the throttle plates and shafts in place. Work the linkage. Do the secondaries open all the way? Mine didn't. They stopped well short of vertical. That will reduce the flow capacity at wide open throttle (WOT). Why won't they fully open? The linkage just needs adjustment. Was it built like that on purpose, or by mistake, or did it get bent somewhere over the last 40 years? I don't have the answer, but I do know that sometimes factories do it on purpose. At one time I had a '68 Firebird with the 400 4bbl. This engine was rated lower in H.P. than the 400 in a larger Pontiac. All the specs looked the same: cam, heads, compression, and carburetor. The Firebird was lighter and for corporate reasons couldn't match the larger car. The "fix" was to manipulate the linkage on the Q-jet so it wouldn't allow the secondaries to fully open. Result: less H.P.
Lesson for us: always make sure the secondaries are vertical at WOT on whatever carburetor you're using. On the 4GC there are 2 spots to make this adjustment on the linkage. First, be sure the choke is disengaged. Otherwise the secondaries won't open. (Make sure the choke linkage is correctly assembled, as it can be put together wrong). Next, invert the carburetor, and work the primary linkage. When the primaries are about one-third open, the linkage contacts a spring coiled around the primary shaft. (See the left-hand photo above.) Bending this up slightly initiates the secondary opening sooner.
Both primaries and secondaries should reach WOT at the same time. (See the right-hand photo above.) If the secondaries don't open fully, bend the small tab that contacts the base plate and prevents the shaft from going farther (as shown in the left-hand photo below.)
Make sure all the passages are open. There are 4 circuits in carburetor s like these: Idle, off idle, accelerator and main (jets). Each is important. The idle and off idle use the same feed passage, but the idle discharges below the throttle plate, while the off idle discharges just above it. This passage can affect the fuel metering well above idle.
With the main body inverted, the auxiliary throttle valve assembly in the secondaries is visible. The whole unit pulls out.
This is an air valve that uses the engine's need for air to push against the spring pressure to allow more air flow into the engine: a kind of vacuum secondary arrangement. If we leave it out do we have mechanical secondaries? Yes. I've done this and was surprised at the difference on a Chevy 283 I had years ago. The body of this valve assembly forms part of the venturi however. Some turbulence may result inside the carburetor so it would be better to leave the body in without the plates and shaft. The 283 has more pull through the venturis, and no problems resulted. Either way quicker response is the result.
Vacuum secondaries are designed to prevent too much air being available to the motor before it needs it. This is ideal for heavy cars, or cars with automatics and high rear gear ratios. Mild camshafts work well with vacuum secondaries, too. But we've changed all that. We're not going to get more CFM with this modification; we'll just get the air sooner when we crack the throttle.
Maybe you've heard some stories where this type of thing produced problems. I've experienced it myself on a Rochester Quadrajet. The Q-jet has small primaries and huge secondaries, with air doors on top. These doors are the mechanism that slowly opens to keep the engine from bogging. Also, they are attached to metering rods which fit in the secondary jets. Removing the doors means you must remove the rods. This gives you a transition from the very small primaries to all the carburetor at once. It's like flushing the toilet....but with gas, not water. Even in a car with low gears there's a noticeable bog; then hang on because she'll go like gangbusters! The fuel consumption suffers in this set up too.
The 4GC does not use the auxiliary throttle valve to meter any fuel. There are no vacuum feeds or lines to worry over either. Just remove the plates and shaft, and you're good to go.
Next, look at the base plate with the throttle plates and shafts in place. Work the linkage. Do the secondaries open all the way? Mine didn't. They stopped well short of vertical. That will reduce the flow capacity at wide open throttle (WOT). Why won't they fully open? The linkage just needs adjustment. Was it built like that on purpose, or by mistake, or did it get bent somewhere over the last 40 years? I don't have the answer, but I do know that sometimes factories do it on purpose. At one time I had a '68 Firebird with the 400 4bbl. This engine was rated lower in H.P. than the 400 in a larger Pontiac. All the specs looked the same: cam, heads, compression, and carburetor. The Firebird was lighter and for corporate reasons couldn't match the larger car. The "fix" was to manipulate the linkage on the Q-jet so it wouldn't allow the secondaries to fully open. Result: less H.P.
Lesson for us: always make sure the secondaries are vertical at WOT on whatever carburetor you're using. On the 4GC there are 2 spots to make this adjustment on the linkage. First, be sure the choke is disengaged. Otherwise the secondaries won't open. (Make sure the choke linkage is correctly assembled, as it can be put together wrong). Next, invert the carburetor, and work the primary linkage. When the primaries are about one-third open, the linkage contacts a spring coiled around the primary shaft. (See the left-hand photo above.) Bending this up slightly initiates the secondary opening sooner.
Both primaries and secondaries should reach WOT at the same time. (See the right-hand photo above.) If the secondaries don't open fully, bend the small tab that contacts the base plate and prevents the shaft from going farther (as shown in the left-hand photo below.)
There's no need to go beyond vertical as that only creates problems. To complement this
modification, be sure the accelerator pump rod is in the hole closest to the pump (there
are 3 choices, as shown in the right-hand photo below.) This position will give the
longest stroke, and thus the most fuel. Should you remove the choke? That is a race-only
modification, and unless you live in Miami you will find the choke comes in very handy.
Leave it automatic or change it to manual, whichever you prefer.
Don't forget the fuel filter. There are small screens in the needle and seat assemblies. The original seats have a boss machined to hold them in place. The replacement ones in the kit do not. I did not put them in because of this. I'm not sure they will stay in place. I use a large filter between the tank and the electric fuel pump. Then I insert a small in-line filter just before the carburetor.
Remember, the idle set screws are to be adjusted out 1 1/2 turns for the beginning point. The primary plates are adjusted with the spring-pressured screw at the front of the carburetor, on the driver's side. Use these two screws and the distributor's initial advance setting to obtain a low, smooth idle. If you open the throttle plate setting to increase the idle RPM, you will most likely need to open the set screws to smooth things out.
At this point we might add that engines with a performance cam tend to like quicker advance mechanisms in the distributor. Not necessarily at idle, but soon thereafter. There are kits available for both the mechanical and vacuum advance in either points or HEI-type distributors. If anyone out there wants to lend me their Weber set up for a few weeks, I'd be glad to test it. I'm hoping make it into at least third gear the next time I have that dream.
Disclaimer: This page was researched and written by Greg Myer. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.
Photographs by Brandon Myer at Brightside Photography and Greg Myer for BritishV8. All rights reserved.
Don't forget the fuel filter. There are small screens in the needle and seat assemblies. The original seats have a boss machined to hold them in place. The replacement ones in the kit do not. I did not put them in because of this. I'm not sure they will stay in place. I use a large filter between the tank and the electric fuel pump. Then I insert a small in-line filter just before the carburetor.
Remember, the idle set screws are to be adjusted out 1 1/2 turns for the beginning point. The primary plates are adjusted with the spring-pressured screw at the front of the carburetor, on the driver's side. Use these two screws and the distributor's initial advance setting to obtain a low, smooth idle. If you open the throttle plate setting to increase the idle RPM, you will most likely need to open the set screws to smooth things out.
At this point we might add that engines with a performance cam tend to like quicker advance mechanisms in the distributor. Not necessarily at idle, but soon thereafter. There are kits available for both the mechanical and vacuum advance in either points or HEI-type distributors. If anyone out there wants to lend me their Weber set up for a few weeks, I'd be glad to test it. I'm hoping make it into at least third gear the next time I have that dream.
Disclaimer: This page was researched and written by Greg Myer. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.
Photographs by Brandon Myer at Brightside Photography and Greg Myer for BritishV8. All rights reserved.
