Workshop advice: Rebuilding a carburettor

By: Paul Tuzson, Photography by: Paul Tuzson

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Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
carb30 carb30
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor
Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor Workshop advice: Rebuilding a carburettor

Paul Tuzson's tips on how to keep your carb in fine fettle...

 

Rebuilding a carburettor

 

Carb

Despite being usurped in fabour of modern fuel injection systems, carburettors are still a core of the classic car world. And they have a hard life. The fuel that constantly passes through them leaves deposits that clog up the system and prevent correct operation. It’s also really hot on top of an engine, and this can cause distortion. Carburettor parts can actually melt because of these operating conditions. Moving parts wear, gaskets leak, rubber components crack and early-style brass floats are prone to leak. After enduring such tough conditions, it’s no wonder that carburettors have to be refurbished at some point.

All carbs have specific service and repair steps, but going through them model by model would take a hefty book; we couldn’t cover it all here.

However, there are some common steps and principles that apply to pretty much any carburettor you care to mention.

The journey to understanding the basics of a carb rebuild starts here...

01 Know your carb components

Carb1

Carb2

Venturis increase air speed and reduce pressure. Primary and secondary venturis are also known as barrels. The small venturis through which fuel is added are called booster venturis, or often simply ‘nozzles’.

Fuel bowls are reservoirs in or on the carburettor that hold fuel ready to be supplied to the inlet air stream.

A float pivots up and down as fuel level changes in a bowl. In doing so it acts on a special valve called a needle and seat.

Jets control how much fuel can flow from a bowl for a given pressure reduction at a booster venturi.

A circuit is a specific set of drilled passages and associated carburettor components that performs a particular role under specific conditions.

The choke restricts airflow so that engine vacuum draws more fuel.

Accelerator pumps add a shot of extra fuel to an engine during periods of sudden demand.

 

WHAT YOU’LL NEED

- A rebuild kit for your carburettor and any extra components that need replacing

- Vice (optional), and a block of wood or set of factory supports

- A bench-mounted belt sander or fine sandpaper and sheet of glass

- A workshop manual for your carburettor (if one exists), or an owner’s manual for the car in question

- Rags and small plastic cups to mop up any spilled fuel to prevent hazards

COST

For run-of-the-mill carburettors like Stromberg single-barrels and similar models, rebuild kits are only around $50. Kits for four-barrel models are generally between $50-$100. Individual components don’t cost much, but $20 here and there can start to add up.

TIME

For a first-timer, about a day should see it done, though stopping to photograph the configuration during disassembly could double that. Regardless, it’s good insurance!.

02 - DISCONNECTION

Carb3

Carb 30

Carb4

The only thing more painful than photographing a job at the same time you’re actually doing it is not remembering how all the parts went together when it’s reassembly time. It’s true that some components are obvious, but just as many aren’t. It may only be a matter of reattaching a small spring the wrong way around, but it can make a difference.

What’s more, if you aren’t doing this sort of job every day, small mistakes can be very difficult to identify when everything has been reassembled and you find that your carburettor doesn’t work like it should.

Getting a carburettor off the engine is the first step. Clamp off the fuel line(s), loosen the retaining pieces (hose clamps or flare nuts, for example), and make sure you have a small cup or bowl handy, ready to catch any fuel that may leak out of the system.

Remove any other lines, like vacuum connections, but first make labels for them and note where they attach. ‘Flags’ made from masking tape serve well for the labels.

Mechanical linkages for the throttle and choke cables have to be removed, but these are often pretty simple. There may also be electrical connectors for the choke.

When these things are done, unbolt the carburettor and remove it from the manifold. Cover the open section of the manifold to make sure nothing can fall into the engine.

03 - INITIAL CLEANING

Carb5

Carb6

Carb7

When throttle plates are fully opened, the edges of the plates invariably extend below the underside of the carburettor base. The plates and shafts on which they rotate have quite fine tolerances, so if they’re knocked in the course of cleaning they can become misaligned and/or damaged.

A block of wood or two can serve to hold the carburettor off the bench and protect the plates. Holley supplies special service pedestals or ‘feet’ for the task. If not, you can utilise nuts and bolts to make stand-offs to accomplish the same thing.

It’s a good idea to get the exterior of the carb as clean as possible before beginning to dismantle it. Aerosol cans of carby cleaner are designed specifically for the task. Use a stiff-bristled parts cleaning brush.

Next, start to separate the main sections. As likely as not, there will be three main sections – the top, the main body and the base.

Often, these parts have linkages between them and these have to be separated. Again, they may come apart easily enough, but getting them back together in exactly the right way can be more difficult than separating them; here’s where your photographs will prove invaluable.

Make sure your tools are in good shape. A worn screwdriver will slip and damage a screw much more easily than a new one.

The main sections may stick together stubbornly and great care is required if you have to prise them apart.

Additionally, the top sections of some carburettors have a number of brass tubes extending from their undersides that can be easily damaged. Rochesters in particular also have fairly delicate metering rods that hang from the underside of the upper section.

 

04 - PARTS REPLACEMENT

Carb8

Carb9

Carb 10

Carb 11

Carb 12

Carb 13

 

 

Rebuild kits often have a number of gasket options for slightly different models. Save the original gaskets when disassembling the carburettor so that they can be used to identify the correct replacements.

This is often easier said than done as they can tear apart during removal, but do your best. Mark which sides of the gaskets faced inwards, upwards, forwards, backwards –whatever is best. This information will be used to make sure that you fit the correct replacements and that they’re correctly oriented.

Completely clean off the gasket surfaces before any other cleaning. A scraper will likely be needed, but be careful not to damage the surface.

Use a straight-edge to see if the surfaces are completely flat. If they aren’t, you might be able to get them even on a bench-mounted linisher/belt sander. If you don’t have one, sheets of fine sandpaper on a sheet of glass will suffice. A speed file can also work.

Whatever method you use, remember to rotate the part 90 degrees every few strokes to make it even.

Some distortion is so severe that the parts can’t be restored this way, in which case they’ll need replacing. The same is true of linkages. They wear and should be replaced if there’s too much play in them.

Throttle plates should pivot freely and easily on their shafts. There shouldn’t be any notchiness, sticking or binding.

By the same token, the tolerance between a shaft and the bore in which it rotates in the body of a carb has to be quite fine to prevent air leaks and lean fuel mixtures. If a shaft is leaking, you’ll usually have to take it to a specialist to have a seal fitted. Similarly, if a shaft is bent and not rotating freely, specialist attention is probably needed.

If throttle plate shafts are in good working order and the plates are sealing when held up to the light for inspection, it’s best to simply give them a clean with your brush and cleaner and leave them alone.

 

05 - SIDE BY SIDE

Carb 14

Carb 15

There are a number of peripheral components, such as vacuum secondary diaphragm housings, automatic choke housings and other items, depending on the model.

Then there are components that could be considered semi-peripheral, like accelerator pumps on Holleys. They are incorporated in the body of the carburettor, but have external covers and levers. Idle mixture adjustment screws probably fall into this category, too.

These components will also have to be removed and/or dismantled to inspect internal sub-components like diaphragms, and you’ll probably need to replace them. Replacement diaphragms and anything else generally needed may be included in the rebuild kit; sometimes they have to be purchased specifically.

The orientation of return springs, pins or any other sub-components should be photographed to make reassembly easier. The removal/refitting of these components may have to occur at particular points in the overhaul/reassembly process, but it all depends on the make and model of carburettor.

 

06 - INTERNALS

Carb 16

Carb 17

Carb 18

Carb 19

 

 

Many carburettors house parts such as accelerator pumps internally. Other components – like floats, jets, discharge nozzles, power valves, needles and seats, emulsion tubes, metering rods – are always internal. These must also be removed, cleaned and refitted, or perhaps replaced. For the most part, whatever items are included in a rebuild kit should be replaced during the rebuild.

When you come to remove idle-mixture screws, first turn them in until they stop and count the number of turns, including fractions, making sure you don’t over-tighten them. Write down the number of turns and then remove them.

Carburettors contain lots of small parts. Some are easy to see, others aren’t. When you remove a screw or other component, check the hole for seals, O-rings or anything else that might be lurking in there.

Again, the rebuild kit can be a helpful guide. If it contains a part, you should be able to find a corresponding part in or on the carburettor somewhere. However, this is not an absolute statement because, as we mentioned earlier, some kits contain parts for variations of the same carburettor that may not be needed for your particular unit.

 

07 - PUTTING IT BACK TOGETHER

Carb 20

Carb 21

Carb 22

Carb 23

Carb 24

Carb 25

 

 

 

Putting your carburettor back together after thoroughly cleaning or replacing everything is, for the most part, simply the reverse of disassembling it.

Some things require special attention, such as floats that must always be reset to specific levels. Sometimes this is done with a spacer like a drill bit, sometimes by ensuring an edge of the float is level with a surface or edge of the float chamber, sometimes by direct measurement, and sometimes by physically checking the fuel level in the chamber.

Some floats are adjusted by screw and lock nut, others require metal tangs to be bent a little. It all depends on your carburettor type, but it’s usually fairly straight-forward.

Turn the idle mixture screw(s) in until they stop and then turn them out by the number of turns you recorded during disassembly. This will probably give you enough of an idle to get the engine running. We’ll cover what to do after that in a future article.

Multiple screws for float bowls, air-horns and the like should be tightened in a criss-cross pattern to ensure even pressure on gaskets. Some screws are the same size but different lengths, and have to go in specific holes. Again, this is something you should note during disassembly.

There is a lot more to say about refurbishing your carburettor, but it starts to become very model-specific. Putting a rebuild kit through your carburettor isn’t brain surgery; you just have to be careful and methodical in your approach.

TOP TIPS

Carb 31

- Take lots of photographs of each step, from a variety of angles. You may feel confident as you start, but it’s easy to lose your way.

- Lay out parts on clean white paper in the order you disassemble them and make notes on the paper.

- If you are short on bench space, use small plastic resealable bags to hold your parts. Place one or two parts in each bag, and label them.

- Screwdrivers with hex handle extensions are a good idea because, although the screws on carburettors are small, they can be extremely tight.

- Have a bench vice, but never use it to hold alloy carburettor components unless you have soft jaws for it.

 

CONTACTS

Eagle Autoparts eagleautoparts.com.au

VPW vpw.com.au facebook.com/MikesDynos - 03 9793 9113

Ace Workshop www.aceworkshop.com.au - 03 -543 5686

 

*****

 

More reviews:

>Workshop advice: Fuel systems explained

> Workshop advice: Garage safety

 

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