For us classic car enthusiasts, there are few automotive components that elicit such an emotional response as the carburetor… or better yet, multiple carburetors. We are intrigued by them, enamoured by them; when we open the hood they are the source of our greatest pride; and at full chat, the sound they emit from their open throats is music to our ears. Many look to the carburetor as the mystical source of all power but when it's all going wrong they are usually the first place we lay the blame. Carburetors are wonderful but they are one of the most misunderstood aspects of classic car ownership. In this edition of ‘Let’s Get Specific’ I will try to dispel some myths and provide some solid, simple advice for living with and getting the most out of your carburetors.

1/2

Photographed by Daniel Lackey

The Basics

A carburetor does two jobs. First, it controls the flow of air to the engine through the throttle butterfly, a rotating metal disc that opens and closes with the throttle pedal. Second, it meters the desired amount of fuel and allows it to atomise in the incoming stream of air. That’s it, that’s all they do, simple right? As we already know, for our engines to run at their best, there is an ideal ratio of air and fuel. That ratio must be maintained in all conditions, be it at idle, partial throttle, full throttle or pulling up a steep hill. To make things a little more complicated, different manufacturers in different parts of the world have all invented different ways to meter the fuel in a carburetor. So, knowing what type of carburetor(s) you have is the first hurdle.

Types of Carburetor

There are two main categories of carburetor, fixed venturi and variable venturi. I will start with the variable venturi carburetors as they are the smaller of the two groups. Variable venturi carburetors are most commonly found on British and Japanese classics and the most common types were manufactured by the SU Carburettor Company, Zenith-Stromberg and Hitachi in Japan. The easiest way to identify this type of carburetor is by the ‘dashpot’, a large aluminum cylinder that sits atop the carburetor. Within the dashpot is a piston and on the bottom of the piston is a tapered needle which slides within a fixed jet. As engine speed increases, a vacuum above this piston allows it to rise, lifting the tapered needle in the jet and allowing more fuel to enter the air stream. The oddball in this category is the American Edelbrock carburetor which looks much like any American carburetor but uses vacuum pressure to actuate a needle (metering rod) through a fixed jet.

1/3

Photographed by Daniel Lackey

The fixed venturi category is somewhat larger and includes most European carburetors such as those by Solex, Weber and Dellorto as well as most American carburetors like those from Carter and Holley. Instead of having a vacuum operated needle to attenuate the fuelling, a fixed venturi carburetor relies upon a number of separate fuel circuits to deliver the correct dose of fuel in different conditions. In simple terms, a fuel circuit is a somewhat intricate system of tiny passages (drillings) and jets. Generally a jet looks like a small brass screw with a hole drilled through the middle. Drillings and jets usually supply fuel to the air stream but may also supply air. We need not go into great detail here, but suffice to say that most fixed venturi carburetors will in the very least have an idle circuit, for closed and partially open throttle, and a main circuit for when the throttle is open. The other feature of this type of carburetor is the accelerator pump. As the throttle is depressed, a small diaphragm squirts extra fuel directly into the air stream. This compensates for the immediate effect of the throttle opening and the increased volume of air.

Both of these types of carburetor have some form of float chamber. The float chamber fills up with fuel and provides the carburetor with a steady, consistent flow. In the float chamber is a small buoyancy device which floats on top of the fuel and whose height determines the level of fuel allowed in the chamber. At the top of the chamber is a needle valve which will shut off the flow of fuel entering the chamber when the float reaches the top.

1/5

Photographed by Daniel Lackey

Living with Carburetors

Carburetors should be, relatively speaking, maintenance free. The only carburetors that require regular maintenance are the types with dashpots on top such as SU, Zenith-Stromberg and Hitachi. On the top of the dashpot is a large hexagonal nut, under the nut is a metal rod and a damper which sits in a bath of oil. Over time, that oil disappears so occasionally it needs to be topped up. Low dashpot oil can cause the engine to run rough, run excessively rich (too much fuel) and foul up spark plugs. 20w50 motor oil usually does the trick.

Most carburetor issues will fall into one of two categories, the first is poor or improper maintenance of the vehicle itself, and the second is improper or incorrect tuning of the carburetor. We will come to the second category a little later. Let’s first take a look at the rest of the vehicle. Carburetors are one part of a fuel delivery system. A system that must be well maintained, starting with the fuel tank. Many of our 40, 50, 60 year-old classics will have their original fuel tanks. Old fuel tanks can be full of rust and other debris which loves to clog fuel lines, fuel pumps and especially the tiny drillings in carburetors. Cleaning out old fuel tanks is only moderately successful so it's worth considering a brand new tank if they are available and it is always a good idea to fit a good quality fuel filter.

1/2

Is your fuel pump doing enough? Most carburetors will require 2-4psi of fuel pressure and a flow rate depending on the engine and application. Bigger engines and racier engines will require a greater volume of fuel but not necessarily more fuel pressure. If your fuel pump is old and tired it may no longer be able to deliver an adequate amount of fuel. Ensure that your fuel lines are clear and free of any kinks that might impede your flow and if the lines are rubber, make sure they are in good condition, not dried out or cracked.

Needless to say, all carburetors need to be in good condition. Old carburetors can have worn throttle spindles which can allow air to leak in and cause rough running and an erratic idle. Old carburetors can also suffer from blockages from debris or dried out fuel, very old fuel can leave a sticky, toffee like residue. Needle valves in the float chamber can wear over time and cause carburetors to flood with fuel. Fuel leaks can occur through old gaskets and seals. If you think your carburetors need attention then sending them to a specialist for a rebuild is highly recommended.

Lastly, is your ignition system up to scratch? Carburetors are often blamed for poor running issues when the route of the problem lies with the ignition system. Worn out distributors will need to be rebuilt or replaced. Contact breakers, rotor arms and distributor caps should all be the best quality parts available and spark plugs should be replaced at each service interval and gapped correctly. Contact breakers should also be gapped correctly and the ignition timing set at the recommended amount. Only when all of the above has been considered is it worth moving on to tuning.

Carburetor Setup and Tuning

A carburetor setup is the adjustment of the basic carburetor settings while the engine is at idle. Typically these settings include idle speed, idle fuel mixture and the balance between multiple carburetors. A competent mechanic should be able to do this for you. These settings rarely change on their own but should be checked as part of your annual service. If your car is bone stock and has not been modified then this should be all you need to keep your motor running sweet. Taking things a little further, if you want to get the very best out of your engine then you will need to consider having your carburetors properly tuned.

If you have made any modifications such as free flowing exhaust, long duration camshaft or gas flowed cylinder head then you will have to have your carburetors properly tuned on a dyno or rolling road. Even if everything in your engine is standard, you can still benefit from tuning. The dyno will provide a resistance against which your engine will push. It is only under this load that your engine's demand for fuel can be gauged and your carburetors adjusted accordingly. For an SU carburetor, this involves changing or re-profiling the needle. Tuning fixed venturi carburetors like Weber, Dellorto or Holley, primarily involves changing the jets. Properly tuned carburetors will not only give you more power, they will also improve your fuel efficiency. Once your carburetors are tuned, they shouldn’t need tuning again unless you change something else on the engine. The only maintenance you should have to do is the occasional top-up of the dashpot oil or the adjustment of basic settings during an annual service.

Carburetor Upgrades

From the 1950s, Weber became the first choice for motorsport applications around the world. As such, many cars were homologated with Weber carburetors even if they weren’t standard fitment. Today, for performance upgrades, Weber carburetors still remain a popular choice, they make good power, they are easy to live with and easy to tune. A lot of engines will benefit immediately from the fitment of larger Weber carburetors, the 2-litre BMW M10 engine comes to mind, but other engines may require internal upgrades first in order to fully realise their potential.

Likewise, with American V8s, you may be tempted to fit that 850cfm Holley double pumper but unless your engine is built for it you will most likely be disappointed. Upgrades in carburetion can be hugely rewarding but they can also make your car less pleasant to drive in day to day traffic so make your choices carefully. The universe of carburetor upgrades is enormous. I can’t even begin to scratch the surface here but what I can say is be honest with yourself about what you really want, avoid internet forums and seek the advice of reputable specialists before proceeding.

Hopefully this overview helps shed some light on the mysteries of carburetors. They demand respect but should never be feared. With a little insight you can ensure that your carburetors are well maintained and tuned for many miles of reliable driving pleasure.

Want to keep reading? Check out:

• Let's Get Specific: Fuel Injection

• Let’s Get Specific: Suspension

Meet our contributors

Daniel Lackey is a long-time photographer, automotive writer, and Porsche Carrera Cup GB Team Manager for CCK Motorsport. Needless to say, he keeps pretty busy when not contributing to Marqued!

Stay up-to-date with the latest action and stories on Marqued's Instagram and Facebook pages.