How do I clean a carburetor

Clean the carburetor

"Sewer Cleaner"

Carburetors are highly sensitive components.
They are particularly sensitive to dirt.
If the engine coughs and spits, it's time
to blow through the mixing plant thoroughly.

Text & Photos: Winni Scheibe

"Six cylinders - six carburettors"
The CBX engine will only run smoothly if the carburetors are working properly

BSA A75R Rocket engine with three Amal Concentric carburetors

Motors can be annoying sometimes. Bad jumping is a warning sign. Misfires, uneven idling and who knows what other inconsistencies put the biker in a lot of stress. Cable connections to the ignition coil, ignition cables, spark plug caps, spark plugs, fuel supply and even the valve clearance are systematically checked. But the unit continues to strike, is the "worm" perhaps in the carburetor system?

"Carburetor view"

This device has long since gone out of fashion in automobiles. Modern injection systems in connection with pollutant-reducing G-Kats have taken their place. Strict statutory emissions regulations make these systems absolutely necessary in the passenger car sector. In the case of motorcycles, this did not appear to be necessary for the majority of manufacturers until the turn of the millennium. With a few exceptions, antiquated carburetors were still in use until well into this period. The spectrum ranges from simple round slide carburettors to constant pressure carburettors to ultra-modern flat slide racing carburettors. No matter how different and from which epoch the mix equipment is, they all have one thing in common: they should supply the engine with the correct fuel / air mixture in every speed range and under all operating conditions. Chemical relationships dictate what is correct. For optimal combustion in the cylinder head, 14.7 kilograms of air must be added to one kilogram of commercially available fuel. This ratio is referred to as lambda = 1.

Amal carburetor from the 500 Velocette Venom Thruxton

In order to come close to this ideal condition (modern carburetors rarely manage this!), The carburetor designers would have to come up with something new again and again. While simple carburettors with simple nozzle systems were sufficient in the early days of vehicle construction, the demands grew enormously over time and with further development of the engines. Regardless of the outside temperature, good starting behavior is always required. In idle, part-load and full-load ranges, the engine should have a good grip on the gas, when accelerating it shouldn't choke, deep in valleys and high up on mountains, where the air is thin, it should still run flawlessly.

Dell'Orto-PHM-40 ​​round slide carburetor of the Ducati 750 SS

These requirements made the carburetor more and more complex. For laypeople, it is now almost inscrutable. A look inside makes the confusion perfect. Tiny bores, narrow channel connections, many different nozzles, adjustable nozzle needles, small valves, floats, membranes, pumps, springs, levers, a number of adjusting screws and much more make the structure a highly complicated component. The time when carburetor systems could be synchronized and precisely adjusted "by ear" is a thing of the past. Modern measuring devices are required for the adjustment work.

But not only incorrectly adjusted carburettors are responsible for poor engine running, dirt in the mixing factory can also cause chaos. If channels are clogged with deposits or if tiny dirt particles prevent the free flow through the nozzle, the mixture preparation can get messed up. The mixture becomes lean, the engine coughs and spits. In the worst case scenario, it can lead to major engine damage.


"Sure instinct"
Synchronize carburetor

Carburetor cleaning campaign using the example of one
Honda GL 500 Silver-Wing from 1983

Honda GL 500

In the past, cleaning the carburettor was part of the basics of motorcyclists - this work was described in detail in every operating manual - in "modern" machines, fine air filters ensure clean breathing air and make cleaning largely unnecessary. Dirty or resinified carburettors are by no means uncommon. Especially when the machine has been standing in the corner for ages and you haven't drained the fuel from the float chambers. The fuel does not evaporate all together in the air. Residues are left over and stubbornly get stuck in the last corner.
The prerequisite for working on the carburetor is a high level of manual skill and a well-organized screwdriver booth. Hobby hobbyists who do not trust themselves to do this job should turn to a specialist with confidence.

The suction rubbers can be brittle or have fine hairline cracks

For a conscientious cleaning operation, it is advisable to dismantle the system. However, it is hardly possible to give general instructions for dismantling. On some machines, the component can be unscrewed in no time. With other bikes, the tank, seat, side cover, battery, air filter box and who knows what else have to be removed in order to get to the carburetor. The throttle and choke cables are unhooked and the ventilation hoses removed.

When removing the carburettors, carefully disengage the choke and throttle cables

To dismantle the system, it is placed on the sparkling clean workbench. As part of this work, you should definitely pay attention to the suction rubbers and connectors to the air filter box. If the bike has been around for a few years, the rubbers are almost always brittle or already torn. They absolutely need to be renewed. Holes in the suction rubbers can be devastating. The engine draws in secondary air, becomes lean and can be damaged.

But back to the mixing equipment. Even experienced screwdrivers are advised to make precise records when dismantling which nozzle or which adjusting screw was screwed in and how many turns were required to unscrew the screw. Once all the individual parts of the "factory" are on the workbench, the big cleaning up begins. The remedies or liquids the restorer uses to tackle the appliance vary greatly.

Carburetor cleaning fluid
Veteran, but now obsolete

For a long time, Yamaha carburetor cleaning fluid was the measure of all things. In a ratio of one part cleaner to three parts gasoline, both liquids are mixed together in a bowl and the carburetor components are soaked in it overnight.
What you fish out of the vat the next day is impressive. Not only do the nozzles flash sparklingly clean and all deposits in the carburetor have disappeared, the outer surfaces of the carburetor are also perfectly clean. For subsequent cleaning, the parts are washed off with fresh petrol and then blown off with compressed air. But please be careful that the small nozzles don't fly away between your fingers.

Everything is "sparkling clean" again

The Asian wonder broth is very aggressive to the skin and must never get into the eyes, but it does not attack any carburetor components made of plastic or rubber. After the procedure, it is advisable to pour the precious juice, for example through a coffee filter, into a clean container. The cleaning power remains well sealed and the mixture can be used again at the next opportunity. If the liquid is no longer required, it must be disposed of as hazardous waste because of its environmental pollution!
So far so good. In the meantime, due to the disadvantages mentioned above, the sale of the special liquid in Germany has been discontinued. In neighboring countries, the Yamaha dealers still have the product on offer on various occasions. Many fine screwdrivers in this country continue to swear by the devil's stuff and don't let the tincture come up against it.

The parts are washed with fresh gasoline
and then blown off with compressed air

A well-established method of carburetor cleaning is the ultrasonic bath. Well-equipped workshops have such a device. Here, too, you have to disassemble the carburetor down to the last screw, then all parts go into the bathroom. Depending on how dirty it is, the treatment takes 10 to 15 minutes, but it can take a little longer for stubborn deposits. As a rule, the workshop experts have a wealth of experience and act according to the circumstances.

Ultrasonic cleaning device here with Amal carburetors

An insider tip is recommended for do-it-yourselfers and do-it-yourselfers. In the drugstore you can get a 500 gram pack of citric acid for two euros. Half of the pack is placed in a disused saucepan with hot water. All dismantled carburetor components are then placed in the alternative cleaning bath for 10 to 15 minutes. Plastic parts do not suffer any damage. When the procedure is finished, normal tap water is sufficient for rinsing.
After a successful cleaning action, all parts are clearly arranged on the workstation and assembly can begin. If any carburetor seals are damaged, they will of course be replaced. When screwing in the nozzles, make sure that they are only pulled "hand-tight". Anyone who slams the nozzles too tightly will quickly over-turn a thread. Which component has to be lubricated with assembly grease during assembly can be found in the workshop manual. The same applies to the float level adjustment dimension and the basic setting of the mixture control screw. In the case of multi-carburetor systems, it is advisable to check the slide synchronism before installation and to adjust it if necessary.

After the cure, the Honda GL 500 runs like clockwork again

Once the system has been screwed together and attached, it is then adjusted. For this, the exact data - which differ from machine to machine - must be available. They are in the workshop manual or can be inquired about in the specialist workshop.