Design and device of the carburetor of the outboard motor

The outboard motor is the heart of your boat, and the carburetor is the regulator of the boat, the regulator of the engine, the regulator of the fuel-air mixture, and understanding how this unit works allows you not only to properly maintain the equipment, but also to quickly diagnose malfunctions that can catch you away from the shore. boat-engine have their own specifications associated with high speed work and the specifics of operation in the aquatic environment.

The main task of the device is to precisely mix gasoline with air in the proportions necessary for efficient combustion in the engine cylinders. The slightest deviation in the composition of the mixture - whether it is over-enrichment or, conversely, excessive poverty - leads to a drop in power, overheating or unstable idling, which is why Construction of the outboard carburetor requires detailed consideration so that each owner can confidently understand its internal processes.

In this article, we will discuss the key components of the fuel supply system, explain the principle of operation of the float chamber and consider the influence of various factors on the quality of the mixture. two-stroke and four-stroke units, providing them with a long service life and reliable operation on water.

Principle of operation and basic scheme

The fundamental principle of the carburetor is based on Bernoulli's physical law, which states that when air passes through the narrowing (diffuser), its speed increases and the pressure drops. It is this pressure difference that creates a dilution effect that causes fuel to rise from the float chamber and spray into the airflow, a process that occurs continuously while the engine is running, ensuring stable combustion.

Modern models, such as popular ones Tohatsu or YamahaThey use a complex system of jigsaws and channels, which allows you to adjust the composition of the mixture in different modes of operation. At low speeds, fuel is fed through one channel system, and when the throttle is fully opened, other circuits come into operation, this separation is necessary to ensure smooth running and responsiveness of the motor throughout the entire speed range.

It's important to understand that the carburetor is not just a mixer, but a dynamic system that responds to the change in load on the screw. When a throttle is opened abruptly, the pressure in the diffuser changes instantly, and the device must have time to supply an additional portion of fuel to avoid failure, and this is the reaction that the accelerator pumps and special emulsion wells are responsible for.

⚠️ Attention: When disassembling a carburetor, never use metal wire or sharp objects to clean the jellyclera. This can change the caliber of the hole and disrupt the estimated throughput, leading to irreparable disruption to the engine.

Float chamber and fuel supply system

The heart of the carburetor is a float chamber that serves as a reservoir for storing a small amount of fuel and keeping gasoline steady. Inside the chamber is a float that is mechanically connected to the shut-off needle. When the fuel level drops, the float goes down, opening the valve, and the gasoline comes out of the tank. Once the level reaches normal, the needle blocks the supply.

Fuel stability is critical to the proper operation of all metering systems. If the level is too high, the mixture will become over-enriched, causing black smoke from the silencer and smoked candles. Too low will lead to poorer mixture, overheating of the piston group and possible jamming of the engine. The design often provides a drain screw to drain the sludge.

Modern systems, especially eco-certified engines, can use additional valves to prevent fuel leakage when the engine tilts or flips over, implemented through a system of ball valves that overlap the channels when the angle of the carburetor body changes relative to the horizon.

• 🔹 Float regulates the level of fuel, is made of brass or plastic, requires a seal check.
• 🔹 Shut-off needle - precision detail, the cone of which should fit perfectly to the valve saddle for tightness.
• 🔹 Drain-drive screw - allows you to drain water and dirt from the bottom of the chamber without removing the node.

Dosing systems: idling and average loads

The engine is operated at low speeds by an idling system, which is a separate channel that bypasses the main throttle, in which air enters through a small hole created by a slightly slightly open valve, and fuel is sucked through a special idle jikler, and the quality of the mixture is adjusted here by a screw that changes the amount of air or fuel supplied.

When you go to medium speed, the main metering system comes into operation, the fuel starts to flow through the main jigsaw, the amount of which depends on the position of the throttle, the cone needle, attached to the throttle, moves inside the sprayer, changing the passage section and thereby adjusting the enrichment of the mixture depending on the position of the gas handle.

The precise configuration of this system allows you to achieve maximum efficiency and purity of exhaust in cruising mode. If the cone needle is set too high, the mixture will be poor, which will cause jerks when the gas is opened smoothly. Too low position of the needle will lead to fuel overrun and gas formation.

Mode of work	Basic element	Adjustable parameter
Idle move	Giller XX, quality screw	Revolving and stability
Average turnover	Cone needle, sprayer	Throttle responsiveness
Full gas.	Head jeckel	Maximum power
Acceleration	Accelerator pump	Smoothness of acceleration

Maximum speed system and accelerator pump

When the throttle is fully open, the maximum speed system comes into operation, at which point the cone needle exits the spray completely, and the amount of fuel supplied depends solely on the throughput of the main jeepler, which is critical, because this is the mode in which the engine develops maximum power, and an error in the selection of the jeepler can cost the piston group its life.

To achieve a sharp set of turns, many carburetors are equipped with an accelerator pump, a mechanical or vacuum device that, when pressed sharply on the gas, injects an additional portion of fuel into the diffuser, which compensates for the inertia of the air flow and prevents dips when the air flow has already accelerated and the fuel has not yet evaporated in the right amount.

Two-stroke engines often use resonant exhaust, which creates pressure pulsations that affect cylinder filling. The carburetor must be consistent with these pulsations so that there is no back-ejection or insufficient flow of the mixture, so the intake path geometry and the size of the diffuser are carefully selected by engineers.

Why does the engine stop when the gas is opened?

This is a classic sign of accelerator pump failure or transient jeeler clogging, and the mixture is drastically depleted, and the combustion process is interrupted until the flow stabilizes.

Environmental systems and automation

Modern outboard motors are increasingly equipped with cold-start automatic enrichment systems, using a thermostatic valve or electromagnetic enricher instead of a hand-pump, and opening an additional channel when power is applied or when it is cold, through which the enriched mixture needed to start is fed into the cylinder.

Fuel vapor return systems (EVAPs) may also be present in the design, although they are less common on small outboard motors than on car engines, and their main objective is to minimize carbon emissions, which is especially important for motors certified to EPA or CARB standards.

An important element is the petal valve valve vacuum actuator, which is often integrated into the carburetor housing, which responds to intake manifold pressure and helps optimize cylinder filling at different revolutions, improving thrust at the bottom and power at the tops.

• 🔸 Electromagnetic valve - automatically controls the start-up enrichment, requires proper wiring.
• 🔸 Thermostat mechanically opens the fuel supply channel at low engine temperature.
• 🔸 Return system Directs the gasoline vapor back to the tank or intake, reducing toxicity.

⚠️ Attention: If there are electrical components in the carburetor (electromagnetic starter), it is forbidden to blow the knot with aggressive chemistry without first removing the coils.

Typical malfunctions and their impact on the work

The most common problem is that the jelly jelly tar deposits formed during long-term downtime of the gasoline engine in the system, the thin channels of the main metering system and idling are clogged first, which leads to the inability to start the engine or run at low revs. Regular use of fuel stabilizers helps to avoid this.

Another major problem is the sucking of air through the crankshaft gaskets or glands. Because the carburetor is working at a dilution, any unaccounted air that enters the carburetor depletes the mixture, manifested in the form of floating idle turns and difficulty starting. Diagnosis is done by spraying the joints with combustible liquid — if the speed changes, then there is a sucker.

The wear and tear of the needle and the shut-off valve saddle causes the float chamber to overflow and the fuel to spill, and the gasoline starts flowing through the drain tubes or, worse, into the engine crankcase, flushing the oil and causing hydraulic shock or bullying, visually visible in the black dim and characteristic odor of the exhaust.

Maintenance and configuration of the device

To properly adjust the carburetor, you must first make sure that the rest of the engine systems are working properly: compression, ignition and intake tightness. Adjustment begins with the installation of the mixture quality screw in the middle position (usually 1.5-2 revolutions from the stop), after which the engine starts and warms up.

Then, by rotating the number or quality screw (depending on the design), you get the maximum and stable idling turns. Then the quality screw finds the position at which the speed is maximum, and slightly enriches the mixture for stability. Full loading is only possible on water or on a special stand with a water pump.

Regular ultrasound cleaning and compressed air blowing of all channels is a mandatory procedure for each seasonal service. Specialized carburetor cleaners allows you to dissolve lacquer deposits that are not removed by ordinary gasoline, restoring the factory throughput of jellyplants.

Why is a carburetor engine better than an injection engine for a boat?

Carburetor engines are easier to repair, cheaper to maintain, and less sensitive to fuel quality, which is critical in remote waters, and they don’t require electricity to run the fuel supply system, which increases reliability.

How often should the carburetor be cleaned?

It is recommended to carry out preventive cleaning once a season or after every 100 motor hours. If the motor is used infrequently, be sure to drain the fuel from the float chamber before long storage.

Can I wash the carburetor with gasoline?

Gasoline will wash away only fresh contaminants, and special chemical solvents or an ultrasonic bath are needed to remove resinous deposits and varnish left over from the evaporation of the light fractions.