Marine Carburettors
Both 4-stroke and 2-stroke carburettors work the same. Fuel is delivered to a small resiviour in the Carb called a float chamber. The fuel level in this chamber is controlled by a needle valve. As the float rises and falls, as fuel is sucked through the Carburettor into the engine, the needle valve opens annd closes allowing fuel to flow as needed. The fuel is then sucked into the Venturi, mixes with the Air, and is drawn into the cylinder. There are many different jets and passages within a Carburettor. There are different systems that come into effect when needed, to provide more fuel or air. Carburettors work using vacuum. More vacumm being produced will operate different systems within the Carburettor.Carburettors come in different sizes and connfigurations. Single, Double and Quad carburettors are common.
Venturi - This is the name given to the main passage of the caburettor. The Venturi is the shape of a straight pipe and has had small lumps, similar in shape to an aircraft wing, shaped into its design. This helps create vacuum in the caburettor ans the air compress's at the narrowest point of the venturi and speeds up as it passes through.What is the purpose of the Venturi & how does it work?
Carburetor componnents:
What is an Emulsion tube?
The Emulsion tube is a carburettor component. A small tube with lots of tiny holes in it. Located within the main dischage nozzle. It is designed to help pre-mix the extra fuel with air when accelerating.Manual & Automatic Chokes:
Manual chokes are simple. A cable which is attached to the choke butterfly, is pulled to close choke. The cable can be pushed back to open then choke. This give the operator the ability to change the desired amount of choke as needed.
Automatic chokes are a little more complicated. There are three different types of automatic chokes, Themostatic, Electric and Wax element. All three close the choke valve when the engine is cold and progressively open it as the engine warms up. There is a choke opening device that opens the butterfly when the throttle valve is open.This prevents the choke from operating when its not needed.
Thermostatic chokes consist of a Bimetal spring, Housing, Vacuum piston. The spring is responsible for closing the choke and opens as the engine heats up. The Vacuum piston is resposible for opening the choke butterfly at higher engine speeds.
Electric chokes have a thermostatic spring and a heating element. As the heating element heats up it also heats the spring. As the spring heats up it opens the choke butterfly. These are usually fitted with a Vacuum diaphram unit, which opens the butterfly when accelerating.
Wax Element chokes operate by expansion and contraction.The choke butterfly is closed. As the engine coolant heats up it expands the wax element pushing a rod attached to the choke butterfly. As the temperature rises and the element continues to expand the choke will open further. This is also fitted with a Vacuum pump for higher speeds and acceleration.
How does a Bimetal spring work in an automatic choke?
A Bimetallic spring is used to convert temperature change into mechanical displacement. The spring will consist of two metals which expand at different rates as heat acts on them. Made from steel and Copper, although in some cases copper can be sustituted for brass.
To adjust the float gently bend the bracket of the float untill the desire angle is achived. This can be measured by placing a feeler gauge between the float and needle valve needle. Check specifications.
This picture shows the difference in thermal expansion of the two metals.
As the bimetal spring heats up, the diffence in expansion rates causes the spring to rise and fall. Opening and closing the choke.
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| Float Chamber. Upside down. |
What is the purpose of the 'Float'?
The Float is part of a needle valve system that controls the the fuel supplied from the fuel pump to the float bowl. The float bowl holds a small amount of fuel and is designed to sustain fuel supply to the carburettor under various conditions. As the fuel supply increases the fuel level drops, as does the float. As the float lowers it allows the needle to rise out of its seat and fuel to flow into the chamber. As the float rises the fuel to the chamber reduces.To adjust the float gently bend the bracket of the float untill the desire angle is achived. This can be measured by placing a feeler gauge between the float and needle valve needle. Check specifications.
What is a Mechanical lift pump & how does it work?
A mechanical lift pump is a diaphram fuel pump operated by a cam on the camshaft. This pump is constantly operating. Its is fitted with a tank return hose to bleed off any excess pressure.
How does a Vacuum fuel pump work on a two stroke:
A Vacuum fuel pump works using a vacuum line from the carburettor. As this vacuum line pulses with the intake strokes of the 2 stroke it acts on a diaphram, the fuel is draw in a one way valve and subsequently push out the other.
What pressures will these pumps produce?
What pressures will these pumps produce?
Mechanical lift pumps generally work with a line pressure of between 3-5psi.
What is the difference between Petrol & Desiel Fuel?
Diesel is produced from the fractional distillation of crude oil at 200-350degrees. Diesel is made up of 75% saturated hydrocarbons and 25% aromatic hydrocarbons. Diesel is more oily than petrol and has less fumes.
Diesel can be derived from sources other than petroleum. Diesel is usually ignited by injecting it into a heated compressed air.
Petrol or Gasoline is a yellowish coloured derivative of petroleum. Has also been used as a solvent. Composed primarily of saturated hydrocarbons. Petrol is often enhanced with iso-octane or aromatic hydrocarbons like Toluene and Benzene to increase the octane level. Petrol gives off large amounts of fumes and is easily combustable. Petrol is ignited with a spark.
Diesel can be derived from sources other than petroleum. Diesel is usually ignited by injecting it into a heated compressed air.
Petrol or Gasoline is a yellowish coloured derivative of petroleum. Has also been used as a solvent. Composed primarily of saturated hydrocarbons. Petrol is often enhanced with iso-octane or aromatic hydrocarbons like Toluene and Benzene to increase the octane level. Petrol gives off large amounts of fumes and is easily combustable. Petrol is ignited with a spark.
Fuel shutoff soloniod:
A fuel shutoff solonoid is designed to open a valve, within the fuel line, as voltage is passed through it. Allowing fuel to circulate. This process is usually initiated by the ignition switch. As the ignition is turned off and power is cut to the solonoid, the valve closes. Cutting fuel supply to the engine.
To check operation of this part; Apply volatge to the solonoid circut, if the unit is operating properly you should hear a ticking sounnd as you connect and disconnnect the power.
To check operation of this part; Apply volatge to the solonoid circut, if the unit is operating properly you should hear a ticking sounnd as you connect and disconnnect the power.
Four stroke cycle:
There are four strokes in a four stroke cycle.
They are:
Intake - A mixture of fuel and air are drawn through the intake manifold and into the cylinder.
Compression - The mixture of fuel and air is compressed as the piston strokes up the cyclinder.
Intake - A mixture of fuel and air are drawn through the intake manifold and into the cylinder.
Compression - The mixture of fuel and air is compressed as the piston strokes up the cyclinder.
Ignition- The fuel mixture has been compressed annd is ignited. The mixture is ignited by a spark just before TDC
Exhaust - After the fuel mixture has been burnt the exhaust valve opens and the piston pushes the exhaust gas out the exhaust.
What does S.I mean?
Spark Ignition, this is when combustion of the fuel/air mixture is ignited with a spark from the spark plug.
What does C.I mean?
Compression Ignition, this is when combustion of the fuel/air mixture is ignited by the temperature of the air due to compression. Ignition takes place as the fuel (usually diesel) is injected into the compressed air.
How is the idle and mixture screw adjusted?
How is the idle and mixture screw adjusted?
This is adjusted by turning the screw with a screw driver. This is usualy done whilst the engine is running as it has an immediate effect. Fuel mixture can be more accurately adjusted when measuring exhaust gasses.
What is the purpose of the power valve?
The purpose of the power valve is to deliver more fuel into the carburettor during acceleration.
What are some causes of high fuel consumption?
Some causes of high fuel comsumption could be: - Float level to high.
- Carburettor flooding
- Idle/Mixture screw needs adjusting
- Choke valve not opening properly
- Accelerator pump link needs adjusting
- Power valve stuck
- Restricted air intake
Carb stripping:
- Carburettor flooding
- Idle/Mixture screw needs adjusting
- Choke valve not opening properly
- Accelerator pump link needs adjusting
- Power valve stuck
- Restricted air intake
Carb stripping:
When dismantiling a carburettor what precautions should be taken?
It is important when dismantiling a Carb that great care is taken. Make sure that your area is clean and tidy. make sure you use the right tool for the job as there are many small fragile parts to remove. Be sure not to turn the carb upside down. Take carefull note of how you pull the carb apart as you may not re-assemble it for a few days.
It is important when dismantiling a Carb that great care is taken. Make sure that your area is clean and tidy. make sure you use the right tool for the job as there are many small fragile parts to remove. Be sure not to turn the carb upside down. Take carefull note of how you pull the carb apart as you may not re-assemble it for a few days.
When re-fitting a three multiple carburettor system what should be done once fitted?
When fitting a multiple carb system it is important that you check all linkages are conected and operating correctly. Check Throttle and Choke butterflies are working correctly.
When fitting a multiple carb system it is important that you check all linkages are conected and operating correctly. Check Throttle and Choke butterflies are working correctly.
References:
Ed May Vol 1
Ed May Vol 2
