CarburetorsCarburetors A carburetor is a mechanical device which mixes air and A carburetor is a mechanical device which mixes air and

fuel together for combustion in an engine at approximately fuel together for combustion in an engine at approximately the correct air fuel ratio.the correct air fuel ratio.

Engine requirements vary widely, depending on weather, Engine requirements vary widely, depending on weather, temperature, load, and speed. And, there are as many temperature, load, and speed. And, there are as many other variations as there are types and sizes of engines, so other variations as there are types and sizes of engines, so carburetor design is dependant on a number of different carburetor design is dependant on a number of different factors.factors.

Carburetors have a hard time compensating for all of these Carburetors have a hard time compensating for all of these factors when compared to fuel injection.factors when compared to fuel injection.

However, a carburetor can be prepared and tuned to However, a carburetor can be prepared and tuned to create both good drivability.create both good drivability.

They are widely available through performance retailers They are widely available through performance retailers and are relatively inexpensive.and are relatively inexpensive.

With the wide spread application of fuel injection, tuning With the wide spread application of fuel injection, tuning of carburetors has become somewhat of a ‘lost art’.of carburetors has become somewhat of a ‘lost art’.

CarburetorsCarburetors The fueling requirements vary depending on the The fueling requirements vary depending on the

current operating conditions.current operating conditions. For example:For example:

While cruising at a steady speed, an air/fuel ratio of around While cruising at a steady speed, an air/fuel ratio of around 14.7:1 (known as ‘stoich’) provides all the power the engine 14.7:1 (known as ‘stoich’) provides all the power the engine needs to propel the vehicle. If the engine can tolerate it, needs to propel the vehicle. If the engine can tolerate it, running leaner, maybe 16:1, will provide increased fuel running leaner, maybe 16:1, will provide increased fuel economy.economy.

WOT (Wide Open Throttle) demands a rich air/fuel ratio. WOT (Wide Open Throttle) demands a rich air/fuel ratio. Generally, the engine will produce the most power at an air Generally, the engine will produce the most power at an air fuel ratio between 12:1 and 13:1. The exact ratio depends on fuel ratio between 12:1 and 13:1. The exact ratio depends on operating conditions and engine design and is arrived at by operating conditions and engine design and is arrived at by experimentation on the dynamometer.experimentation on the dynamometer.

When the engine is first started on a cold morning, the air/fuel When the engine is first started on a cold morning, the air/fuel ratio needs to be exceedingly rich due to ‘fuel dropout’ onto ratio needs to be exceedingly rich due to ‘fuel dropout’ onto the cold manifold surfaces. A 6:1 air/fuel ratio would not be the cold manifold surfaces. A 6:1 air/fuel ratio would not be uncommon.uncommon.

CarburetorsCarburetors

A Matter of MatterA Matter of Matter Matter can exist in three different states: Matter can exist in three different states:

solids, liquids, and gas.solids, liquids, and gas. Two of these states, liquid and gas, as Two of these states, liquid and gas, as

known as fluids.known as fluids. This means that they will flow readily This means that they will flow readily

from one container to another.from one container to another. For example, water is a fluid and will For example, water is a fluid and will

flow easily through a garden hose. Air is flow easily through a garden hose. Air is a fluid, and will flow easily through an a fluid, and will flow easily through an air hose in your shop.air hose in your shop.

CarburetorsCarburetors What causes flow?What causes flow?

Movement of any fluid is the direct result of difference in Movement of any fluid is the direct result of difference in pressure.pressure.

Direction of flow is always towards the point of least Direction of flow is always towards the point of least pressure.pressure.

Water flows through a hose because the end is exposed Water flows through a hose because the end is exposed to atmospheric pressure. The other end of the hose is to atmospheric pressure. The other end of the hose is subject to the greater pressure in the water line.subject to the greater pressure in the water line.

An engine is an air pump.An engine is an air pump. Every time a piston goes down on an intake stroke, it Every time a piston goes down on an intake stroke, it

creates a vacant space.creates a vacant space. Atmospheric pressure tries to fill this vacant space by Atmospheric pressure tries to fill this vacant space by

flowing through the carburetor.flowing through the carburetor. The rate of flow is controlled by the driver, when he The rate of flow is controlled by the driver, when he

selects the desired throttle opening.selects the desired throttle opening.

CarburetorsCarburetors Vacuum versus Low PressureVacuum versus Low Pressure

Vacuum is just another word for low pressure, or pressure Vacuum is just another word for low pressure, or pressure that is lower than atmospheric.that is lower than atmospheric.

Vacuum is measured in inches of Mercury (“Hg).Vacuum is measured in inches of Mercury (“Hg). There are 3 different ‘types’ of vacuum found in an engine.There are 3 different ‘types’ of vacuum found in an engine.

Manifold vacuum – vacuum port which is connected to the Manifold vacuum – vacuum port which is connected to the intake manifold between the intake valve and throttle plate in intake manifold between the intake valve and throttle plate in the carburetor. At idle, a vacuum gauge hooked up to a the carburetor. At idle, a vacuum gauge hooked up to a manifold vacuum port will show a high vacuum. Once the manifold vacuum port will show a high vacuum. Once the throttle is opened, the vacuum will drop to near atmospheric throttle is opened, the vacuum will drop to near atmospheric pressure.pressure.

Ported vacuum – vacuum is picked up from a port directly above Ported vacuum – vacuum is picked up from a port directly above the throttle plate. If a vacuum gauge were hooked up to this the throttle plate. If a vacuum gauge were hooked up to this port, at idle there would be no vacuum. However, when the port, at idle there would be no vacuum. However, when the throttle is cracked open the gauge would begin to register a throttle is cracked open the gauge would begin to register a vacuum. If the throttle is opened to WOT, the gauge will again vacuum. If the throttle is opened to WOT, the gauge will again go to near atmospheric pressure.go to near atmospheric pressure.

Venturi vacuum – vacuum is picked up from a port in the Venturi vacuum – vacuum is picked up from a port in the carburetors venturi. The amount of vacuum that registers on a carburetors venturi. The amount of vacuum that registers on a gauge is proportional to the amount of air flowing through the gauge is proportional to the amount of air flowing through the venturi.venturi.

CarburetorsCarburetors Venturi PrincipleVenturi Principle

The most important law of physics involved in The most important law of physics involved in carburetors was discovered by Daniel Bernoulli carburetors was discovered by Daniel Bernoulli in 1775. It deals with the venturi.in 1775. It deals with the venturi.

The venturi constant.The venturi constant. The ‘constant’ condition in a venturi is made up of two The ‘constant’ condition in a venturi is made up of two

primary factors, velocity and pressure.primary factors, velocity and pressure. The two factors, multiplied together, form the constant The two factors, multiplied together, form the constant

for any given condition and a particular time.for any given condition and a particular time. Pressure times velocity equals the constant.Pressure times velocity equals the constant. The factors of pressure and velocity depend on the The factors of pressure and velocity depend on the

pressure differential that is causing the flow. The pressure differential that is causing the flow. The constant will be different for different flow conditions.constant will be different for different flow conditions.

If the constant does not change, if velocity goes up, If the constant does not change, if velocity goes up, pressure will go down. If pressure goes up, velocity pressure will go down. If pressure goes up, velocity goes down.goes down.

CarburetorsCarburetors Sizes and SpeedsSizes and Speeds

When the engine draws air through the carburetor, the speed When the engine draws air through the carburetor, the speed of flow is determined by engine demand.of flow is determined by engine demand.

The speed of flow governs the amount of fuel that is drawn into The speed of flow governs the amount of fuel that is drawn into the cylinders.the cylinders.

When air flows through the venturi, it speeds up. When air flows through the venturi, it speeds up. That is because the same amount of air is flowing through the That is because the same amount of air is flowing through the

barrel of the carburetor, including the restricted venturi barrel of the carburetor, including the restricted venturi portion.portion.

The venturi is a narrow portion of the carburetor barrel. In The venturi is a narrow portion of the carburetor barrel. In order for the same amount of air to travel through this portion, order for the same amount of air to travel through this portion, it must speed up.it must speed up.

Daniel Bernoulli proved that as air, or any other fluid, passed Daniel Bernoulli proved that as air, or any other fluid, passed through the narrowest part of a venturi, it increases in speed, through the narrowest part of a venturi, it increases in speed, but decreases in pressure.but decreases in pressure.

This is contrary to the appearance of the tube. It would seem This is contrary to the appearance of the tube. It would seem that when the air goes through the narrow part of the venturi, that when the air goes through the narrow part of the venturi, the air would be compressed and the pressure would rise, but the air would be compressed and the pressure would rise, but this is not true.this is not true.

CarburetorsCarburetors

Six basic systems in a carburetorSix basic systems in a carburetor FloatFloat ChokeChoke IdleIdle Low speedLow speed AccelerationAcceleration High speedHigh speed

Each has a specific functionEach has a specific function Some of the functions overlapSome of the functions overlap

CarburetorsCarburetors Float circuitFloat circuit

The float bowl is a storage reservoir which maintains a The float bowl is a storage reservoir which maintains a precise level of fuel.precise level of fuel.

This assures there will be an ample supply of fuel for the This assures there will be an ample supply of fuel for the various fuel delivery requirements of the engine to meet various fuel delivery requirements of the engine to meet any operating condition.any operating condition.

The fuel is supplied to the float system by the fuel pump, The fuel is supplied to the float system by the fuel pump, which supplies a constant fuel pressure (usually 3 – 9 psi which supplies a constant fuel pressure (usually 3 – 9 psi depending on carburetor model, consult owners manual for depending on carburetor model, consult owners manual for your spec.)your spec.)

Needle, Seat, and FloatNeedle, Seat, and Float The opening and closing of the needle and seat is controlled by The opening and closing of the needle and seat is controlled by

the float and fuel pressure.the float and fuel pressure. Float is hollow and rides on the fuel.Float is hollow and rides on the fuel. As long as the fuel level in the bowl is below where is should As long as the fuel level in the bowl is below where is should

be, the needle and seat is open and fuel flows into the bowl.be, the needle and seat is open and fuel flows into the bowl. When the amount of fuel in the bowl reaches the proper level, When the amount of fuel in the bowl reaches the proper level,

the rising float shuts the needle and seat and blocks further the rising float shuts the needle and seat and blocks further fuel flow into the bowl.fuel flow into the bowl.

As the fuel in the bowl is consumed, the needle and seat open As the fuel in the bowl is consumed, the needle and seat open again and the cycle repeats.again and the cycle repeats.

CarburetorsCarburetors Float level is very importantFloat level is very important

If set too low, the engine will run lean.If set too low, the engine will run lean. If set too high, the engine will run rich.If set too high, the engine will run rich.

Also possible for fuel to flow down throat of carburetor after car Also possible for fuel to flow down throat of carburetor after car is shut off and ‘flood’ it. The result will be a hard or impossible is shut off and ‘flood’ it. The result will be a hard or impossible to start engine.to start engine.

Float bowl has a vent which goes to the atmosphere. It is Float bowl has a vent which goes to the atmosphere. It is sometimes called a ‘bowl vent’ or a ‘balance tube’.sometimes called a ‘bowl vent’ or a ‘balance tube’.

It is possible for a hollow float to get a hole in it. This It is possible for a hollow float to get a hole in it. This condition is called a ‘sunk’ float and will result in an condition is called a ‘sunk’ float and will result in an extremely flooded engine. Plastic floats that are solid are extremely flooded engine. Plastic floats that are solid are made to prevent this.made to prevent this.

‘‘Percolation’ is when fuel boils in the float bowl after Percolation’ is when fuel boils in the float bowl after the engine is shut off and heat soaks. This is usually the engine is shut off and heat soaks. This is usually seen as a hard start after the engine has sit ten or seen as a hard start after the engine has sit ten or more minutes after the engine is shut off.more minutes after the engine is shut off.

‘‘Vapor lock’ is when fuel boils in the fuel line, and the Vapor lock’ is when fuel boils in the fuel line, and the pump can no longer supply sufficient fuel to fill the pump can no longer supply sufficient fuel to fill the bowl.bowl.

CarburetorsCarburetors

Idle systemIdle system Essentially, a passage that bypasses the closed Essentially, a passage that bypasses the closed

throttle plate.throttle plate. Air is mixed with fuel through and ‘idle air bleed’ Air is mixed with fuel through and ‘idle air bleed’

and travels down to the idle mixture adjustment and travels down to the idle mixture adjustment screw.screw.

Idle air bleeds ensure that the air is volatilized Idle air bleeds ensure that the air is volatilized enough to be distributed to all cylinders.enough to be distributed to all cylinders.

Idle mixture adjustment screws ONLY adjust mixture Idle mixture adjustment screws ONLY adjust mixture at idle. It does not affect the mixture off idle at all.at idle. It does not affect the mixture off idle at all.

Any mixture adjustments off idle must be done by Any mixture adjustments off idle must be done by changing jets/metering rods.changing jets/metering rods.

CarburetorsCarburetors

Low speed systemLow speed system Low speed system can be considered a bridge Low speed system can be considered a bridge

between the idle system and the high speed between the idle system and the high speed system.system.

As throttle is opened slightly, vacuum moves up As throttle is opened slightly, vacuum moves up carburetor barrel slightly above throttle plate.carburetor barrel slightly above throttle plate.

A ‘transfer port’ is slightly about the throttle port A ‘transfer port’ is slightly about the throttle port and works similarly to the idle system, except and works similarly to the idle system, except there is no adjustment.there is no adjustment.

As high speed system comes into operation, both As high speed system comes into operation, both the low speed system and the idle system are the low speed system and the idle system are phased out and no longer supply fuel.phased out and no longer supply fuel.

CarburetorsCarburetors Acceleration SystemAcceleration System

When the throttle is opened suddenly, from the When the throttle is opened suddenly, from the low speed or idle position, a couple of things low speed or idle position, a couple of things happen that prevent the engine from getting the happen that prevent the engine from getting the richer mixture it needs. If it does not, the engine richer mixture it needs. If it does not, the engine will stumble and may stackfire. This may result in will stumble and may stackfire. This may result in a carburetor fire. a carburetor fire.

Fuel is no longer drawn from the idle and low-speed Fuel is no longer drawn from the idle and low-speed systems due to loss of a low pressure area near them. systems due to loss of a low pressure area near them. This leans the mixture.This leans the mixture.

The high speed system cannot supply the extra fuel The high speed system cannot supply the extra fuel immediately. Since this is a mechanical device, it takes immediately. Since this is a mechanical device, it takes a moment for fuel to get flowing. This leans the mixture a moment for fuel to get flowing. This leans the mixture temporarily.temporarily.

As the pressure inside the manifold suddenly increases, As the pressure inside the manifold suddenly increases, fuel ‘drops out’ of the air/fuel mixture and condenses on fuel ‘drops out’ of the air/fuel mixture and condenses on the manifold walls. This leans the mixture.the manifold walls. This leans the mixture.

CarburetorsCarburetors Therefore, fuel must be force-fed into the intake Therefore, fuel must be force-fed into the intake

air stream until the high speed circuit can come air stream until the high speed circuit can come into operation.into operation.

An ‘accelerator pump’ is what does this.An ‘accelerator pump’ is what does this. Located in a separate well in the float bowl.Located in a separate well in the float bowl. Sprays fuel directly into the air stream to make up for Sprays fuel directly into the air stream to make up for

lean mixtures during ‘transient’ conditions.lean mixtures during ‘transient’ conditions. At idle, the throttle linkage pulls the pump to its highest At idle, the throttle linkage pulls the pump to its highest

position. position. When accelerator pedal is depressed, the linkage When accelerator pedal is depressed, the linkage

releases the pump operating rod. This allows the releases the pump operating rod. This allows the accelerator pump spring to actuate the pump.accelerator pump spring to actuate the pump.

The throttle linkage does not depress the pump, the spring The throttle linkage does not depress the pump, the spring does. It just allows it too.does. It just allows it too.

If the throttle is quickly opened to WOT, fuel may flow from If the throttle is quickly opened to WOT, fuel may flow from the accelerator pump nozzles for several seconds.the accelerator pump nozzles for several seconds.

Generally only works during the 1Generally only works during the 1stst half of throttle travel. half of throttle travel.

CarburetorCarburetor High Speed SystemHigh Speed System

As air requirements increase, the idle and low speed As air requirements increase, the idle and low speed systems are phased out and the high speed system comes systems are phased out and the high speed system comes into play.into play.

The high speed system is the main metering system and The high speed system is the main metering system and is what uses a venturi.is what uses a venturi.

A basic system includes a discharge tube, venturi, main A basic system includes a discharge tube, venturi, main well passage, and main metering jet.well passage, and main metering jet.

The discharge tube extends from the main fuel well to the The discharge tube extends from the main fuel well to the narrowest part of the venturi.narrowest part of the venturi.

It is possible for a carburetor to have a ‘double venturi’.It is possible for a carburetor to have a ‘double venturi’. The fuel in the main well is at the same level as the fuel The fuel in the main well is at the same level as the fuel

in the float bowl as long as pressure remains equal.in the float bowl as long as pressure remains equal. Once significant air starts flowing through the venturi a Once significant air starts flowing through the venturi a

vacuum is developed at the discharge tube. vacuum is developed at the discharge tube. Since the float bowl has atmospheric pressure above it, Since the float bowl has atmospheric pressure above it,

fuel will begin flowing out the discharge tube.fuel will begin flowing out the discharge tube.

CarburetorsCarburetors Choke SystemChoke System

Choke provides the rich mixture needed to start a Choke provides the rich mixture needed to start a cold engine.cold engine.

A ‘choke plate’ partially blocks off the top of the A ‘choke plate’ partially blocks off the top of the carburetor.carburetor.

This causes a vacuum in the idle, low speed, and This causes a vacuum in the idle, low speed, and high speed systems, and causes fuel to flow from high speed systems, and causes fuel to flow from all which causes a rich mixture.all which causes a rich mixture.

Choke must be gradually opened as engine warms Choke must be gradually opened as engine warms up.up.

A manual choke uses a cable and knob to open and A manual choke uses a cable and knob to open and close the choke.close the choke.

An automatic choke is either electric or heated by An automatic choke is either electric or heated by the temperature of the engine.the temperature of the engine.

Carb will also have a ‘fast idle cam’ which Carb will also have a ‘fast idle cam’ which increases the idle while the choke is on for easier increases the idle while the choke is on for easier running when cold.running when cold.

CarburetorsCarburetors How to start a carbureted engine:How to start a carbureted engine:

Unlike a fuel injected engine, there is a special Unlike a fuel injected engine, there is a special starting procedure for carburetors.starting procedure for carburetors.

Pump gas to the floor once and release pedal all the Pump gas to the floor once and release pedal all the way.way.

Crank engine to start.Crank engine to start. If engine does not start, pump pedal once more and If engine does not start, pump pedal once more and

crack throttle about ¼ open and crank engine until it crack throttle about ¼ open and crank engine until it starts.starts.

If engine does not start, it either has not gotten If engine does not start, it either has not gotten enough fuel or it is flooded.enough fuel or it is flooded.

Pump the gas three times and see if it will start.Pump the gas three times and see if it will start. If it doesn’t, it is likely flooded. Press the gas all the way If it doesn’t, it is likely flooded. Press the gas all the way

to the floor and crank the engine until it starts.to the floor and crank the engine until it starts. Lower volatility of modern fuels and different Lower volatility of modern fuels and different

distillation curve has made fuels for carbureted distillation curve has made fuels for carbureted vehicles less than optimal, especially when it comes to vehicles less than optimal, especially when it comes to starting.starting.

Starting a carbureted car may require practice.Starting a carbureted car may require practice.

CarburetorsCarburetors Tuning carburetorsTuning carburetors

There are many good brands of carburetors on There are many good brands of carburetors on the market with a good variety of supporting the market with a good variety of supporting parts.parts.

HolleyHolley Make wide variety of 2 and 4 barrel carbsMake wide variety of 2 and 4 barrel carbs The standard in racingThe standard in racing

EdelbrockEdelbrock Now makes the ‘AFB’, which was formerly made by Now makes the ‘AFB’, which was formerly made by

Carter, Magnetti Marelli, and Weber.Carter, Magnetti Marelli, and Weber. WeberWeber

Side draft WeberSide draft Weber 4 4 WebersWebers on V8 on V8

QuadrajetQuadrajet Installed on many, many GM vehicles.Installed on many, many GM vehicles. When tuned correctly they are an excellent performance When tuned correctly they are an excellent performance

carburetorcarburetor There is a lot of support for themThere is a lot of support for them

CarburetorsCarburetors Tuning carburetorsTuning carburetors

First, rough tune idle so it won’t stall.First, rough tune idle so it won’t stall. If engine idles, bump idle up to about 1000 rpm.If engine idles, bump idle up to about 1000 rpm. Spray a short burst of carburetor cleaner into the carb. If idle Spray a short burst of carburetor cleaner into the carb. If idle

increases, the mixture is lean. If engine bogs down then increases, the mixture is lean. If engine bogs down then mixture is too rich.mixture is too rich.

Turn off engine. Turn idle mixture screws in, counting the Turn off engine. Turn idle mixture screws in, counting the number of turns it takes until the mixture screw stops turning. number of turns it takes until the mixture screw stops turning. Do not force it. Record number of turns.Do not force it. Record number of turns.

If carb has multiple mixture screws they should be turned out If carb has multiple mixture screws they should be turned out an equal number of turns. an equal number of turns.

If mixture was too lean, turn screws out ¼ turn more than they If mixture was too lean, turn screws out ¼ turn more than they were previously.were previously.

If mixture was rich, turn screws out ¼ turn less than they were.If mixture was rich, turn screws out ¼ turn less than they were. Spray another burst of carb clean in carburetor and see what Spray another burst of carb clean in carburetor and see what

happens. happens. If engine is still too lean, back off all idle mixture screws an equal If engine is still too lean, back off all idle mixture screws an equal

number of turns ¼ turn at a time.number of turns ¼ turn at a time. If engine is too rich, do the same but turn the screws inward.If engine is too rich, do the same but turn the screws inward.

Some Holleys work in reverse! If you are not getting the Some Holleys work in reverse! If you are not getting the desired results, try going the opposite way with the screws.desired results, try going the opposite way with the screws.

The same procedure is used when setting idle to final spec.The same procedure is used when setting idle to final spec.

CarburetorsCarburetors

To tune a carburetor properly you need an To tune a carburetor properly you need an emissions analyzer (4 or 5 gas analyzer).emissions analyzer (4 or 5 gas analyzer). What goes in, must come out.What goes in, must come out. Lets you know how efficient combustion is.Lets you know how efficient combustion is.

Air/fuel ratio monitors do not seem to work Air/fuel ratio monitors do not seem to work well for tuning carbureted cars.well for tuning carbureted cars. Great for precisely tuning load/rpm points on Great for precisely tuning load/rpm points on

fuel injection.fuel injection. Since a change in jet size in a carb affects fuel Since a change in jet size in a carb affects fuel

metering at all rpm you do not need something metering at all rpm you do not need something so precise.so precise.

CarburetorsCarburetors 4 gas analyzer4 gas analyzer

CO (Carbon Monoxide)CO (Carbon Monoxide) Measured in percentageMeasured in percentage Lean mixtures will have nearly no COLean mixtures will have nearly no CO CO will go up progressively as mixture gets richer.CO will go up progressively as mixture gets richer.

HC (Hydrocarbons)HC (Hydrocarbons) Measured in PPM (parts per million)Measured in PPM (parts per million) Lean mixtures will have high HCLean mixtures will have high HC Correct mixtures will have low HCCorrect mixtures will have low HC Overly rich mixtures will have high HC (and high CO)Overly rich mixtures will have high HC (and high CO)

CO2 (Carbon Dioxide)CO2 (Carbon Dioxide) Measured in percentageMeasured in percentage Measure of combustion efficiencyMeasure of combustion efficiency When tuning non-catalyst vehicles I usually ignore thisWhen tuning non-catalyst vehicles I usually ignore this

CarburetorsCarburetors

Setting idle mixture with HC and COSetting idle mixture with HC and CO Set idle to desired RPMSet idle to desired RPM Check exhaust gases with warm engineCheck exhaust gases with warm engine

CO should be 3-4%. The higher the CO the CO should be 3-4%. The higher the CO the richer the mixture is. So if the CO reading richer the mixture is. So if the CO reading is 5%, you need to lean out the mixture by is 5%, you need to lean out the mixture by turning in the screws.turning in the screws.

HC - On a stock street vehicle with no cats, HC - On a stock street vehicle with no cats, HC at idle should be below about 300 ppm. HC at idle should be below about 300 ppm. With a cam with a lot of overlap, HC can be With a cam with a lot of overlap, HC can be as high as 1000 ppm.as high as 1000 ppm.

CarburetorsCarburetors

Tuning mixture at cruise (high speed Tuning mixture at cruise (high speed circuit) with gas analyzer.circuit) with gas analyzer. Bring engine up to 3000 rpm.Bring engine up to 3000 rpm.

CO should be between 1.5% and 3% COCO should be between 1.5% and 3% CO HC should be between 0 and 300 ppmHC should be between 0 and 300 ppm If CO is above 3%, carb needs smaller jets, If CO is above 3%, carb needs smaller jets,

or larger metering rods.or larger metering rods. If CO is below 1.5%, carb needs larger jets, If CO is below 1.5%, carb needs larger jets,

or smaller metering rods.or smaller metering rods.