Carburetor Theory

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Carburetor Carburetor Theory Theory A.N.Sainath A.N.Sainath

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Thory of Carburetor

Transcript of Carburetor Theory

Page 1: Carburetor Theory

Carburetor Carburetor TheoryTheoryA.N.SainathA.N.Sainath

Page 2: Carburetor Theory

Variable Venturi or “slide” Variable Venturi or “slide” carburetorscarburetors

The goal of providing The goal of providing the stochiometric the stochiometric ratio, (14.7:1) is ratio, (14.7:1) is tempered with the tempered with the goal of providing goal of providing extra fuel to cool (by extra fuel to cool (by evaporation) the evaporation) the engine.engine.

Carburetor tuning Carburetor tuning must be done to must be done to provide max power provide max power without overheating without overheating the 2 stroke engine.the 2 stroke engine.

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VM CarburetorVM Carburetor Slide carburetor Slide carburetor

provides smoother provides smoother variation and better variation and better adjustment than a adjustment than a throttle plate throttle plate carburetor.carburetor.

VM is a Mikuni round VM is a Mikuni round slide carburetor.slide carburetor.

Various circuits control Various circuits control fuel and air flow for fuel and air flow for different conditionsdifferent conditions Starting, (idle) pilot, and Starting, (idle) pilot, and

main main

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Integrated carburetor Integrated carburetor systems systems

The carburetor has The carburetor has multiple adjustments multiple adjustments for different throttle for different throttle positions;positions; 0-1/4 Pilot jet0-1/4 Pilot jet 1/8-1/2 Throttle 1/8-1/2 Throttle

cutawaycutaway ¼-3/4 Jet needle (clip)¼-3/4 Jet needle (clip) ¼-3/4 Needle jet¼-3/4 Needle jet ½-Full Main jet½-Full Main jet ½-Full Air adjustment ½-Full Air adjustment

screwscrew ½-full Power jet ½-full Power jet

(optional)(optional)

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Starter CircuitStarter Circuit Mini- slide carburetor Mini- slide carburetor

used for startingused for starting ““Choking” the Choking” the

carburetor actually carburetor actually opens plunger to opens plunger to allow gas flowallow gas flow

Gas is drawn from Gas is drawn from the bottom of the the bottom of the bowl through a bowl through a starter jetstarter jet

Air is drawn from in Air is drawn from in front of the slidefront of the slide

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Starter circuitStarter circuit

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Starter CircuitStarter Circuit

Starter can only function when slide is Starter can only function when slide is closed and vacuum is highclosed and vacuum is high

Starter jet may need to be changed if Starter jet may need to be changed if using a snowmobile carb on a motorcycle, using a snowmobile carb on a motorcycle, or vice-versaor vice-versa

Plunger positions – Open (choked), half-Plunger positions – Open (choked), half-open, closedopen, closed

Starter circuit can be used to enrichen Starter circuit can be used to enrichen mixture at other times to check mixture at other times to check carburetor problems.carburetor problems.

Page 8: Carburetor Theory

Pilot circuitPilot circuit Used at idle to ¼ Used at idle to ¼

throttlethrottle Gas flow is controlled Gas flow is controlled

by pilot jet size, this jet by pilot jet size, this jet is the first to plug if is the first to plug if gas thickensgas thickens

Air flow is controlled Air flow is controlled by air adjustment by air adjustment needleneedle

At idle, air flows in at 5 At idle, air flows in at 5 to further mix with fuelto further mix with fuel

At just off idle, mixed At just off idle, mixed fuel-air flows out at fuel-air flows out at both 6 and 5 both 6 and 5

Page 9: Carburetor Theory

Pilot CircuitPilot Circuit Stumble coming off Stumble coming off

idle indicates plugged idle indicates plugged pilot jet or air screw pilot jet or air screw too tighttoo tight

Idle speed controlled Idle speed controlled by a separate screw by a separate screw that determines how that determines how tightly the slide tightly the slide closes.closes.

Slides with different Slides with different shape cutouts are also shape cutouts are also sold for tuning sold for tuning applications.applications.

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Mid range operationMid range operation ¼ to ¾ operation is ¼ to ¾ operation is

controlled by needle controlled by needle jet and jet needle. As jet and jet needle. As slide pulls up the slide pulls up the long tapered needle long tapered needle is drawn out of is drawn out of needle jet.needle jet.

The air jet controls The air jet controls the air bled in to the air bled in to atomize the fuel.atomize the fuel.

The main jet is so The main jet is so large that it does not large that it does not limit flowlimit flow

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Mid range operationMid range operation Both needle jet and Both needle jet and

jet needles can be jet needles can be changed, as can the changed, as can the position of the clip on position of the clip on the end of the needle.the end of the needle.

Needles can have up Needles can have up to four different to four different tapers on them for tapers on them for different performance different performance throughout the slide throughout the slide travel.travel.

Page 12: Carburetor Theory

TuningTuning Simple tuning can Simple tuning can

begin with moving the begin with moving the needle up or down one needle up or down one clip position. This can clip position. This can be done before be done before changing needles or changing needles or needle jets.needle jets.

Any tuning Any tuning adjustments should be adjustments should be made by stepping jets made by stepping jets up or down up or down oneone size at size at a time.a time.

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Full throttle operationFull throttle operation At ¾ to full throttle, At ¾ to full throttle,

the needle is so far the needle is so far removed from the removed from the needle jet, that it no needle jet, that it no longer limits flow. longer limits flow.

At this throttle range, At this throttle range, the main jet limits gas the main jet limits gas flow.flow.

Air jet limits air flow.Air jet limits air flow. Always jet main rich Always jet main rich

and work down.and work down. Air jet is typically not Air jet is typically not

changedchanged

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Power jetsPower jets Not found on all carbs – Provides richer Not found on all carbs – Provides richer

mix at top endmix at top end

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Jetting adjustmentsJetting adjustments To make proper To make proper

carburetor carburetor adjustments the adjustments the mixture ratio at each mixture ratio at each range will have to be range will have to be determined. There determined. There are various methods:are various methods:

Plug reading – plug Plug reading – plug chopchop

Piston washPiston wash Pyrometer readingsPyrometer readings Condition correctionsCondition corrections Exhaust soundExhaust sound

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Plug Reading – Plug Plug Reading – Plug ChopChop

To get an accurate read To get an accurate read of the condition, perform of the condition, perform a “plug chop”a “plug chop” Install a new plugInstall a new plug Warm engine upWarm engine up Run engine at selected Run engine at selected

throttle setting for 30 throttle setting for 30 seconds – 1 minute.seconds – 1 minute.

Cut ignition and Cut ignition and simultaneously close-off simultaneously close-off carbcarb

Study insulator down Study insulator down toward the base of the toward the base of the insulatorinsulator, not on the tip!, not on the tip!

Appearance of tip better Appearance of tip better for determining detonationfor determining detonation

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Plug appearancePlug appearance

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Center electrodesCenter electrodes Another valuable indicator on

the spark plug is the center firing electrode; as the plug color starts to lighten up when properly jetted, the center firing electrode will start to have a “silver” tip, or crown. As the mixture becomes leaner, this silver crown will start to creep down the side of the electrode; this is your target. This metallic appearance on the end of the electrode should not extend any further than 1/4 - 1/3 of the way down the tip. Many tuners are happy with the margin afforded by simply seeing the silver tip; then they’re close enough for trail riding with a bit of margin to spare.

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Side electrodeSide electrode The ground electrode is

also an indicator; on many engines you will see a “shadow” (darker area) just up to the radius (bend) that will usually coincide with the color and firing electrode appearance. If this shadow is further down the ground strap towards the plug threads, you’re likely too lean. If this dark shadow is all the way across the strap to the center of the plug, you’re too rich.

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NGK plug numbering NGK plug numbering systemsystem

The plug heat range can be The plug heat range can be adjusted to keep a plug tip adjusted to keep a plug tip clean, or avoid detonating on clean, or avoid detonating on an engine that otherwise has an engine that otherwise has the right mixture. the right mixture.

For NGK and most foreign For NGK and most foreign manufacturer’s spark plugs manufacturer’s spark plugs there is a simple rule of there is a simple rule of thumb:Low heat rating number thumb:Low heat rating number (for inst. BP4ES) "Hot spark (for inst. BP4ES) "Hot spark plug".high heat intake due to plug".high heat intake due to long insulator tip.long insulator tip.

.High heat rating number (for .High heat rating number (for inst. BP8ES) "Cold spark inst. BP8ES) "Cold spark plug".Low heat intake, due to plug".Low heat intake, due to short insulator tip.short insulator tip.

For Champion and most For Champion and most domestic manufacturer’s spark domestic manufacturer’s spark plugs (e.g. in ATVs, the plugs (e.g. in ATVs, the numbering is opposite.numbering is opposite.

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Piston readingPiston reading

Normally done for an engine that has been run for ¼ Normally done for an engine that has been run for ¼ mile at constant speed. By looking down the spark mile at constant speed. By looking down the spark plug hole with piston at BDC, a clear look at the top plug hole with piston at BDC, a clear look at the top of the piston can determine how much carbon is left. of the piston can determine how much carbon is left. A ½” ring of wash (clean) is “normal”, typically a A ½” ring of wash (clean) is “normal”, typically a little variation at transfer and exhaust ports will be little variation at transfer and exhaust ports will be noted.noted.

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Exhaust gas pyrometersExhaust gas pyrometers Uses the Exhaust Uses the Exhaust

temperature as a check temperature as a check on mixture, and relative on mixture, and relative changes, not for changes, not for baseline tuning. baseline tuning. Thermocouples are Thermocouples are typically installed typically installed permanently 5-10” from permanently 5-10” from exhaust port and they exhaust port and they must be centered in must be centered in pipe. Manufacturers pipe. Manufacturers will often give you an will often give you an exact location.exact location.

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Interpreting EGT valuesInterpreting EGT values

Each gauge is used Each gauge is used for relative for relative readingsreadings Guidelines Guidelines

Below 1100 – RichBelow 1100 – Rich 1200-Ideal1200-Ideal Above 1250 LeanAbove 1250 Lean

Changing a Mikuni Changing a Mikuni carburetor jet one carburetor jet one size will cause about size will cause about a 50 degree a 50 degree temperature changetemperature change

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Relative Air Density Relative Air Density correctioncorrection

Once an ideal jetting Once an ideal jetting has been developed for has been developed for one condition, it will one condition, it will need to be corrected for need to be corrected for changing environmental changing environmental conditions:conditions: Temperature (Loss of 2% Temperature (Loss of 2%

per 10 degree F increase)per 10 degree F increase) Altitude (loss of 3.5% per Altitude (loss of 3.5% per

1000’ increase)1000’ increase) Barometric pressureBarometric pressure Relative Humidity – Moist Relative Humidity – Moist

air is less denseair is less dense

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Relative Air density Relative Air density calculationscalculations

Calculating Air DensityCalculating Air Density Air density is expressed in units of mass per unit volume, in this Air density is expressed in units of mass per unit volume, in this

case kg/m3. The formula for this calculation is derived from basic case kg/m3. The formula for this calculation is derived from basic physics.physics.

Air Density (kg/m3) = 1.2929 * (273.13/(T+273.13)) * Air Density (kg/m3) = 1.2929 * (273.13/(T+273.13)) * ((P-MN*RH)/760)((P-MN*RH)/760)

Simplified = 0.46464*(P-MN*RH)/(T+273.13)Simplified = 0.46464*(P-MN*RH)/(T+273.13) where where T = temperature in Celsius T = temperature in Celsius P = barometric pressure in mm of mercury P = barometric pressure in mm of mercury MN = moisture number from the following chart - include chartMN = moisture number from the following chart - include chart RH = relative humidity, expressed as a decimal (0.55, not 55%) RH = relative humidity, expressed as a decimal (0.55, not 55%) For example: For example: T = 13C T = 13C P = 770 mmHg P = 770 mmHg MN per the chart for 13C is 11.24 MN per the chart for 13C is 11.24 RH = .21 (21%)RH = .21 (21%) Air Density = 1.25 kg/m3 Air Density = 1.25 kg/m3

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Relative air density useRelative air density use In simple form, the change in relative air In simple form, the change in relative air

density corresponds to the change in jet size.density corresponds to the change in jet size. e.g. – If relative air density goes from 100 to 103 (a 3 e.g. – If relative air density goes from 100 to 103 (a 3

% change) the jet size should increase by 3%. If you % change) the jet size should increase by 3%. If you are using a 300 jet, the new jet size will be (300 x are using a 300 jet, the new jet size will be (300 x 1.03) = 309. Since jets only come in multiples of 5, 1.03) = 309. Since jets only come in multiples of 5, you could use a 310. you could use a 310.

Jetting changes in direct proportion to changes in absolute Jetting changes in direct proportion to changes in absolute temp on the Rankine scale (460 + Farenheit temp)temp on the Rankine scale (460 + Farenheit temp)

Jetting changes only 70% as much as absolute barometric Jetting changes only 70% as much as absolute barometric pressure changes. This is due to the fact that the bowl is pressure changes. This is due to the fact that the bowl is vented, and less pressure is exerted on the gas in the bowl as vented, and less pressure is exerted on the gas in the bowl as wellwell

Hex head jets are marked in area, round head jets in Hex head jets are marked in area, round head jets in diameter.diameter.

Every change of 3 on a RAD gauge, changes the main jet one Every change of 3 on a RAD gauge, changes the main jet one sizesize

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Correcting jet size for Correcting jet size for RADRAD

For instance, if you get the jetting perfect at AD = For instance, if you get the jetting perfect at AD = 1.05 and the air density changes to 1.08, you know 1.05 and the air density changes to 1.08, you know that you will need to go up at least one size. If it that you will need to go up at least one size. If it goes from 1.05 to 1.06, is it time to change a jet or goes from 1.05 to 1.06, is it time to change a jet or not? This is why you need to check all the way not? This is why you need to check all the way through the range. I know that for my bike, AD = through the range. I know that for my bike, AD = 1.03-1.05 is one jet size and 1.06-1.08 is the next 1.03-1.05 is one jet size and 1.06-1.08 is the next size up. In the above example, I would have known size up. In the above example, I would have known to go up one size. If it had gone up to 1.08 and to go up one size. If it had gone up to 1.08 and looked like it was going to continue rising, I would looked like it was going to continue rising, I would go up two sizes.go up two sizes.

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TM CarburetorTM Carburetor TM is a flat slide carbTM is a flat slide carb Less disruption of air Less disruption of air

flow in venturi gives flow in venturi gives carbs better response carbs better response and power.and power.

Rack mounted for Rack mounted for twins and triples twins and triples reduce throttle effort, reduce throttle effort, and are held in and are held in synchronizationsynchronization

Circuits similar to VMCircuits similar to VM

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Mikuni CV CarbMikuni CV Carb Used on four strokes, Used on four strokes,

this is a “Constant this is a “Constant Velocity” or Velocity” or “Constant Vacuum” “Constant Vacuum” carb.carb.

Butterfly throttle is Butterfly throttle is combined with a combined with a slide to give the carb slide to give the carb good response to good response to quick throttle quick throttle opening.opening.

Butterfly controlled Butterfly controlled by operator, Slide is by operator, Slide is controlled by vacuumcontrolled by vacuum

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CV Carb OperationCV Carb Operation Operator controls Operator controls

butterfly from idle to ¼ butterfly from idle to ¼ throttle, during this slide throttle, during this slide is held ¼ openis held ¼ open

Past ¼ throttle, the Past ¼ throttle, the engine vacuum is applied engine vacuum is applied to the slide, and the slide to the slide, and the slide movement is controlled movement is controlled by vacuumby vacuum

Slide will not open until Slide will not open until sufficient engine vacuum sufficient engine vacuum has been developed.has been developed.

Opening throttle quickly Opening throttle quickly will not produce “flat will not produce “flat spot”spot”

Setting Idle speed too Setting Idle speed too high will cause high will cause carburetor to “hang”, or carburetor to “hang”, or not return to idlenot return to idle

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Carburetor sizeCarburetor size

Carburetor size Carburetor size must be matched must be matched to flow for to flow for displacement and displacement and rpms that engine is rpms that engine is used. Changing used. Changing either will require either will require an adjustment to an adjustment to the carburetor to the carburetor to match.match.

Carburetor Throttle Carburetor Throttle Bore DiameterBore Diameter

D = K x SQRT( C x N ) D = K x SQRT( C x N ) D is throttle bore D is throttle bore

diameter, in millimeters diameter, in millimeters K is a constant ( approx. K is a constant ( approx.

0.65 to 0.9, derive from 0.65 to 0.9, derive from existing carburetor existing carburetor bore) bore)

C is cylinder C is cylinder displacement, in liters displacement, in liters

N is RPM at peak power N is RPM at peak power

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Tuned intakeTuned intake

Intake (airbox) can Intake (airbox) can also be “tuned” to also be “tuned” to match resonance match resonance of intake with peak of intake with peak power band.power band.

Resonance EffectsResonance Effects   F = Vs / 2¼ * the square F = Vs / 2¼ * the square

root of A / Vc (L + 1/2 the root of A / Vc (L + 1/2 the square root of  ¼ A square root of  ¼ A

Vs is the sonic speed Vs is the sonic speed Usually about 1100 ft/sec) Usually about 1100 ft/sec)

A is the cross-sectional A is the cross-sectional area of the inlet area of the inlet

L is the inlet pipe length L is the inlet pipe length Vc is the flask (crankcase) Vc is the flask (crankcase)

volume volume

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Ignition safetyIgnition safety To avoid having the To avoid having the

slides freeze open and slides freeze open and customer stuck on a customer stuck on a runaway sled, the runaway sled, the safety switch on the safety switch on the side of carb measures a side of carb measures a magnet on the slide to magnet on the slide to be sure the slide closes be sure the slide closes when throttle is when throttle is released. released.

When replacing throttle When replacing throttle cable, BE SURE TO cable, BE SURE TO SET FREE PLAY or SET FREE PLAY or engine may not startengine may not start