Carburetor Theory

Carburetor Theory

Variable Venturi or “slide” carburetorsThe goal of providing

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

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

VM CarburetorSlide carburetor

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

VM is a Mikuni round slide carburetor.

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

main

Integrated carburetor systems The carburetor has

multiple adjustments for different throttle positions;◦ 0-1/4 Pilot jet◦ 1/8-1/2 Throttle

cutaway◦ ¼-3/4 Jet needle

(clip)◦ ¼-3/4 Needle jet◦ ½-Full Main jet◦ ½-Full Air adjustment

screw◦ ½-full Power jet

(optional)

Starter Circuit

Mini- slide carburetor used for starting

“Choking” the carburetor actually opens plunger to allow gas flow

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

Air is drawn from in front of the slide

Starter circuit

Starter CircuitStarter can only function when slide

is closed and vacuum is highStarter jet may need to be changed

if using a snowmobile carb on a motorcycle, or vice-versa

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

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

Pilot circuit

Used at idle to ¼ throttle

Gas flow is controlled by pilot jet size, this jet is the first to plug if gas thickens

Air flow is controlled by air adjustment needle

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

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

Pilot Circuit

Stumble coming off idle indicates plugged pilot jet or air screw too tight

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

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

Mid range operation¼ to ¾ operation is

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

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

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

Mid range operation

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

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

Tuning

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

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

Full throttle operation

At ¾ to full throttle, the needle is so far removed from the needle jet, that it no longer limits flow.

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

Air jet limits air flow.Always jet main rich

and work down.Air jet is typically not

changed

Power jetsNot found on all carbs – Provides richer

mix at top end

Jetting adjustments

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

Plug reading – plug chop

Piston washPyrometer readingsCondition correctionsExhaust sound

Plug Reading – Plug ChopTo get an accurate read

of the condition, perform a “plug chop”◦ Install a new plug◦ Warm engine up◦ Run engine at selected

throttle setting for 30 seconds – 1 minute.

◦ Cut ignition and simultaneously close-off carb

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

◦ Appearance of tip better for determining detonation

Plug appearance

Center 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.

Side electrodeThe 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.

NGK plug numbering system The plug heat range can be

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

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

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

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

Piston reading

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

Exhaust gas pyrometers

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

Interpreting EGT values

Each gauge is used for relative readings◦ Guidelines

Below 1100 – Rich 1200-Ideal Above 1250 Lean

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

Relative Air Density correctionOnce an ideal jetting

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

per 10 degree F increase)

◦ Altitude (loss of 3.5% per 1000’ increase)

◦ Barometric pressure◦ Relative Humidity –

Moist air is less dense

Relative Air density calculations Calculating Air Density 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 physics.

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

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

Relative air density use

In simple form, the change in relative air density corresponds to the change in jet size.◦ e.g. – If relative air density goes from 100 to

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

absolute temp on the Rankine scale (460 + Farenheit temp)

Jetting changes only 70% as much as absolute barometric 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 well

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

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

Correcting jet size for RADFor instance, if you get the jetting perfect at AD =

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 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 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 size up. In the above example, I would have known to go up one size. If it had gone up to 1.08 and looked like it was going to continue rising, I would go up two sizes.

Engine sound

An engine running way too rich will “four cycle” or miss ignition on every other stroke.

An engine running way too lean will be running very fast and not return to idle

TM CarburetorTM is a flat slide carbLess disruption of air

flow in venturi gives carbs better response and power.

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

Circuits similar to VM

Mikuni CV Carb

Used on four strokes, this is a “Constant Velocity” or “Constant Vacuum” carb.

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

Butterfly controlled by operator, Slide is controlled by vacuum

CV Carb OperationOperator controls

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

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

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

Opening throttle quickly will not produce “flat spot”

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

Carburetor size

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

Carburetor Throttle Bore Diameter

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

diameter, in millimeters K is a constant ( approx.

0.65 to 0.9, derive from existing carburetor bore)

C is cylinder displacement, in liters

N is RPM at peak power

Tuned intake

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

Resonance Effects F = Vs / 2¼ * the square

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

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

A is the cross-sectional area of the inlet

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

volume

Ignition safetyTo avoid having the

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

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