Carburetor's 101

Carburetor's 101 - (why it works and what makes it better)

Theory:
Most people know how fuel injection works, but many people wonder how the fuel ends up in the engine without the assistance of a pump and fuel injectors.  The answer is MAGIC!!!  This magic can be demonstrated by blowing across the top of a straw that is sitting in a drink.  As you blow, the liquid is drawn up the straw and eventually sprays on the girl sitting next to you.  This is known as suction. (yes I know suction doesn't exist and its simply equalization of differentiating pressure zones)

So a carburetor basically consists of a float bowl, which is effectively the cup; jets, which are effectively little straws sticking up out of the cup; and the venturi, which is the tube that air flows through.  As air is sucked through the venturi of the carb, it rushes across the top of the jets, which in turn draws gas up the jets and sprays into the engine.  This process of combining fuel with air is known as atomizing the fuel.  Simple right?

So if the amount of atomized fuel being fed to the engine is in direct relation to the amount of airflow going through the carb, how is the airflow regulated?  Well the answer is what separates one carb from another.

There are basically two types of carburetors; the Butterfly Carb, and the Slide Carb. (& one more)

• The Butterfly Carb: (left pic) uses a pivoting throttle plate which turns sideways to increase airflow.  Butterfly carbs include the SU, S&S, Linkert, Zenith/ Bendix, and Keihins between 1976 & 1989.  Butterfly carbs are simple and easy to tune, however they can only handle being opened up so much before the low speed becomes mushy.  Over-carburetion is a common problem with people trying to get more power out of butterfly carbs.
• Pro's: they are very responsive at high RPM's, and the modern addition of secondary circuits and accelerator pumps has helped them to maintain smooth overall responsiveness.
• Con's: even wide open, the throttle acts as an obstacle for airflow, also butterfly carbs are highly sensitive to changes in altitude.

• The Slide Carb: (right pic) uses a slide to open or constrict the size of the venturi, in order to control airflow through the carb.  The slide carb also uses a needle to better control flow from the main jet, which extends straight down from the slide.  Slide carb's include the Mikuni, the Edelbrock/ Quicksilver, and most Keihin carbs.  The slide carb has potential to create significantly more power than the slide carb, and a smother transition from idle to full throttle.  This is due to the slide forcing airflow down and near the jets, as well as not having any obstacles for the airflow to traverse around at full throttle.
• Pro's: responsive throttle feel, high airflow, and less sensitivity to changes in altitude.
• Con's: because fuel can't react as quickly as air, low speed throttle response often requires a bewildering number of circuits with individual jets to control the airfuel mixture.  This can make tuning a slide carb one hell of a job, and if its not tuned right all the advantages go out the window.

Note* circuits & air fuel mixture: complete combustion is achieved in a gasoline engine at an air to fuel ratio of roughly 15:1 respectively.  Circuits are used to maintain this ratio as RPM's climb from idle to full throttle.

• The CV Carb: (above) the CV, or Constant Velocity carb, is essentially a hybrid carb which combines a throttle plate with a venturi slide.  Essentially the throttle plate is controlled by your wrist, while the slide is controlled by vacuum.  (I know its not really vacuum; the slide is driven by its need to equalize the pressure inside and outside the slide)  So the same phenomena that sucks fuel up through the jets also raises the slide, lifting the needle and opening up the main jet.  Because the main fuel jet in the CV carb is directly controlled by airflow, this carb is able to compensate for changes in elevation by only reacting to the amount of air available.  (if you climb in elevation you'll still loose power, but you wont be running overly rich)  
• Pros: this carb is the go-to for stock applications and 60's race cars because its partially self regulating, it forces airflow down and across the jets making for very respectable power, and its easy to tune.
• Cons: this carb has two obstacles for air to flow around giving it the slowest air velocity of the three, and because the slide is controlled by something other than your wrist it has less throttle response. 

Tuning and Jet Kits:
The primary way to tune your carb and boost performance, is to change those stock fuel jets.  Jet kits are available from many brands such as Yost Performance and of course Dynojet.  While some are meant to be used in combination with performance parts, others are simply meant to fix trouble spots in the stock application.  These trouble spots are not necessarily the result of negligence by the highly paid factory engineers, but are purposely placed there to make environmentalists happy.  (this is why most new bikes will experience an immediate power gain from installing a jet kit)
In order to maintain the 15:1 air to fuel ratio discussed above, any performance modifications that change the amount of air going into or out of the engine, will require you to change the size of your fuel jets in order to keep to this ratio.  Most kits will provide a table matching up your bikes modifications with the proper jet size, however a trip to your local dyno-testing center can give you a much more accurate picture of what size fuel jets your carb really needs.

Note*  jet kits for Slide and CV carbs include a needle that directly matches the shape of that brands main fuel jet.  Don't mix and match unless you know what you're doing.

If you are normal and you don't have the resources to get your bike dyno-tested.  The next best way to tune a carb is to find an open stretch of road and take notes on the bikes performance.  Heres a rundown of this process:

First, to check the high speed hold the throttle fully open, bringing the bike above 75% to redline, and then drop the throttle to 7/8 open.  If the bike accelerates slightly, the bike is too lean and you should switch to a larger main jet.  If the engine hesitates or misses slightly, the bike is too rich and you should switch to a smaller main jet.  Ideally, your bike should just slow a slight amount.
Second, to check the midrange hold the bike steady between 40% - 70% to redline.  If the bike is hunting or surging the main or intermediate jet is incorrect.  To find whether it is too lean (small) or too rich (large), snap on the throttle.  If the engine pops or cuts out, its too lean.  If the engine blubbers and farts, its too rich.
Last, check the low speed idle jet.  This is a different process, because the idle jet is fine-tuned using the mixture screw.  The process of setting the mixture screw is slightly different for each carb, but here is the basic process:

• Starting from scratch: turn the mixture screw in counterclockwise until it bottoms out with light pressure.  Turn the screw back out 1 3/4 turn.  Now warm the bike up to normal operating temp, and set the idle speed between 900-1000 RPM's.  Turn the mixture screw inward (clockwise) in slow 1/8 turn increments (wait a few seconds in between turns) until the engine begins to falter.  Next turn the mixture screw outward (counterclockwise) in the same manor as before, and stop when the engine again begins to falter.  Give the screw 1/8 turn inward and you should be done.  
• If from the 1 3/4 starting point you had to go more than 1 full turn inward before the engine began to falter, the idle jet is likely too small.
• If from the 1 3/4 starting point you actually had to go outward more than 1/2 turn to find the first falter point, your main jet is likely too large.
• Additionally, if after the bike feels sluggish off the line, try turning the mixture screw inward 1/8 turn.

Chang of Altitude:
Lastly its worth wild to discuss the effect of altitude.  Weirdly enough, it seems that those of us who live near the ocean are always taking rides up into the mountains.  While the people who live in the mountains always want to take rides out to the ocean.  Because we're never content at our present elevation, our air fuel mixture is constantly being stressed.  This due to the fact that air pressure drops roughly 1 psi for every 1000 feet climb in elevation.  With a starting point of 15 psi at sea level, this means that at the end of a trip from LA to Tahoe your bike will have 20% less air available for it to breath.  Your bike now has 20% less power and, if your not using a CV carb, you're likely running too rich.  If this is a quick day ride you have nothing to worry about, other than potentially getting passed by Honda's, but if you're going to be riding at that elevation for long you should re-jet to avoid engine damage.

So thats it..... hopefully this was helpful to demystify carburetors, and build up your d.i.y. confidence.  Please comment or send me an email if you have any question.

Sources & Links:

• Sportster/ Buell Engine Hop-Up Guide - Kip Woodring
• CV Performance
• Catalog - Sudco
• Video - CV Carb Demo