Carburator Theory and Tuning
For some reason everyone seems
to think tuning a carb is just real easy. Change a jet or two and boom,
your there. Yeah, right ! There are quite literally millions and
millions of jet combinations. A rough check on Bing carbs shows there
are at least 13,860,000 different combinations of jets. If you are going
to change carbs you'd better be prepared to spend some time and money
on the job.
If you look at a carburetor,
you will notice a rather large hole going from one side to the other.
This is called a Venturi. Air passes into the engine through this hole
(Venturi). As the velocity of the air entering the carb (and then the
engine) increases, it's pressure decreases, creating a low pressure or
vacuum in the venturi. This vacuum moves around in the venturi, as the
throttle is opened, and sucks gasoline through the different jets in the
carb. The gas then mixes with the air going through the venturi. The
way the jets are made
causes the fuel to vaporize as it goes into the venturi. Where the jets
are placed in the carb and where the jet's outlet is located in the
venturi, determines what part of the throttle opening that jet controls.
The idle jet system (comprised of pilot air jet, pilot fuel jet and
pilot fuel screw) controls from 0% to about 25% of the throttle opening.
The throttle valve controls 0% to 35% of the throttle opening. The
needle jet and jet needle control from 15% to 80% of the throttle
opening and the main jet controls 60% to 100%. This means that when you
open the throttle about one eighth of the way open, all of the gas/air
mixture going into your engine is controlled by the idle jet. As you can
see, the different jets over lap the operating range of each other.
That is, the jet needle starts to effect things before the effect of the
idle jet ends. This is something to remember when working on carbs...
everything is interconnected. Change one thing and it will effect other
OK, let's go over the different systems in
the carb and see what they do.
level. The fuel level is controlled by the fuel floats and the
fuel float valve. The floats are hollow or made of something that will
float on gasoline, such as cork. Part of the float presses against the
float valve, sometimes called a needle and seat. Most times the part of
the float that touches the float valve needle is bendable so you can
adjust the level of the fuel in the floatbowel. All plastic floats are
not adjustable. If this level is way too high, gas can leak out the carb
overflow tube or into the engine. If fuel gets into the engine it will
thin out the engine oil, ruining it's ability to lubricate. This will,
sooner or later, blow up your engine ! If a full tank of gas in the
evening turns into a half tank by morning, check your oil. If it's thin
and smells like gas, change it and replace your float valve and/or check
your fuel level. If the oil is OK, check under the overflow tube. If
it's OK, then check where you are parking your bike 'cuse someone is
walking away with your gas !
If your fuel level is just a bit high, the mixture will tend to be a
bit rich. If it's low, the mixture will tend to be a bit lean. This is
because a high level takes less vacuum to **** fuel into the engine and a
low level takes more vacuum to do the same.
- Pilot or
idle jet system. The idle jet controls the idle and on up to quarter
throttle, give or take a bit. On some carbs, like Mikuni there is an
air jet too. In conjunction with the idle jet there is an idle jet air
screw. This screw leans or richens the fuel mixture for a smooth idle
and on up to one quarter throttle. From the idle jet, there are little
passages cast into the carb that lead to holes just in front of the
throttle valve or plate. There can be just one hole or there can
depending on the carb design. They effect the mixture as long as the
vacuum, in the venturi, is over them. As the throttle opens further, the
vacuum moves to the needle jet and jet needle.
- The Throttle Valve.
The big slide that opens and closes your throttle has a bevel angle cut
in one side of the big round (can be flat, too) slide, toward the air
cleaner. This angle comes in several sizes and helps control the fuel
mixture from idle to about 35% open throttle.
- Needle Jet.
This jet doesn't really even look like a jet, but it is ! It controls
the fuel mixture from 15% to 60% open throttle. It sets in the center of
the carb, right over the main jet.
- Jet Needle.
This is the needle that rides in the throttle slide and goes into the
needle jet. This needle controls the fuel mixture from 20% to 80% open
throttle. It can come in many different sized tapers. Sometimes, one
needle can have several tapers on it. The top end of the needle has
grooves cut in it, usually five, and you can move the little clip on
the end up or down to lean (down) or richen (up) the mixture. Most late
model bikes have needles with only one groove cut in them. This is so
you can't richen the mixture, thereby keeping the EPA happy.
- Main Jet. This
jet controls the fuel mixture from 60% to 100% open throttle.