A carburetor's job is to feed the engine a perfect mix
of fuel and air as commanded by the pilot. There
are two basic types of carburetors: Float bowl and Membrane. About 70% of
all paramotors use membrane carbs which are cheap and lightweight. Each
type has it's
advantages and drawbacks but this article only addresses Membrane carbs.
2-stroke motors have two types mixture that you'll read
about: fuel:oil mixture and fuel:air mixture. Carbs work to porvide the
appropriate fuel:air mixture. Rich means too much fuel and lean means too little fuel.
Metering Fuel and Air
Air is typically metered using a circular valve called a
butterfly valve. As you open the throttle, the butterfly valve
opens to allow more airflow. Fuel is sprayed into the incoming
airstream in a carefully metered amount determined by a variety of
mechanisms in an effort to keep the mixture relatively constant from low
airflow to maximum.
2-stroke Motors tend to lean out as they warm up so it's
best to be slightly rich when starting. They need to be richer when cold.
See more information on
Walbro carbs have a built-in fuel pump, needle & seat
controlled by a float diaphragm and a low end needle set. Most carbs also
have a high-end needle set and a few have chokes. Paramotors normally use
the WB and WG series although there are others out there such as WT, WL,
WS, WTL. Fortunately, they share a common tune-up
It's helpful to understand what's going on and what part
of the carb is being used at that time. Any one of several functions can
affect operation in sometimes strange ways. What could sound like a rich
low end needle setting could easily be a float setting on
the needle & seat. A stiff diaphragm will make it run rich too. It could
also make it run lean. The high end and low end needles interact
throughout the entire throttle range. So if you adjust one needle, you
will likely need to adjust the other slightly.
Understanding Fuel & Air Flow
Starting from the gas tank, fuel is pumped in through the
carb inlet into a little diaphragm pump which is controlled by 2 one-way
flapper valves. The fuel then passes through a needle & seat that is
controlled by the float diaphragm. The float diaphragm manages how
much fuel is available for the rest of the fuel circuit by opening &
closing a passage utilizing a small lever attached to the needle. How much
pressure it takes to move this needle off of its seat is called pop-off
pressure. Less pressure means more fuel goes into the chamber and will
enrichen the mixture. If it takes more pressure to pop off its seat then
less fuel is available in the chamber ergo a leaner mixture.
Fuel in the float cavity area is waiting for a vacuum
signal at the various jets. The lever setting is very critical since it
controls the available fuel to the jets. If the lever is too low (or
pop-off pressure too high), the engine will run lean, if the lever is too
high (or pop-off pressure is too low), the engine will run very rich and
will likely flood out at idle.
All diagrams and photos Copyright
© 2007 M. B. Fuess
History: why so complex?
Float bowl carburetors were,
for a long time, the standard method to deliver an engine's fuel
and air. But they require being upright so gravity keeps the float floating and metering.
When designers started putting small gas engines on
tools, they needed a way to work in unusual orientations. Viola, the
membrane carb. You can hold it in any attitude and it will function.
served as the root and graphics for this article. Details on specific
tuning of several carbs can be found on
Alex Varv's site as
All of these parts reside in the float cavity area as well
as the fuel ready to be fed through the jets as needed. The amount of fuel
available in the cavity is regulated by the lever and its relationship to
the float diaphragm. So it's critical that the lever be set properly.
Within the cavity, there are distribution holes that are managed by the
low end and high end needles. Plus the idle circuit, which is a fixed
Another separate fuel circuit provides a small amount of
fuel flow into the throat continuously which helps at the lowest throttle
settings. At high power this circuit is still injecting fuel but
represents a very small percentage of the total flow. This circuit helps
keep a correct mixture for acceleration.
A few notes:
The low end needle is always the one closest to the
engine, the high end needle is the closest one to the intake/choke.
On most carbs, both needs adjust the fuel flow but on a
few, the low needle actually adjust the air inflow.
Carbs also have an idle stop screw which merely sets
where the throttle stops. Screwing it in is just like slowly squeezing
Both low end AND high end needles feed the top end fuel
supply but the low screw affects the high end fuel flow much less.
1. The fuel starts its journey through
the pump assembly first. 2. Then the fuel is regulated by the float
diaphragm that controls the needle & seat.