Two Brothers Racing Honda XR50R - Manual (Page 7)

Tuning Tips and General Information on how to tune your XR50 Carburetor from
Two Brothers Racing:
How does a carburetor work?
All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure press-
es on everything at about 15 (PSI). As the piston goes down a low-pressure area is formed above the
piston. This low pressure also causes a low pressure inside the carburetor. Since the air pressure is
higher outside the engine and carburetor, air will rush inside the carburetor and the engine until the
pressure is equalized. The moving air going through the carburetor will pick up fuel and mix it with
the air. The ratio of air and fuel is called an air fuel ratio, this ratio varies but the optimum ratio is
about 12.8 (12.8 grams of fuel to 1 gram of air) the action of getting to this ratio is called jetting.
Basic Carburetor Adjustments:
Carburetor adjustments on your XR50 carburetor are made based on throttle position and not engine
speed. There are four basic adjustments that can be easily made on your carburetor; we list them
here based on throttle position. These are basic adjustments that can be easily applied based on
simple diagnostic techniques.
Idle Adjustment:
By turning the idle screw in or out you can decrease or increase the speed of the idle. The idle screw
can either be located on the right or left side of the carburetor (depending on the manufacturer) The
idle screw will sometimes have a spring behind it and or a plastic know so that you can adjust it by
hand. To achieve an accurate adjustment the engine must be at normal operating temperature. Ten
minutes of normal riding should do the trick. The idle speed should be set at around 1700 RPM give
or take 100 RPM.
Idle -> 1/4 Throttle – air screw and pilot jet:
Off idle to ¼ throttle is monitored by the airscrew and pilot jet. The air screw regulates how much air
enters the pilot circuit. If the screw is turned in, it reduces the amount of air and richens the mixture.
If it is turned out, it opens the passage more and allows more air into the circuit that results in a lean-
er mixture. If you have to turn the air screw out more than 2 turns to get good performance you need
the next size smaller pilot jet.
Air screw turned in = RICHER
Air screw turned out = LEANER
1/8 -> 3/4 Throttle – jet needle and needle jet:
The jet needle is the most effective component in this range. The jet needle is a long tapered rod that
controls how much fuel can be drawn through the carburetor venturi. The thinner the taper the richer
the mixture. The needle jet is where the jet needle slides. The needle jet and jet needle work together
to control the fuel flow between the 1/8 and ¾ throttle opening range. Most of the tuning is done with
the jet needle, the needle jet rarely needs to be changed. The jet needle has grooves cut into the top.
A clip goes into one of these grooves and holds the needle from falling or moving from the slide. The
clip position can be adjusted to make the engine run richer or leaner.
Jet Needle Clip UP = LEANER
Jet Needle Clip Down = RICHER
1/4 -> Full Throttle – main jet:
The main jet is located under the float bowl in the middle of the floats. Sometimes the main jet will
have a plastic shroud around it called a “jet baffle” The main jet comes into play as the throttle opens
up past ¾ throttle. At ¾-full throttle when the jet needle is pulled far enough out of the needle jet the
size of the hole in the main jet begins to regulate fuel flow. The bigger the number on the main jet the
bigger the hole. The bigger the hole the more fuel that can flow which results in a richer mixture.
Main Jet = Bigger the number, bigger the hole, RICHER the mixture.
Main Jet = Smaller the number, smaller the hole, LEANER the mixture.
Jetting Considerations:
Air temperature, altitude, and humidity all affect the jetting and how your bike will run.
Air Temperature:
When the temperature goes up the air density decreases, thus you have less air available for com-
bustion and your air fuel ratio becomes richer. When the air temperature goes down the air density
goes up and your engine runs leaner.
Air Temp High = Less Air and a richer mixture
Air Temp Low = More Air and a leaner mixture
What do I do if the air temp is higher than normal?
Air screw turned out = LEANER
Jet Needle Clip UP = LEANER
Main Jet = Smaller the number, smaller the hole, LEANER the mixture.
What do I Do if the air temp is lower than normal?
Air screw turned in = RICHER
Jet Needle Clip Down = RICHER
Main Jet = Bigger the number, bigger the hole, RICHER the mixture.
Altitude:
The higher the altitude the less dense the air becomes, hence a richer mixture. The lower the altitude
the more dense the air and the leaner the mixture.
High Altitude =Less dense air so you will want lean the mixture if you normally ride at a lower altitude
Low Altitude = More dense air, so you will want to richen up the mixture if you normally ride at a high-
er altitude.
What do I do if I normally ride at 5000 feet and I go down to sea level to race?
Air screw turned in = RICHER
Jet Needle Clip Down = RICHER
Main Jet = Bigger the number, bigger the hole, RICHER the mixture.
Humidity:
When the humidity in the air increases the air-fuel mixture becomes richer so you should lean out the
mixture.
What do I do if I normally race in dry air conditions (like Arizona) and I go to a very humid climate to
race (like Florida)?
Air screw turned out = LEANER
Jet Needle Clip UP = LEANER
Main Jet = Smaller the number, smaller the hole, LEANER the mixture.
Throttle Position:
Idle Set at 1700 RPM + or – 100 RPM
Idle -> 1/4 Air screw Turn in for Richer out for Leaner
1/4 -> 3/4 Needle Jet Clip up =Leaner
Needle Clip Down = Richer
3/4 -> Full Main Jet (bigger the number richer the mixture)
Conditions:
Air Temperature High = (less air so you have lean out the mixture)
Air Temperature Low = (more air so you have to richen the mixture)
Altitude High = (lean the mixture)
Altitude Low = (Richen the mixture)
Humidity High = (Lean out the mixture)
Humidity Low = (Richen the mixture)
XR50R 88cc Engine Build Instructions.qxd PAGE 7