Riva XC125 carburetor
Understanding the Yamaha Riva 125 scooter carburetor This carburetor was taken
apart for cleaning. The carburetor is a Y22V Teikei.
The carburetor has an automatic choke and five jets, a starter jet, a CSN jet, a pilot jet, a
needle jet, and a main jet. All the jets have an associated path for air to bleed into the fuel.
The starter jet works in conjunction with an automatic electric choke. The electric choke
is shown on the left in figure 1. Labeled is a plastic cam that is shown in the cold
position. When the scooter starts, the choke coil heats a wax filled cavity that expands
pushing the plastic cam out of the photo. It presses against the assembly at the lower left
labeled with the starter jet needle. The assembly is pushed into the body of the carburetor
cutting off the flow of starter jet fuel.
The starter jet is unusual in that it has its own air filter attached to the electric choke
body. The short black rubber hose seen attached to the electric choke goes to the air filter.
The air from the filter follows the path of the green arrow to the starter jet while the fuel
from the fuel bowl follows the blue path.
The fuel path for the starter jet is more clearly shown in figure 2. Fuel enters the side of
the brass tube and pools in the starter jet cavity which is capped off with a black rubber
plug. This forms its own mini-fuel bowl. The jet itself can be examined simply by pulling
the rubber cap off. The mixture follows the path of the pink dotted line and exits from the
hole at the tip of the pink arrow. There are two additional air bleeds to the left of the hole
for the starter mixture. These air bleeds get air from the cavity below the vacuum
diaphragm. The air source is the main air filter. One air bleed is large and the other tiny.
The tiny one is visible as a black dot at the end of the arrow labeled air bleeds. The starter
jet needle has a shiny metal cylinder surrounding it. As this cylinder is pressed into the
carburetor body, the cylinder cuts off the large air bleed, a rubber gasket surrounding the
starter jet needle at the base of the cylinder cuts off the tiny air bleed, and the starter jet
needle cuts off starter jet fuel.
Figure 3 shows where the starter jet fuel mixture comes out in front of the butterfly valve.
When the engine is not operating, the butterfly valve is closed as shown. When the engine
is cranked, engine vacuum created in front of the butterfly valve pulls the starter fuel
mixture into the engine. The same vacuum is used to open the petcock.
The CSN system provides extra fuel to the engine during acceleration while cold as
shown in figure 4. The system consists of an air bleed, a jet and a jet needle attached to
the choke jet assembly. The automatic choke assembly pushes the CS needle into the
body of the carburetor when warm cutting off the CS fuel supply just as it cuts off the
starter fuel supply.
Unlike the fuel output of the starter jet, which comes out in front of the butterfly valve,
the fuel output of the CSN system is behind the butterfly valve. The Venturi vacuum that
occurs when the butterfly valve opens draws in the CSN mixture.
The CSN system receives fuel from the cavity in the fuel bowl shown in figure 5. Since
sediment tends to gather at the bottom of the bowl, this passage shown in figure 5 is
sometimes obstructed as it was in this carburetor. Make sure these passages are clear.
The fuel output from the CSN jet is as shown in figure 6 below.
The pilot jet is is the third jet. Unlike all the other jets. the pilot jet has an adjustment
screw shown in figure 7. The adjustment screw is under a cover which was drilled and
tapped to pull out.
The fuel flow is as shown in by the pink dotted line. The fuel exits at the top of the
carburetor throat in front of the butterfly valve as shown in figure 9. One exit hole (the
one nearest the front of the carburetor throat) is set by the pilot screw. The other three
holes are just under the top of the butterfly valve. Adjustment of the butterfly valve
exposes more or less of these three holes to engine vacuum.
Figure 8 shows the pilot jet itself and the pilot screw. The jet has a very small hole which
must be cleaned thoroughly. Spray cleaners are not a guarantee that the hole will be
cleaned. A small wire like a guitar string does a much better job than chemical cleaners.
Figure 9 shows the openings through which the idle mixture comes into the throat of the
carburetor. Use a spray cleaner and spray into the pilot jet cavity with the pilot jet
removed. Make sure cleaner comes out the holes shown.
Needle and Main jets
The needle and main jets are the fourth and fifth jets. They are shown in figure 10. The
openings in these jets are much larger than the pilot jet and pilot screw openings making
them less susceptible to clogging.
Figure 11 shows where the main and needle jet fuel mixture enters the throat of the
carburetor in back of the butterfly valve.
Figure 12 shows the air sources for all but the CSN jet. Notice that there are two air
bleeds for the starter jet.
Setting the fuel level in the carburetor bowl
Figure 13 shows the parts of the float system. The float height is adjusted by bending the
metal tang on the float assembly. This is a very important adjustment because it affects
the mixture at cruising speeds. There is no other adjustment for this. Notice that the top of
the float valve needle assembly is spring loaded.
Figure 14 illustrates how to set the float level. The float must be set to 27 mm +/-1 mm
above the lip of the carburetor. When making this adjustment, it is important that the float
valve needle spring not be depressed but continues to be fully extended as shown in
The final check of the fuel level is done with the clear tube attached to the carburetor fuel
bowl drain. Open the screw to the drain and with scooter idling, the fuel level should be
adjusted to be 5 mm below the fuel bowl lip as shown in figure 15. The float height had
to be adjusted several times to achieve this. This adjustment determines the mixture at
Figure 16 shows all the jets after they are assembled. Note the rubber cap over the starter
Fuel goes to the engine not directly because the throttle is operated but rather, the throttle
opens the butterfly valve. When the butterfly valve opens, air enters from the air box.
This air entering the engine from the air box creates a vacuum via the Venturi effect. This
Venturi vacuum lifts up the vacuum piston with jet needle. There is a hole shown in
figure 17 at the bottom of the piston though which vacuum travels to the top of the rubber
diaphragm lifting the piston.
Figure 18 shows the parts of the vacuum piston and the jet needle. The rubber diaphragm
around the piston is thin and should be inspected for pinholes. A leak will not allow the
vacuum piston to move up as much as it should causing a lean mixture.
Figure 19 shows the remaining parts of the vacuum piston assembly. Care should be
exercised when replacing the vacuum chamber cover so that it does not pinch the rubber
diaphragm and damage it.
Fuel Bowl Overflow
This is not mentioned or shown in the Yamaha factory service manual for some reason.
The overflow tube allows too much fuel in the bowl to exit via a clear hose at the bottom
of the fuel bowl as shown in Figure 20. The fuel bowl also has a screw to drain fuel
completely from the bowl.
The size of the jets are imprinted on the parts as shown in the figures below.
The stock factory part numbers are,
Main jet = 108
Jet needle = 4C10
Needle jet = 85
Pilot jet = 34
This carburetor has only three adjustments. The idle speed adjustment on the left side of
the carburetor, the pilot screw (idle mixture) adjustment at the top front of the carburetor,
and the float level.
The float level should be set before the other two. The initial setting of 27 mm as shown
in figure 14 is a good starting point but it is more important to get the fuel level 5 mm
below the lip of the carburetor bowl as shown in figure 15. The fuel level in the bowl sets
the mixture at cruising speeds.
As an example, this carburetor initially had the fuel level set exactly level with the top of
the fuel bowl ( position of the bowl gasket ). The result was a rich cruise mixture.
Starting with a clean plug and riding about 20 miles with the engine already warmed
would cause the plugs to blacken with carbon. The bike would stall at red lights but start
right back up. Maximum speed on a level road was 53 mph.
This being an older bike, it was trickier to set the fuel bowl level than one would expect.
Increasing the float height as in figure 14 worked up to about 30 mm. At 31 mm, the
floats actually hit the bottom of the bowl which closed off the float valve.
A procedure that worked is with two bends. Bending the tab to raise the floats then
bending the floats where they attach so the floats are parallel to the carburetor lip.
Bending the tab alone causes the floats to tilt up reducing their buoyancy. The second
bend increases the buoyancy. This allowed the floats to be set at 29 mm above the
carburetor lip which resulted in the fuel level dropping to 5 mm below the bowl gasket
line. Maximum speed on a flat road increased from 53 mph running rich to 56 mph.
Attach a tachometer and set the idle speed for about 1,400 rpm. Then adjust the pilot
screw to peak the engine speed. Go back to the idle speed adjustment and lower ( or
increase ) the peak found with the pilot screw adjustment to 1,400 rpm. It might be
necessary to go back and forth a few times.
When the pilot screw is set up properly at the peak, the rpm change for +/- 1 turn from
peak is about +/- 200 rpm. The peak will occur between 1-3 turns out from the seated
position of the mixture screw.
It is important to set the initial idle speed around 1,400 rpm. Too high an initial idle speed
will defeat the idle mixture screw adjustment. The idle speed adjustment simply opens
the carburetor butterfly just as opening the throttle so more fuel from the idle fuel ports
not governed by the mixture screw and fuel from the needle jet swamp the fuel coming
from the idle mixture screw rendering it ineffective. Figure 9 shows the only one fuel port
governed by the pilot (idle mixture) screw.