Low Speed Tuning; Mid-Range And High Speed Tuning - Yamaha BR250F Service Manual

4. Slowly loosen slow adjuster. As the slow
adjuster is loosened, the mixture will
become leaner and engine rpm will in-
crease. Adjust the slow adjuster to attain
the highest posstble engine speed.
5. Finally, turn the throttle stop screw
counterclockwise so that the engine runs
at 1,100 rpm.
Idle speed: 1,100 ± 100 rpm
1. Throttle stop screw
NOTE: _
If the engine shows poor performance in high
elevations or in an extremely cold area or
when it is suddenly accelerated or run at low
speeds, proceed as follows:
a. Replace the slow jet according to operat-
ing conditions.
Standard slow jet: # 78
F. Mid-range and High Speed Tuning
No adjustment is normally required, but
adjustment is sometimes necessary depending
on temperatures and/or altitude.
Mid-range speed and high-speed tuning (from
1/4 to full-throttle) can be done by adjusting
the main jet.
1. Run the engine at high speeds, and make
sure the engine operates smoothly.
2. Check the spark plug. The main jet
should be adjusted on the basis of the
following table. (Refer to spark plug
color samples.)
Standard main jet: # 108
Spark plug color
No.1 Good (Carburetor is turned properly.)
No.2
Bad (Mixture is too
rich.l
Replace main jet with one-step small one.
No.3
Bad (Mixture is too lean.)
Replace main jet with one-step larger one.
Bad (Due to too lean a mixture, piston is
damaged or seized.)
No.4 Replace the piston and spark plug.
Tune the carburetor again starting with
low-speed tuning.
Bad (Due to too lean a mixture, the
engine knocks.)
Check the piston for holes or seizu reo
No.5 Check the cooling system, gasoline octane
rating, and ignition timing. After replac-
ing the spark plug, tune the carburetor
again starting with low-speed tuning.
Bad (Due to lean a mixture, the spark
plug melts.]
Check the piston for holes or seizure.
No.6
Check the cool ing system, gasoline octane
rating, and ignition timing. After replac-
ing the spark plug with colder type, tune
the carburetor again starting with low-
speed tuning.
1. Slow jet 2. Main jet
b. By repeating steps 1 to 5 above, adjust
the idle speed.
Idle speed: 1,100
±
100 rpm
3-6