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RE: 1978 xs 750 77 Yamaha XS 750 discussion forum:
RE: 1978 xs 750

RE: 1978 xs 750 
zoli said 2005-08-29 11:07:26 
Adjusting the Idle Mixture

Carburetor idle mixture adjusting procedure:
The ´lean idle drop´ method of adjusting the idle mixture is designed to
eliminate the need of using a gas analyzer to set the idle mixture
adjustment. If done properly each cylinder is adjusted to its peak idle
performance and should idle smoothly. Please remember that the base
settings are only a place to start. You will need to adjust every bike
to a slightly different setting. Likewise each cylinder will require a
slightly different setting.

Tools required:
* A set of vacuum guages. If you use mechanical/dial type vacuum guages,
connect all of them to one cylinder at the same time and calibrate them
to all read the same while in use, not at zero.
* A tachometer that will show a change of 50 RPM or better. The stock tach
will NOT do this. A good automotive tach/dwell meter may work. Remember
that the number of cylinders is likely to be wrong, so the reading may
be off by a factor of two or four.
* An auxillary fuel supply.
The procedure:
1.Set the pilot screws to a "base" setting. The manual may have a spec
for this. If not, try 1 turn from seated.
2.Hook up the vacuum guages and the tach.
3.Run the motor (have cooling fans, this takes a while) and warm it up.
4.Set the idle speed to the factory spec.
5.Adjust the carb sync. Set to be within 5mm Hg (mercury).
6.Reset the idle speed.
7.Re adjust the carb sync if nessecary.
8.Repeat step 6 and 7 until both are correct.
9.Now turn all the pilot screws the same amount in the rich direction,
about 1/4 to 1/2 turn is good.
10.Watch for an increase in the RPM. Readjust the idle speed and carb
sync as necessary.
11.Repeat steps 9 and 10 until the maximum idle speed is attained. This
should coincide with the maximum vacuum reading.
12.Turn one pilot screw in the lean direction until the engine speed
drops 50 RPM.
13.Reset the idle speed.
14.Repeat steps 12 and 13 on each cylinder.
15.Reset idle speed and check carb sync.
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