Canon multipass c3000 Service Manual (en)

(10) Loosen screw B until the carriage guide frame moves forward under the carriage’s
own weight around screw A.
(11) Lightly push the screw B end of the carriage guide frame once or twice and check
that the carriage guide frame moves around screw A as shown by the arrows.
If it does not return to its original position, screw A is too tight.  And if it moves on
the screw A end as well, screw A is too loose.  Refasten the carriage guide frame
lightly again and repeat the procedure from 
C-1) Adjustment preparation
(12) Lightly tighten screw B.
(13) Push the paper thickness lever to the right and move the carriage to position A.
Then place the gap gauge at position B.
(14) Repeat steps 2) through 13) twice.
(15) Tighten screws A and B alternately for final tightening.
Do not use your hand to move the carriage.  It may cause the carriage guide
frame to shift.  To move the carriage, use the carriage belt instead.
On the platen, there is an elevated step and paper guide.  If the gap gauge is
placed on either part, an accurate measurement will not be obtained.
d) Adjustment check
After completing the adjustment, execute print test operation and check the image.
Chapter 3: Maintenance & Service
Figure 3-12 Head Gap Adjustment (2)