TM 10-3930-671-24 pressure  rises  and  moves  the  piston  in  the  advanced direction. Advance piston movement is related to speed. Total   movement   of   the   cam   is   limited   by   the   piston length. A "trimmer screw" is provided to adjust advance spring preload which controls start of cam movement. It can be incorporated at either side of the advance mechanism and may be adjusted on the test bench while running (Figure 1.13). SPEED ADVANCE OPERATION FIGURE 1.12. FIGURE 1.13. F-222