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Oft times when fixed or rebuilt over a lifetime of the machines this washer (white arrow) against cabinet is missing, making wobble enough for belt to jump out at green arrow. Same effect if screw at blue arrow is stripped. The treadle wheel itself should not be over-exaggerate in its wobble. If so, again the belt jumps the track. Also, before troubleshooting further, be sure all oil points have been oiled “down below”. Is there any rust build ups.
Do not get iscouraged, I LOVE my White Rotary Treadle! Once you resolve this the machine will most likely be maintenace free from here on out. It is a blast to “drive”!