They're only self-adjusting up to a point. The plastic pawl only has a few teeth on it and it doesn't take the cable long to stretch past the adjustment it will provide. You can try adjusting it by pulling upward from the bottom side of the pedal with your toe then depressing the clutch. If you hear a 'click', then it moved on to the next adjustment. If not, then its probably out of adjusting teeth. I'd still recommend an adjustable cable.
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Jeff Chambers
Trophy Stock #3
11.86 Seconds / 113.8 MPH
Chambers Racing Team