Looking through the '85 FSM, the diagnostic flow chart lists the following probabilities:
1. Improper adjustment of speed control throttle cable
2. Vacuum leak
3. Improper speed control servo lock-in adjustment (max 2 turns in either direction).
4. Defective servo.
I'd say it'd be either 1 or 2 above because the lock-in adjustment, as the manual says, only corrects for minor speed variations, hence the 2 turn max adjustment.
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