In 90% of the cases, intake vacuum leaks are the culprits that cause a bad idle.  Please check out our intake vacuum leak page here:
FC3S Pro - Problems #1, Engine, Intake Vacuum Leaks

Since this problem is very similar to a low-idle problem, please check out out Low Idle page for more troubleshooting info:
FC3S Pro - Problems #1, Engine, Low Idle

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