Here is per the service manual...sure is pointing to the rpm range where people are having issues.
These models are equipped with the Exhaust Controlled Timing (ECT) system utilizing an exhaust pipe temperature sensor. NOTE: The engine has a three-stage exhaust valve system. The stages are closed, mid-open, and full-open. These models have a designated RPM of 4500 at which the system is synchronized and checked by the ECM for proper operation occurring once each time the engine is started. This system automatically adjusts the ignition timing to provide maximum performance through a variety of operating conditions. The ECM receives input on engine RPM (demand) and exhaust temperature (engine condition) and adjusts the ignition timing accordingly. This system is not adjustable and is maintenance free. If a system fault is suspected, use an ohmmeter to check continuity of the exhaust pipe temperature sensor located in the expansion chamber. A reading of either 0 ohm or infinity indicates a failed sensor. A reading at room temperature should be between 200-220 ohms. NOTE: A disabled ECT system WILL NOT cause engine damage; however, a failed ECT system will have slower throttle response and may produce slightly less top-end performance.