Prop/Governor Problem

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I have an RV-7 with a Hartzell Blended Airfoil C2YR-1BFP and an MT-860-4 Prop Governor. All work normally until I reduce RPM to about 2300 RPM. At that point, further selection of a lower RPM has no effect. The only way to get RPM below 2300 is to reduce MAP. When MAP is reduced, the prop RPM will decrease and the prop acts a though it's a fixed pitch prop -- prop follows MAP.

The cable to the governor works fine. Governor can be controlled from HI to LO stops. Runup prop check is normal -- drops to about 1500 RPM during cycle.

Would greatly appreciate your troubleshooting recommendations...

Thanks

John