Stationaly engine. Viscous fan drive?
Had it off lately? If so did you stand it on the drive hub or, as recommended, on the radiator end? If the former, delta T may have forced some or all of the silicone oil past the seal.
Verify that the thermocoil spring is in place, undamaged and that the shaft is not sticking in the housing.
If all OK, VERIFY proper operation of the centrifugal spark advance.