1977 Mercury 1150 - Debris in the Cavity next to Water Pump Housing

[kck8385]

I have a 1977 Mercury 1150.

Quick summary.
1. I have had the motor since 1981. Always used in a freshwater Florida lake.
2. Motor sat unused for @ 15 years until 2014 at which time it was serviced for use by a mechanic who knew old mercs. Motor runs fine and at the proper temp.
3. When I run the motor in the lake only and drop the lower unit to service the water pump there is nothing (no debris) in the housing in the area of the water pump.
4. When I run the motor in a tub there is a mineral type deposit that is left in area around the pump housing. It is NOT sand. Tub is clean before adding water.
5. I have attached a picture. The material crushes between my fingers into a powder so fine it disappears. Again it is NOT sand or anything in the tub.
6. Some kind of deposit is coming out of the prop exhaust and recirculating into the lower unit and settles beside the pump housing. I looked up the exhaust tube and it is clean.
7. It is not getting in the water pump. Impeller and inside of housing is clean.

Any ideas what is could be?

Thanks,
Kevin

 

[jheron]

Looks like scale to me, nothing to worry about typically.
Is there any sign of foam coming out of the exhaust ports?
Cheers,
Jon

 

[kck8385]

Jon,

No foam coming from the exhaust ports.
Kevin