i have a merc 165hp inline 6 and removed the side plates from the manifold and found alot of rust inside of it how to clean that out and get better water flow??
If the manifold is plugged up, you replace it not just clean it out. Since it is the cooling system, rust is normal. Do you have a cooling problem? Being in Ohio with fresh water, I can't imagine it being plugged up. If you do have a cooling problem, have you checked the raw water pump, thermostat, riser etc.?
i ran some water threw the namifold and it flowed pretty good some rust came out i read some place to rod it out?? as for the thermostat and water hoses all looks good but still going to replace the thermostat. was thinking about running a gallon of mirax acid threw it just to help clean it some more. the last time i ran the motor it never over heated on me and did run good..