03 25hp 4strk cooling system clogged

Joined
Mar 20, 2012
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27
4 stroke Merc 25.
After replacing the waterpump impeller I discovered I was not pumping water. Motor overheating, indicator not clogged.
I took lower unit off motor and blew compressed air up the water tube. With the thermostat out, I could not get air to move freely through the cooling system. I could hear a slight bubbling in the engine block. Looking for some good ideas on how to unclog the system.
 
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