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Remove the exhaust pipe and the spark plug wire. Drain the barrel by removing the drain bolt in the side of the barrel. Once the water is drained out, reinstall the drain bolt. Remove the hose clamps and disconnect the coolant hoses from the cylinder head. Disconnect the carburetor from the intake manifold.
Using a 12mm socket, remove the cylinder head nuts. Remove the head and examine the combustion chamber, piston top and head gasket.
Look carefully at the mating surface of the cylinder head. Burn marks and deep scratches are trouble. You will have to resurface the head to assure you have a smooth, flat surface. Look at the squish band area and hopefully you will not see any areas (usually shiney)where the piston is coming in contact with the head. An easy fix it to add a couple of .001's of base gaskets to raise the barrel. The better fix is to machine down the squish band on a lathe. Look for burn marks here on the barrel mating surface with the head. If you see one(or more), you will have to remove the barrel studs and surface this area. (piece of glass and 400 grade wet/dry works well).
Use a 12mm socket and remove the barrel nuts. Slowly rock the barrel and lift the barrel off the engine cases. Try not to rip the base gasket.