Reply To: Douglas SV54

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If you’re sure that the valves are sealing make sure that the head is flat by getting a sheet of emery cloth, lay it out on something that you know is flat (like a piece of glass) and rub the underside of the head on it using a circular motion. It’s surprising how much some of these heads can distort over time and the rubbing marks will give you an idea how much- or how little- it’s out of true.

If it’s not the head, head gasket or valves then it has got to be the piston rings. Cylinder bores tend to wear more at the top than the bottom and if your engine has been standing it’s a good probability that the old oil will have glued the piston rings in. Turning the engine over will compress the rings on the piston as it moves down the tapered bore, then they don’t spring out again as the piston comes back up. Have you tried some engine oil down the sides of the piston to see if it raises the compression?