It may be worn rings, but not necessarily.
Its good you have verified your gauge, that would be step 1.
Ensure that the compression release is not activating, or the valves are not stuck open, which would limit your compression readings.
Its also good that the compression readings are very similar. I would start by closely inspecting the head gasket between the cylinders, to see if there is leakage there. Also, dress the head, by laying emory on a flat surface like plate glass, and doing figure 8s with it. You will watch the head get true, by continually checking it. In between cylinders always seems to get surfaced last when I do them. If that's the case, it was an easy fix of some time, and a new head gasket.
If that doesn't do it, its still not necessarily the rings. You will need to measure the bore, to see if it is worn oversize or not. The Javelin bore should measure 3.0625". If you don't have a bore gauge, and a micrometer, you could cheat and install new rings in the bore, and measure the end gap. The end gap should be between .007 and .017, any bigger a gap, and the cylinder is worn and will need to be bored oversize.
Now, if your bore is good, it could very well be the rings. Remove them, and the check their end gap when inserted into the cylinder.
Ill post some pics of a Javelin powerhead im rebuilding. Ive done dozens of restos on Javelins, they are one of my favourite motor. I have about a dozen of them!
There are a few candidates that you could use to swap powerheads if you end up having one that's no good, but lets verify that first.
If you need help with NLA parts or rebuild, PM me. The rings and/or head gasket, I believe you can get here at iboats, if need be
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