I thought about the intake valve pocket too, but never heard these heads were prone to cracking there. I've never heard of a head gasket that didn't show up as bad on a leakdown or compression test. And it would have definitely got worse over 3 years. And there's definitely no way for ater to get into the intake manifold on #3 cylinder.
The reason I'm suspecting the exhaust is that this started after a "substandard" winterization where the block, coolers and risers got drained, but not the exhaust manifolds. Turns out they do not drain when the block is drained.
The cleaning of the plug could be due to reversion. Cam overlap causes the intake valve to be open for a short time during the intake stroke. This can suck in water if it enters too close to the exhaust valve.
BTW, the water pump, hoses, bypass valve, etc are all working properly.
And no Barrs leak in my engines!
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