Hello,

I ran this engine again, it ran great until it warmed up. the symptoms then were:
#missing at times.
#occasional spat through the carb; and
#white/light grey smoke at times from exhaust that seemed to come in waves.

After pulling it down, the intake valve stem still had residue on it, so cleaned it off with an oily scourer pad.
I am confident the valve seats are good.
Reassembled them with assembly grease on the valve stems.
I'll start it up today and fingers crossed it will be ok.
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While it was apart I investigated how the oil gets in and drains from the rocker cover area.
[Linked Image]

You can actually see the decompressor through the oil entry hole, so you could inspect for a broken spring through this hole without removing the bottom plate.

Also, a theory, does the decompressor have a secondary role of throwing oil into this hole, once it flattens out at speed?
Still a bit unsure of how all of the components under the rockers get oiled.

I cant see inside the engine when its running, but imagine the oil flicker with the addition of the pressure changes inside, creates an oil mist and this area under the rockers is oiled by a mist of oil?

Happy is he who penetrates the mystery of things.