Being a quiet day, I decided to run the dial gauge on the valve rockers at various positions.
Both rockers/tappets had dimples from the valves so I ground them flat again, only needing to take off about 0.04mm.
I'm pretty sure I put new cam bearings 6 months before the problem started, making them ~ 2 years old now but not having done much work. The dial gauge tossed up some interesting results. When the cam was loaded ie during a valve lifting part of the cycle, the clearance on the other valve dropped quite a bit.
0.06mm of clearance (at TDC) on the exhaust valve reduced to 0.01mm when the intake valve was being opened.
0.07mm of clearance (at TDC) on the intake valve reduced to 0.02mm when the exhaust valve was being opened.
Not sure why this is happening but I'm guessing this isn't "normal" for a new engine otherwise YS wouldn't specify the clearance to be set between 0.0mm-0.1mm. It might be something that creeps into an older engine like mine. This is something I'll review when I change the cam gear bearings. I'm guessing the bearings have been letting the cam shift when under load, unseating the other valve and allowing it to leak.
Anyone else out there notice their valve clearances change like this?