Power loss

Thanks forum for a great resource.
This Grubee 66cc lost it's mojo (power) kind of gradually (over a couple of maybe 3 weeks).
I couldn't determine any leaks with the starter fluid, or the liquid soap test. I tried swapping out the piston and also the piston chamber. That made it worse. Spark plug etc... Here's some pics of my crank seals. Will a failed leaky seal always be something you can see, or can it look good and be bad? Thanks in advance

 

[Fabian]

What is your main jet size?

Please show us a "CLEAR HIGH RESOLUTION" pic of a brand new spark plug after the engine has been run at full power for 30 seconds, then killing the engine immediately and at the same time pulling in the clutch lever.

Please show us a "CLEAR HIGH RESOLUTION pic of "both sides of your cylinder head gasket.

Make sure that the centre case gasket hasn't blown out.

Make sure that you don't have carburettor air leaks.

Make sure that the main jet or the main jet tube hasn't come loose.

 

[Fabian]

From the photos, your crank seals look like they are correctly seated, and from my experience they always weep a little oil.

 

[Deleted member 12676]

the southern way is using soapy water to check the seals.
the correct way is pressurizing the crank and looking at a gauge to see that it maintains the pressure.
you can't tell by looking at them.
all these engines seal poorly at the head. use sandpaper and glass and plane the mating surfaces flat. and make your own fiber gasket.