I cleaned up an LA 25 after the Brodak contest. This head goop pattern looked interesting. The black stuff on the squish band runs from the glow plug to the cylinder, over where the exhaust port is.
I've been running the engine, the second oldest I've got, for several years on either 23% synthetic, 20% synthetic plus 3% castor, and 10-15% nitro. The rest of the engine is pretty much clean of any cabon build up.
Any useful thoughts?