Is this what ash buildup looks like? It doesn't match any of the "reading your spark plug" charts but the ash looks the most similar. There are globs of buildup that scrape off easily with the lightest pressure from a knife. If it is ash, the cause listed is leaking valve seals - how could that happen on an engine on a 400hr engine? Is it just fouling because my front cylinders are too cool?

The spark plugs in the photos were changed 100 hours ago. The plugs on the back two cylinders look fine but the plugs on the front cylinders definitely do not look normal. On previous plug changes, the front plugs have always been much darker than the back plugs but this is the first time it's looked this ugly. I have always used the NGK DCPR8E plugs gapped to 0.025" and I change the plugs at every 100hr inspection. My EGT sensors are on cyl 2 and 3 and run about 1200F with about 50 degrees difference between the two of them. I have always used 100LL with Decalin in every tank. Oil used is always Aeroshell Sport Plus 4.

Any suggestions what to do?

Thanks!

lumpy-deposits.jpgTop-plugs.jpg

Note: Although it appears that there is thermal paste around the first thread on the Cyl 3 plug, I dont think that is what it is. This is how I always put the paste on the plugs:
plug paste.jpg