If it is carbon buildup then you should be able to see that with a borescope. I got one shown on www.provision100.com and it is pretty good. The 45 degree mirror lets you see to the side and I could see the cylinder head and valves. I got it at my local auto parts store for 229.00. Not positive if you could see carbon but I assume you could.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra