the problem is that where the ports terminate under the exaust valves the carbon in that extreme enviorment is super hardened carbon. you must remove the heads take the valves out and use a drill bit to dig out the carbon. I used a #30 drill bit on my SL500 heads M119 eng. The carbon was very hard there would have been no way to desolve it with any chemicals

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg