The problem is not in the intake or combustion chamber or anywhere near the injectors. The only thing that should ever pass through these ports is air pumped by the air pump. It goes through passages in the head and comes out after the exhaust valve in the exhaust. The carbon should mainly be at the intersection unless the check valve has been bad and exhaust has bee going up the passages toward the valve and then the pump.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician