Ive just took the inlet manifold and there is a huge build up of carbon near wear injectors are and around injectors was a bit of carbon. the motor has done about 175,000 km I bought it with a blown head gasket and have done compression test and was good 140 equal on all. Had engine running and was blowing water through exhaust on right bank, The engine was running smooth . The oil has not been changed in a while and had a look at air filter was totally full of crap. Is it normal to have a build up of a lot of carbon near the inlet? If so what would have been the cause? cold it be resticted air ?, or overheating? engine? any suggestions