I've had a bit of a headache since getting my Nissan sr20 det rebuilt! It's got new low comp je pistons. Since I've tried to get the engine started it's only fired on two cylinders. After a lot of checking, one of the issues was; I had some fuel left in the tank, which had been there for two years and this had gone off, it looked like lime scale at the bottom of tank. As its only ever gone on the back and front cylinders, fuel was getting into the cylinders and the oil has got fuel in. I changed the oil and filters, the fuel tank and pump is changed, I sent the omex 500 ecu and loom to omex and they checked the ecu, this was okay, there was a diode for the rev counter that was suspect, its been tested and omex said this is okay on all four cylinders. Before I try to run the engine back up there's some black stuff on top of the pistons which looks a bit gummy, I think its some of the old fuel! How can I clean this best without removing the head? Any ideas would be appreciated, I'm stumped!!

Jim