After repairing a timing chain and doing a valve job on left head, the #4 plug gas fouls almost instantly upon startup, I have double checked timing of valves and ignition, swapped injectors, swapped wires, ran compression tests, even swapped pipes from injection dist from cyl 3 to 4. The trouble always returns to cylinder 4. I removed the intake manifold to see if there was an error in installation. Looks right and the gasket is well imprinted. Any ideas?