I think if the engine is running well and the compression is good then I would be looking for leaks in the fuel system. I recently had a broken plug where the insulator had cracked and this sprayed loads of slightly coloured fuel / oil out. Took me ages to track; the fact that it was slightly discoloured suggested an exhaust related issue. I believe you say that this is not the case with your problem? Hence anything pre-injection would be suspect including the one way valve and the rocker box cover. Look also for missing / loose screws. Finally if you still can't track it then try siliconing up the joints for a quick test to see if you have poorly mating parts.

Hope this helps

Keith