Not certain but I think you have a pump like this on your engine, Try changing the cam box oil. When used on the road these pumps had a drain tube to allow excess diesel fuel to drip, hence the black slippy bit in the centre of the carriagway, if you are a biker you will know what I mean. Also when you are ticking over does the engine hunt? a sure sign of no oil in the cam box. Do not be surprised or worried if you get pure diesel run out when you remove the drain plug. They need checking about every 8 hours running.
Are you sure that there is no drain plug as indicated on the drawing, the boost control pipe goes into the inlet manifold and the depression in that operates the boost control, alleged to prevent black smoke when opening throttle up, (not a very successful modification). Check that line for air leaks as well. Hope you find the gremlin soon. Did you want a photo of the rocker shaft I have going spare??