Last year (2007) my 1GM10 was stripped down and new main bearing shells, a new conrod, gudgeon pin, piston, and rings installed. The cylinder was deglazed. The valves were reground in (everso slightly) and the whole thing rebuilt. Timing was verified. Etc. And then after all that the engine would only start after a long haul on the batteries. Once it started and got warmed up then it would happily chug away at 2,700 rpm and use just 3/4 litre/hour (5.1 knots with a 2 blade Euro pattern 13 x 12 prop) (Compression before: 260 psi. Immediately after rebuild: 320 psi) Next day it would start easily. Give it a week then I was back to the long haul of wearing out the stater motor ..... Forget the rest of a long saga of finding that there was never any return fuel from the injector. Let's get to the 
bit .....
The Yanmar 1GM10 workshop manual tells you that you can't rebuild the high pressure fuel pump. You can. It's only a question of finding a land based tractor diesel repair shop with no connection to the word 'Marine' that repairs Yanmar or Kuboko generators. He will have access to the secret world of how to acquire spares for the innards of the 'unrepairable/servicible' injector pump ie element without having to add the extra zeros that the swindleries demand. The land based tractor man found that there there was some slight scoring in the element that reduced the pressure by ~18% (hence, as he said, once you got the engine going it was very economical.)
I was told by a boatyard (who happened to be Yanmar Agents) that my 1GM10 with 1,564 hours clocked from new was "beyond repair" and that for the equivalent of USA dollars ~$9,400 (exchange rate at Feb 2008) they would take out the old defunct lump and replace it with a new one which would start ...........
The element and pump repair cost me $120 net of tax. The pump was fitted. The engine bled and primed. The key was turned and the starter button pressed .... It started without hesitation
After 100 hours under 2/3rds running load the compression is now ~385 psi* ..... The Tractor Diesel man said that given the lack of wear on the injector pump he reckoned it should last another 5,000 hours .... The Yard says that if you get 2,000 hours out of a 1GM10 then 'you are doing good' !!!!!
*Russian Xado snake oil was used. It seems to work. The compression will be monitored. Fuel consumption is now 1 litre/hour at 3200 rpm and 5.4 knots.