I had exactly the same problem with a small Perkins. The opening pressure in
one injector was way too low. When a second injector reached the same state
the engine refused to start. It has 3 cylinders.
Have your injectors checked, it will probably cure the problem.
When I had that problem it was only while idling and disappeared when I
increased engine speed.

C.S.

"Drew" wrote in message
oups.com...
I have a chb trawler with a lehman 120, the motor starts great and runs
well initially, however after a half hour or so, the engine starts
idleing very rough, boat shakes quite a bit, is this an issue with the
injectors, could this be air getting in somewhere, can anyone please
help me out.... how should I diagnois this?