The timing device could be the culprit, however I do not believe I have ever heard of one giving problems. I've only been at this a year or so though so my knowledge is not all encompassing.
If the injection pump checks out the timing device would be the next logical step. It is located under the vacuum pump (is there a vacuum pump on a marine 616?) and it is a fairly involved procedure to remove it.
Did you double check that there are no restrictions in the intake?