Don't know, but I had the same problem. There must have been just enough voltage loss to the solenoid on the one engine that it wouldn't engage without applying power from both sets of (new) batteries. Replacing the solenoid corrected the problem on mine.
You may want to check the voltage at the run/fuel solenoid first. Have someone depress the ignition switch while you read the voltage and then do it again with the emergency start switch engaged. If there is only a slight difference in voltage, it may be a flakey solenoid. They are fairly easy...
Another thing to check is that the engine is coming up to speed. If the run solenoid is sticking, the engine will start and run but not put out any juice. You can push on the solenoid linkage and see if the RPM increases to a point that you start generating power.
Greg
Any good marine mechanic should be able to do any of the yearly maintenance items you don't want to tackle yourself and their hourly rate is almost always less than Cat's. There are some items as Frank said such as valve adjustments. and in my case a fuel rail adjustment, that require special...
When I was looking for my 370 in Florida a few years back, there was a huge difference in the overall condition of the various boats we drove to see. Of course all were rated as "8's" and "9's" when talking to the seller's broker, but once on the boat it was obvious that some were never...