Took battery off charge, connected it, and it fired right up....
I did not have water on it so I shut it down after a few seconds, but at least it is seems not to be suffering catastrophic failure.
Next step is to check anti siphon Valve, as this seems very likely and easy ( and cheap) to check/repair. Fuel filter check / replacement as well, but I have not filled up since the last time I was out and it ran ok then. Considering the incredibly low fuel consumption and general lack of power I am suspecting the Anti Siphon Valve / Fuel pump as an issue.
The IAC was checked and I installed a new filter on the IAC inlet last week as the original was missing and it was making a rather loud suction noise. Noise now gone and filter still in place.
In a previous post I have been trying to work out if this motor is lacking power and performing poorly, or if it is just a small motor in a big boat and there is nothing more to be done.At present I would classify it as monumentally gutless with all tests showing it is slower than my previous 130hp 4 cyl in a Sea-Ray 180. This search meant that I have already replaced Plugs, checked plug leads which are one season old. Checked fuel, fuel filter, oil, oil filter, drive lube and flame arrestor. I have also had my mechanic plug in his computer to look for issues and the printout looks to manufacturers spec.
Next step after anti siphon valve is fuel pressure test / Fuel pump/regulator tests. Seeing as the boat just started I may not need to check the ignition system but the following is on the list: ignition testing, distributor check, Ignition control module, coil test.
Still looking for the manual for the 4.3L MPI - a free download would be nice but I will pay for it if a free one does not exist.