I would be interested to know if the OP ever solved the mystery of why his engine would not start. I have had a similar experience, though I don't think it is fuel
-related, because the primary filter is always clean when I pull it and change it - twice a year.
Sailing down from Brisbane (sub-tropical) to Hobart (cool temperate) my Yanmar 2 GM-20 started every time with no trouble.
Then one fine day it just didn't want to go. It would fire, but die. After a lot of cranking with ever more throttle applied, it eventually stumbled to life.
Now it is really stubborn, so I have had to resort to 'Aerostart' (ether?) to help it along.
Once up and running, it produces plenty of power, and no more black smoke than one would expect at wide open throttle. At 2200 to 2300 rpm it runs smooth with no smoke and pushes my 30 ft boat at near hull speed. After that first reluctant start of the day, it always starts easily enough without resorting to the Aerostart, unless it gets really cold again.
A local Yanmar mechanic reckons that these salt-water cooled engines commonly get corrosion in the exhaust
valves, which means removing the head and getting the valves reground etc. But the compressions feel OK - at least I can hardly turn it on the hand crank unless I de-compress the valves. This engine is about 5 years old (although the 2GM series was superseded well before 5 years ago, it was in the crate until then) and it's only done about 400 hours, with plenty of loving maintenance - oil changes etc - and used throughout the year (even in Tasmania it ain't that bad you can't sail most months of the year). I have always been careful to shut off the raw water when not using the engine for more than a few days and also regularly check that no surplus water is anywhere in the exhaust
Any ideas, or actual experience much appreciated. Plus two questions in particular:
1. In colder climates do they really need full throttle to initiate the start?
2. Just how much damage does a squirt of Aerostart do - surely if it's that bad they wouldn't market it?