Hi Mike, let's see if we can take this step-by-step"
Originally Posted by mhfowle
I have a yanmar ysm8 with the following problems: It won't start warm (it starts instantly when cold)
This says to me that your problem is specifically related to a change in the engine when it's hot. You don't mention whether it spontaneously stops when hot and won't restart, or just doesn't want to start after you've stopped it and it's warm.
Since there are really only 3 things a diesel engine needs to run happily: Fuel, Air and Compression, let's assume that it's one of those that's causing the problem.
The air I don't think is likely to be an issue unless your engine has some kind of fancy intake - if it's basically a hose with a filter inline somewhere I can't see how heat would affect it.
Fuel however - now that could be affected by excessive heat depending upon how close your fuel lines run to the hot engine. If they get warm enough to vapourise the fuel in them you could get vapour-lock in the fuel system, whereby a bubble of fuel vapour is stopping the liquid fuel flowing. Not likely, but possible. You should also go back over all of your fuel unions and just make sure that they're nipped up tight. Air can get in through the smallest spaces!
Lastly, and I think most promisingly: compression. You say you haven't tested it. I think you should, both hot and cold. As you're aware, when metal gets hot it expands, and I wonder whether your rockers may not be adjusted too tightly - what are the gaps? a good rough starting point should be 12 thou inlet and 15 thou exhaust, when cold - or whatever it says in your manual. remember to re-check the gaps after tightening the screws of course, so you can be sure you haven't changed them in locking off the adjusters.
What state is your oil in and how long is it since your engine was last rebuilt? the oil film may be creating compression when cold that it can't when hot. Do an oil and filter change and see what you get (be sure to put good thick diesel-friendly oil back in).
Lastly, test that thermostat! If everything worked before installing it, pull it out of the engine, put it in a pan, with a thermometer, on your stove and boil it gently until it opens. Compare this temperature to the stated operating temperature, and the temp. stated in your manual for your region. Diesel engines run cooler than petrol generally I think, and I think the 'stat on my Volvo opens at 60 - I'm based in the tropics too.
Soot: Could be a variety of factors: 1) knackered engine oil being flashed off and going through the re-breather system is one. I assume this little unit doesn't have a turbo or supercharger, so we don't need to worry about that?
2) inaccurate metering by the metering unit (wouldn't expect this to be the case in an engine as new as yours though unless it'd been tonked. 3) Choking the airflow of the engine preventing complete combustion. (actually, if the exhaust only smoked as you accelerated, it would have to be the metering unit, not the lack of airflow) What colour is the smoke? blueish-white or black?
The battery has nothing to do with the current problem - assuming it will spin the engine over when it's warm - although, you may be storing problems up for the future if you're gently simmering your batteries and/or if you have long, thin, unsupported cable runs, for the reasons raised before.
Ignition timing is fixed I should think - if your injector pump runs off the camshaft like mine.
hope this helps