I have the DC version of the FP 4000 and have had the same problems - BUT I have discovered that the installation
was at fault!!!
The problem comes from water getting back up into the exhaust elbow and into the engine. FP's instructions for fitting the waterlock was covered in one small paragraph in my manual. In their later manuals
their recommendations go to 16 pages!!!
The problem happens when the boat is sailing and healing over. If the waterlock is not 600mm directly under the generator
then depending on the angle of heal water can get back up to the waterlock and into the engine. This has happened twice to me. The more the waterlock is not inline with the genny then the worse the problem may be.
Also if the engine is not used overwinter then salty air from the water in the waterlock gets circulated up into the exhaust and valves.
If you carefully read the maintenance
manual for these Kubota engines there are clear instructions about what should be done - sometimes every 50 hours! I suspect nobody does this.
One instruction is to empty the waterlock when winterizing the engine. I now do this everytime
I have run the engine. If access to the tap is not easy connect an extension pipe so you can get to it. I have now re-installed my waterlock and added an extra Vetus one to give more volume.
Instead of buying
new exhaust elbows that seem to use very low grade S/S I had mine remade in Turkey
with A4 S/S for just 90 Euros instead of about 500 Euros for a new one.
I also freshwater flush the genny if I know it will not be used for a month. This stops salt water
deposits in the rectifier cooling
block which finally causes overheating
and needs cleaning
I'm afraid this is what you get for a very small - quiet -"over engineered" generator - at least I have managed to do all the work on the engine myself so my repair costs have been very low.