Well, did some mowing yesterday. Didn't go well. Didn't seem to have an overheating problem though -- IF we go by the temperatures (as measured from an infrared thermometer pointed at radiator by the upper radiator hose, which has been shown to be within a couple of degrees from actual coolant temp as measured with a thermocouple on a Fluke meter). The temp did get to about 216, but if we didn't cut grass as fast (i.e., let load), they would stay in the 195-208 range. I have not been able to get a good feel for what acceptable temp ranges are, but from the fuzzy indication I have got from reading, this seems OK. Seems like they should definitely be > 180 (the thermostat opening temp) and >220 is getting kinda hot. 230 means something went wrong and you need to let it cool off and shut it down. (Can someone tell me if these temp ranges are OK?) I looked at the coolant level this morning ant it was normal. So, no boiling over. Prior to head gasket fix, the coolant level would have been quite low and I would have to add some. So, I think the temp gauge/sensor is just reading kind of higher than it should be.
BUT...
It started doing something different. It would act like it was running out of fuel or something. But it wasn't. It started running very rough, like it was only running on 1 cylinder or something, and then it would stall out and stop. Thought it might just be some air still in fuel system, but I don't think so. Started doing this often. Got some video of it, but don't have it handy at the moment. This morning, cold, it started right up and ran normally for maybe 60 sec, then it did the same thing. Started again and it didn't stall out, but would periodically run rough and then recover. But it was acting intermittently. Does not seem to smoke when it does this. Injectors have been rebuilt by diesel shop with good reputation. Didn't drop them or anything. I have not messed with the injector pump at all. I did remove the hard lines when doing the head. I did remove and clean the fuel filter -- didn't seem dirty enough to replace (cleaned in kerosene and blew off with air). I did bleed the fuel system afterward (fuel bowl, line to pump, caps on injectors).
Any ideas?
It sort of seems to me that the cooling system is right on the edge of being insufficient. This Kubota engine is a 14 HP engine with a 2.2 QUART coolant capacity. The Yanmar is a 19 HP engine with a 1.1 GAL coolant capacity (about twice as much coolant). Yanmar radiator seems quite a bit bigger too, but haven't measured.
I would like to do the pressure test that Eric talked about. Don't have a pressure tester though. Maybe Advance Auto has one I can rent. I did notice that coolant flows EASILY out the temp sensor hole in the head. Found this out by accident by trying to add coolant with temp sensor out. And the temp sensor is on opposite side of head from the water pump.