The radiator drain plug is on the bottom of the radiator on the left side. You may have to remove the left body panel to gain access to it. You need to clean the radiator screens frequently, remove the screens and spray the radiator fins from time to time with a garden hose. This is something you have to pay attention to, because Kubota will not warrant damage from overheating the engine due to a clogged radiator. It is hard to imagine how you clogged the radiator fins after 4 hours use on a brand new tractor, it must have been some severely dusty work you were doing there. I have owned mine for 6 years and not had that kind of problem. Sounds like you took care of that part of it. I would not change the coolant unless the dealer tells you to.

Well, one way to determine if the head gasket is leaking compression/combustion into the cooling system is to connect a common radiator pressure tester to the radiator filler port before starting the engine when cold. DO NOT pump any pressure into the radiator with the tester pump, you are only going to use the pressure gauge to see how soon, and how much pressure, registers on the pressure gauge. If the pressure gauge shows a rise of pressure up to or near the pressure rating of the cap within a minute or so after starting, that indicates the system is being pressurized by something other than normal thermal expansion. Another way to test for compression/combustion leakage into the cooling system is very "old school" but still valid, however, it requires caution and a stop watch or timer. For this test, begin with a cold engine. Remove the fan belt from the pulleys and get the belt away from the crank pulley. Removing the belt disables the water pump and this is necessary because the swirl of the pump could spoil the "quiet stillness" needed to observe the escaping gases (if they are present). Remove the radiator cap. Top off the radiator with coolant so that there is no empty space below the inner radiator cap seat flange. Start the engine and your timer (or note the time on your watch). DO NOT RUN THE ENGINE MORE THAN 2 MINUTES without a fan belt. With the engine at idle RPM, watch the coolant in the radiator filler opening. If you see what look like "air bubbles" begin to burb out of the radiator opening within a few seconds after starting the cold engine, that is "compression/combustion gases" escaping from the cooling system. Be advised that on some engines and depending on how bad the compression/combustion leak actually is..........what you might see instead of a few bubbles............could be a geyser of coolant shooting up from the radiator filler opening, so don't stick your face too close to the opening, or directly above it, and you would be wise to wear some kind of approved eye safety goggles or face shield. If you see gas bubbles or a geyser within 2 minutes, you definitely have compression/combustion leaking into the cooling system. Now, about the tendency for the "grass screen" to clog quickly. That is unusual unless there are some other circumstances or conditions that are promoting a "cloud of debris" in front of the radiator while you are mowing. Have you looked really close at what the screen is catching? How tall is the grass or weeds (as I don't know what you normally are cutting) that you are mowing? Does the grass or weeds usually have "seed heads" or dandelion fluff? What about other types of plants such as milkweed (that also have airborne seeds connected to "fluff parasails")? If the grass/weeds you are mowing is fairly tall, has seed heads (or both) and you are mowing with the front bucket down low enough to "skim" the plants.......when the plants rebound after the bucket passes them, they are flinging fluff and seeds into the radiator air inflow zone where the screen can capture them. If that scenario does not match what you are doing, then you need to look closely at the exact circumstances that ARE taking place when you mow. Something is causing the tractor to draw debris filled air into the radiator and something is causing that debris cloud to be present. If you are cutting at a higher level in sparsely vegetated turf, it's likely that all this "chaff" is escaping all around the edges of the deck instead of being neatly discharged out the directional discharge chute. If that is what is happening, you (or someone) might need to engineer some sort of skirt in front of the deck to stop the chaff and clippings from escaping toward the front of the tractor.

At 265f your engine is seriously overheating. The fact that it cools down when you stop suggests first that the cooling fan is running, which you should easily be able to verify just by looking. It also suggests that you have either a coolant flow, or air flow problem as the main issue. Question, is the plastic air dam that fits below the radiator support under the front of the car in place? This air dam forces air up through the radiator, while also helping to create a low pressure area behind the radiator in order to help force as much air as possible through the radiator. The coolant fan is not part of the problem when you are driving. By design it turns off at about 25mph, because there is more air going through the front of the car by then than the fan could ever pull. Check for debris that could be blocking airflow through the condenser and/or radiator. Things such as leaves, cottonwood poplar "cotton", paper, cardboard etc could restrict airflow and prevent the system from cooling the engine. From here we get into the possibilities that the engine isn't actually overheating and maybe the gage is bad. Is the computer system turning on the check engine light? If so you should see a P0114 setting. A technician with a scan tool would be watching the coolant sensor's temperature input to the PCM. A technician would also directly measure the engine temperature with an infrared thermometer. With reported problems like this damage to the water pump impeller potentially comes into play, however traditionally GM does not use plastic impellers as many of the imports do. Many imported vehicles suffer failure of the plastic impellers and that results in the pump not actually being able to pump the engine coolant fast enough to allow the engine to cool under load. But it may still cool when stopped and idling. A thermostat frozen in one position could do this, but they are usually over replaced in practice, with the person doing the work ensuring to fill the coolant system fully, so it then appears that replacing the thermostat fixed the overheating, when in fact the system was actually low on coolant for an as yet undetermined cause. I don't expect there to be a connection to the missfire/backfire and the overheating situation. UNLESS, the engine is not actually overheating, and there is found to be a reason that sensor inputs to the computer are being corrupted in some fashion.

Thermostat on most cars I have had live near where the top hose from the radiator joins on to the cylinder head, and there is a panel you can unbolt to replace it. Some of the newer cars I have had (ie post 1970's) have had this obscured by the cam belt, I think this is to deter the more amateur home mechanic and get him spending at the main dealer instead.