The YM series Yanmar tractors are really pretty simple so there is not a whole lot that it can cause overheating.
The most common cause is grass and dirt clogging the external fins but you said you already took care of that.
The other common cause is just the little fins being bent over so that air can not flow through. You can buy a little plastic tool just for that purpose to straighten them if that is the case.
Another common cause is the tiny passages INSIDE the radiator get clogged with decades of minerals from the coolant. You can test for this by getting the tractor up to operating temperature and then running your hand along the front side of the radiator. If you feel spots that are warmer or colder than other spots (the hot water not getting through the fins) then you have an internal clog. These minerals can be VERY hard to remove. You can try to take the radiator to a radiator repair shop to get cleaned out but a lot of the time they will recommend you just replace it.
The less common cause is a collapsed radiator hose which can look normal on the outside but is balooned up on the inside.
Finally just be sure that you are not overworking the tractor. The harder you work the tractor, the more fuel the governor sends to the injectors and the more heat the engine produces.
You didn't mention what model tractor you have but, if you have a model without a water pump (YM135, 155, 1500, 1700) then you need to be sure you are not using more than 25% antifreeze during the summer. A 50/50 mix in those tractors that use a thermosiphon system is too thick and will not circulate correctly.
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https://brandmparts.com/pages/why-is-my ... verheating
