The change to the 82c (180f) thermostat was to increase thermal efficiency, therefore spending less fuel just trying to keep the engine warm. You want a cooler (160f) thermostat for what reason?

Bushleague is "spot on" with this. If you somehow think that a cooler opening point will provide a benefit, you are mistaken.
If your engine is already exhibiting signs of overheating, loss of coolant, or "boil-over"........it ain't due to the opening point of the thermostat!
After a thermostat opens at its "set point", the engine will run at about that temperature.......unless there is a problem of other nature in the cooling system.
So, if you are experiencing problems such as coolant loss, overheating, or violent boil-over, do not look at the thermostat.
Instead you need to be checking for evidence of coolant in the lube oil (which usually occurs due to a defective water pump, or head gasket) and/or signs of coolant in the exhaust (defective head gasket).

The problem I have is that the tractor runs great until it gets warm. Then it barely runs if the PTO is not engaged, and if it is engaged, coughs and spits black smoke every time you hit a bump. I have read that this condition arises due to the carb getting hot and vaporizing fuel. Apparantly, after the thermostat changed, the engine was given a new carb to combat this condition, but my unit is an older one with the older carb set up.
I just did an oil change, and saw no sign of coolant in the oil. Also, the coolant in the radiator doesn't go down.
When the tractor was new, the gage barely went into the green on the gauge, but now runs halfway up the green zone and just doesn't run as well.

bushleague and mowie,

You guys are incorrect here. JD changed the thermostat to comply with tighter EPA standards. The tractors that came from the factory with the hotter thermostat came with an updated carburetor. On tractors that required a replacement thermostat, Deere made an upgrade kit that included the new carburetor. Many years have gone past (I think the last year for the 425 was 2001) and though it is possible some upgrade kits are floating around, it is still a better solution to find the lower temp thermostat (if possible) as the tractor will run much better.

Cheers

JDFANATIC

Hi guys. Will I get any problems if I completely remove the thermostats and run without? Thank u

You will lose the benefit of having the engine operate at a consistent temperature.
Engine temperature without a thermostat will vary depending on ambient air temperature and the loading conditions of the engine.
Engine warm up after a cold start will be delayed somewhat.
I would expect any "problems" from not having a thermostat would occur during cold weather operation more than during summer time operation.
Thermostatic control of coolant flow has several myths that arise in popular culture.
One myth being that a "cooler rated" thermostat will result in the engine automatically assuming the new lower OPENING TEMPERATURE as its OPERATING TEMPERATURE.
The rating of the T-stat only governs the temp where the T-stat begins to open allowing coolant circulation through the radiator.
And under ideal conditions, that rating may in fact be the temperature the engine will operate at.
But higher ambient air temps, dirty radiator cooling fins obstructing air flow, anything blocking air flow through the radiator, a collapsed coolant hose, etc can and will result in the engine temperature rising higher than the opening temp rating of the thermostat.

Operating without a thermostat during cold weather will cause a number of problems, the most insidious being degradation of the engine lube oil due to moisture in blow-by gases condensing into the oil and never being driven out again by the beneficial "normal operating temperature" being attained due to "over cooling" the engine.

Jdfanatic is right on the money and this thermostat situation is an acknowledged reality with the JD 425 and other models with the same generation Kawasaki LC engine.

Posts regarding the inability to find the original temp thermostat pop up from time to time and all note that using the replacement stat results in a bad running engine as it approaches normal operating temperature.

If a 425 owner can't find the upgrade kit to cure the disease instead of not treating the symptom cause the original lower temp stat is NLA then they should contact JD and see if a kit can be found... bear in mind that the kit includes a carb so it will be not just be pricey but SO pricey!

Running the LC Kaw twin without any thermostat will result in an incredibly bad running engine... I've seen it done... but only for a very short time till the stat was replaced.