1994 125 yz overheating

stuck thermostat.. replace.. check for air entering the cooling system..if there is a weeping head gasket the presure usually enters the cooling system and will force all the antifreeze out with pressure..fill at the radiator and tale the overflow hose out of the catch bottle and place the end in a glass of plain water 3/4 full with the car running at idle watch the end of this hose for air bubbles to come up in the glass from the hose..there are many reasons why engines overheat..too small of a radiator(not enough cores) bad waterpump, cooling fans defective..warped heads/block//bad gaskets, cracked castings,, if you really want to save the car, put a salvage motor in it and stop fighting the current issue. it will actually be cheaper for you..I would think

How long has it been going on? You need to check if you have a bad head gasket. A compression test of the cylinders would tell you. Not saying that's what it is, but that can be a cause of overheating. And it can also be an expensive result of prolonged overheating. That's why mechanics caution against running an overheating engine. If you find the head gasket is good, it will give you great peace of mind, knowing the Shadow's engine warrants a few more dollars in repairs. Compared to the cost of a new head gasket and having the head checked for straightness and for cracks, other repairs for overheating is minimal.
First check the coolant level and keep it full. It could be a worn water pump. Feel for pressure behind the coolant when you squeeze the upper radiator hose for a second or two. The engine needs to be warmed up and running, so the thermostat is open. Try to squeeze the hose closed, then let off and feel for a surge of coolant, and watch the coolant in the radiator for pressure pushing coolant on through the radiator. Kind of a rough test if pump is circulating coolant. You might also try a new thermostat. They are cheap and easy to replace. Clean the outside of the radiator of leaves or dirt. It should be checked for blockage. When upper hose is hot, the lower hose should also be hot or quite warm, just not as hot as the upper hose. If it is, radiator seems not blocked.
Don't worry about having no heat at this time. When the overheating is solved, and coolant level is full, the heater will return.

CAR OVERHEATING FIRST CHECK FOR LOW COOLANT IN THE RADIATOR COOLANT OVERFLOW JUG.IF COOLANT LEVEL IN OVERFLOW JUG LOW REFILL ADD COOLANT BACK TO THE FULL COLD MARK.PUT RADAITOR CAP ON COOLANT OVERFLOW JUG START ENGINE LET IT IDLE UNTIL IT OPERATING TEMPERATURE.MAKE SURE TOP RADIATOR HOSE GETTING HOT WHILE ENGINE WARM UP.IF TOP RADIATOR HOSE DONT GET HOT WHILE ENGINE IDLING YOUR THERMOSTAT COULD BE STUCK CLOSE CAUSING CAR TO OVERHEAT.IF TOP HOSE IS GETTING HOT.CHECK FOR COOLANT LEAKS CHECK TOP RADIATOR HOSE AND BOTTOM RADIATOR HOSE FOR LEAKS.CHECK FOR LEAKS AT THE RADIATOR CORES AND PLASTIC SIDE CONTAINERS.IF ALL IS GOOD ENGINE OVERHEATING WHILE IN A LONG TRAFFIC LINE, COOLANT FANS NOT COMING ON.COULD HAVE FAULTY ENGINE COOLANT TEMPERATURE SENSOR OR BLOWN COOLING FAN FUSE OR RELAY.COULD HAVE PCM FAULT.MAKE COOLING FAN ELECTRICAL CONNECTORS AND WIRING NOT DAMAGE OR BROKEN.

There is a blockage in the engine coolant path causing insufficient cooling. When the coolant flows, it removes the heat produced from engine combustion to radiator to cool. If there is no flow then the coolant becomes stagnant, therefore the heat is built up without being dissipated to nowhere hence the engine gets overheated. If this is not fixed then the heat will cause the head-gasket blown due the the metal expansion.
Repair at this stage becomes very expensive.
1- Take off the thermostat.
2- Put back the hose connection.
2- Run the engine and feel the upper hose to see if cooling flowing through it.
4- IF Yes. Run engine like this for few days, if no over heat , buy a new thermostat and put it back. Your problem is resolved.
5 IF No. You have a bad radiator. It clogs up so you don't feel the cooling flowing. Buy a new radiator, don't ever use a rebuilt one.
Good luck.

If it gets hot quick, could have a problem with the cooling system, blocked radiator,leaks?
Pressure test radiator and cooling system.
They will backfire if they get too hot.
If its just been rebuilt, check head bolts, valve clearances

The older YZF's radiators were a tad small so they would overheat when sitting still. Even the newer ones will do this if sitting still for too long as there is no air flow across the radiators to dissapate the heat.

The overheating problem is more severe on the older bikes. The best bet is to make sure you avoid sitting still for too long. I know it's not much of a solution, but it's just the way it is with the older YZFs.

If the cooling fans, water pump, radiator, and thermostat are all working properly and the engine is still overheating, I would take a look at something a little more simple, like a defective radiator cap. Try this inexpensive fix and see if it works. Sure hope this helps and good luck.