One thing I forgot to mention, I replaced the temp sensor about a year ago because I noticed the one that was in it had the wires cut right at the sensor. It never overheated since I’ve owned it though.

 

So far no one has responsed so I'll put forth my thoughts.
I suggest a first move as using a thermometer to check the coolant temperature to be certain it truly is overheating. One minute seems like a really short time from a cold start to see an overheat.
Bleeder screws are not effective in removing all air in many cases. Seeing a flow of coolant from a bleeder screw does not mean all air has been purged. One more sure means of bleeding air from the cooling system is to raise the front end a couple feet so air escapes and makes it to the radiator.
If it is truly overheating it may be that your thermostat is not opening and the circulation you see is caused by the coolant by passing as it's pumped via the bypass hose. I'm not familiar with the 500 specifically so i don't know the routing of the coolant to know whether you would see bypassed coolant or not. In addition, air trapped in the thermostat housing will not allow the thermostat to work properly. It must be immersed in liquid coolant to open at it's rated temperature.
You mentioned working on the carburetor. If somehow something is amiss there that causes a lean mixture, it could be the reason for an overheating condition. Reading the spark plug should provide a clue.
It may be that although you see some coolant flow the flow may not be adequate. I've read posts where the plastic impeller on some water pumps gives up causing overheating. If the impeller got too hot it may be slipping on it's shaft or have vanes misshapen or missing. Once again, I am not familiar with the specifics of the 500 so I can't say whether that is possible or not.
Moving on to more serious possibilities, a blown head gasket can introduce exhaust gases into the cooling system creating gas (air) pockets. Water pumps are centrifugal pumps and will not move gasses. Gasses do not cool effectively so the engine overheats. Testers are available to check for exhaust gasses in the cooling system for a small outlay of cash. Try Amazon.
It's worht repeating, overheating after one minute of operation from a cold start, or even after it's been sitting for 10 - 15 minutes is highly suspect. My first guess would be that the temperature sensor is sitting where coolant isn't flowing due to an air pocket or that the sensor is bad. If your 500 has any hoses that had to be disconnected to work on the carburetor you might check them for kinks or improper placement.

 

Thanks for the info, I appreciate it!
I didn’t use the bleeder screw, I jacked up the front end and looked for air bubbles coming from the radiator. There weren’t any.
I also tested the thermostat and it’s opening . I even removed the thermostat and ran it - the temp light still came on. I’ll try a thermometer.
I suppose the temp sensor could be bad.
The spark plug had a bit of carbon on it, but wasn’t too bad. Definitely not lean.
I didn’t remove the carb, just the diaphragm and slide.
None of the coolant hoses are linked.
I’ll look into a block tester.
Thanks again!

 

"And how full should the radiator be? "

Hmmmmmm, why, clear full of course and the recovery bottle should be half full. You didn't mention the all important fan. Have you seen or heard it kick on ? The new thermister .........OEM or aftermarket ?

 

Roger that.

Someone put a switch on the dash for the fan, so it’s manual. Not sure why.

Temp sensor is aftermarket.

 

"I replaced the temp sensor about a year ago because I noticed the one that was in it had the wires cut right at the sensor "

"Someone put a switch on the dash for the fan, so it’s manual. Not sure why.

Temp sensor is aftermarket."

Errrrrrrrrrrr, I'd say probably either the Power Module has half azzed taken a dump and they hot wired the fan to a switch to be able to have fan function
OR

The previous owner after somehow whacking the wires on the thermister and the result being full time fan wired in the switch to be able to intermittently cut power to the fan.

So , in a nutshell, it looks like your new aftermarket thermister is NOT controlling fan function at all....

 

This machine was missing a lot of parts when I got it. Mainly air intake. Kind of strange. When things are disconnected or not working, or “rigged” it doesn’t surprise me. I’ve had if for almost 4 years and it just started overheating. Worked fine until now.

 

I did a block test this morning and the fluid turned slightly green. I assume there’s a minor leak somewhere, probably the head gasket. I also did a compression test and it’s at 50 psi. Sounds like at one point it overheated enough to cause some damage. Looks like I’ll be doing a top end.
Several people around here use the machine so who knows what happened.

 

Before I went any further I would also do a cylinder leak down test, but that's just me. Such a test will tell you whether valves are leaking and if/how bad the rings are, provided the valves aren't leaking.

You can test the valves for leakage on the bench after the head is off by turning the had upside down and filling the combustion chamber with mineral spirits to see if it leaks into the intake and exhaust ports. Also check valve guides while the head is off and check the head for warpage. Better to find it now than after a top end with new head gasket and have it blow again.

 

Sounds like a top end job to me. Fix the air filter and don't let smo drive it....

 

Thanks, I’ll do that next. I appreciate it.