why is my antifreeze bubbling up and coming out my radiator overflow?

could possibly be a blown head gasket, where the piston compresses, the compressed air blows by blown gasket into the cooling system, therefore pressurizing the cooling system... just first thought.

====

edit: yes, could be a bad radiator cap or weak spring in the radiator cap ... and FYI, a CR80 doesn't have a thermostat, so mark that idea off the list... and a blown head gasket doesn't mean there would be water in the oil (water in oil most likely happens when water pump oil seal starts to leak not head gasket), but look for any oil in the radiator with the bike cool to see if some of the pre-mix oil might have blown by the head gasket into the cooling system... but would be ever so slightly noticeable if even traces are there.

On an 01' CR80 there are only 2 possible reasons. Reason 1:Bad or improperly installed head gasket or a warped head. Reason 2:Overheating due to lean jetting, mud in the radiator fins, water pump not functioning, bad bearings somewhere in the transmission or in the crank bearings. The only reason you would check to see if you had coolant in the transmission oil is in the case that the ceramic seal at the water pump was bad but that shouldn't cause your coolant to run over. The best thing you could do is grab a 14mm wrench and pop the head off, and check to see if there is any signs of coolant in the cylinder. If the piston is very clean and moist then it is your head gasket, if all is dry and had a slight carbon build up on the top of the piston then your problem becomes an over heating issue.

Flush the radiator, Disconnect the hoses from the motor and use compress air to make sure there is no debris in the motor, just go ahead and replace the head gasket and replace the radiator cap and replace the seals at the water inlet at the motor and make sure you use "recommended coolant" back in your bike when you put it back together.....This is not really a major problem, it's a easy fix so don't worry.

I don't know anything about dirt bikes but I did have this happen once.

it was on an old pontiac fiero, turned out when I shut the car off or when it got hot it would cycle lots of coolant through the reserve tank, I don't know why but every time it would boil over and spill coolant everywhere and I would refill it thinking it's low, turned out it was just way too full the whole time, dumbass thing I know, but it may be the way the engine takes up the coolant and the lack of pressure after the engine is off, just a thought.

possibly you have the fluid level too high and upon the engine warming up, the liquid expands causing it to overflow but that would be a relatively small amount.

bad radiator cap would be easiest & cheapest to check ... but blown head gasket is also very likely. I had that happen on one of my son's KX80s several years back, with similar symptoms as you described. alot of coolant forced out.

Make sure the thermostat is hooked up correctly and working. This happened to me and that was the cause. Make sure the rubber grommit around the radiator cap is good. If the coolant isn't getting into the oil, it doesn't really have to be a blown head gasket.

Probably a head gasket

could be one of two things that come to mind either head gasket gone (warpped head) or thermostat stuck closed and coolant is boiling at point of comming out of rad, hope this helps

tha kanutin valve is stuck,

otherwise bad head gasket and or water pump.

That's normally caused by a thermostat that is stuck. replace it and the problem should go away.