I assume by 'blowing' you mean out the exhaust, or did you just see smoke in your mirrors and you're not sure exactly where it came from?
There are coolant hoses under the carbs, which can fail and spill coolant onto the hot engine, and you'll see smoke in your mirrors. Depending on how big a leak you have that might explain your symptom. Another possibility of sudden smoke is coolant in your exhaust, which will have a sweet smell if you stand behind the bike while its smoking. That would be relatively bad news as it would likely be caused by a failed head gasket if its coming out the exhaust. A failed head gasket could also allow oil into the cylinder as well, so excess smoke in the exhaust could be from oil.
The first thing I'd do is check the coolant level at the radiator (not at the overflow tank). If its low, then I'd fill it back up and run the bike in the garage and look for any signs of coolant leaking onto the engine. If you see coolant dripping off the back of the engine, then you have the typical coolant leak under the carbs, and you can get a lot of advice for doing that job here. If you see smoke coming from the exhaust, and it smells sweet, then your coolant loss could be from a bad head gasket, but those failures are far less common.
So first step is to determine if the smoke is coming from the exhaust or not, and if so, does it smell more like coolant or oil? If not, then see if you have signs of a coolant leak anywhere.