I'm in the process of getting my 1982 Honda Magna back in running condition. I've cleaned out the carb and put in a new battery. It now runs, but dumps a TON of coolant out the tailpipe. Almost a constant stream. It blows a bit of white smoke, has trouble idling, and overheats rather quickly.
What steps should I take to diagnose and fix this issue?
You can spend time identifying which cylinder is bad but in the end you need to replace both head gaskets
If both head gaskets aren't failing the other one will fail soon. You may not know it yet but you don't want to replace just the single head gasket. You want to do both.
If I were in your position I'd just order two new head gaskets and get on with it but I've worked on a ton of these things so I have a different outlook on getting from point A to point B.
Identifying the bad head gasket
You could do a leak down test to identify the bad cylinder(s).
You could perform a compression test to see what cylinders have poor compression and which do not.
You could turn the engine over a few times with the starter and pull out your spark plugs to see which ones are wet and which are dry.
Here are a couple of links that may be helpful to you.
Conclusion
Clearly you have a bad head gasket if you have coolant coming out of your exhaust.
IMO consider replacing both head gaskets.
Get a factory manual for your bike.
Spec
184 +- 28 PSI with a maximum difference between any two cylinders of 57 PSI. This from the Honda factory shop manual.
