That XL600R is one of my favorite motorcycles, big bore 2 carbs what is not to love. The '83 is great looking in red, I almost had a nice '87, the red white and blue, and the guy sold it before I got there. I will own that bike one day. But back to your question....When first started is usually valves. That is the oil in the head that leaks down after you turn it off. What concerns me is that it is white smoke, that is usually water, blue smoke is oil. If there is no chance that rain water is going in your exhaust, I would start by checking the compression, should be 175, then check the valve clearances. I will bet it is valve seals. If you want to check if the valves are leaking down, just remove the head from the cylinder. Fill the area around the valves with WD-40 and put it on a clean piece of paper towel, you will see the WD on the paper if they are not sealing. Keep in mind that this information comes to you from a half assed backyard mechanic. There is a very good thread with some great mechanics on ADV rider. Go to Thumpers, then look for XL600. Also you call it an XR twice and an XL once, whichever it is there is a lot of good info on that site. Post more info, I would like to see the bike.