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Just throwing out ideas... You may have a bad float valve. I am not certain of the geometry of the 580CK's intake manifold, but I know on other gravity-fed carbs, a bad float valve will let the carb overflow into the cylinders, which will then leakdown past the rings (this is actually less of a problem with updraft carbs). It doesn't take a lot of gas to contaminate the oil. If it coughs out black smoke when you first start it, that's a sign of a leaking float. Even if it doesn't dump gas, the high gas level in the float bowl will make it run rich. Another possibility is if you run it a long time with the choke on. It shouldn't need it more than 10-15 seconds after it fires up. If it needs warmup, warm it up without the choke. Also, if it has to crank a long time on choke before it fires, excess gas dumped into the cylinders will wash down the cylinder walls (this also washes off oil and accelerates ring wear). I had a 4 cylinder dumping a lot of gas into the exhaust when it had a bad condenser (complete with backfiring bad enough to split the welds on a muffler!). I like electronic ignitions, and I'd put one on just for GPs, but I seriously doubt that's your problem. Properly tuned, a points ignition will burn the gas just as completely as an electronic. There are two advantages to an electronic: They have a longer dwell and thus better spark above about 4,000 RPM (probably not an issue on a tractor), and they just about never have to be adjusted. Points have the advantage of being easier to adjust & replace in the field, but since I've never had an electronic ignition go bad (yes, I'm sure other people have, but I haven't), that's not been a big feature for me. I also don't buy the "problem with updrafts" story. Updraft, downdraft or side-draft, carbs really don't care that much, as long as they're sealed and adjusted right. Never hurts to look. I had a friend who had a bad backfire once, and had partly melted his venturi. It still ran, but not great. This was on a Dodge truck. On a low-compression low-RPM engine, he might never have noticed it. Try replacing your points and condenser (filing points just removes the tungsten and accelerates their decay), get the gap right on the points, clean, check or replace your spark plugs AND wires AND rotor AND cap, and make sure the timing is spot on, with a strobe (static timing is only good enough to get it fired up the first time). If the CK has an automatic advance, make sure it's advancing at the correct RPM (something else you can't do without a timing light). Finally, it sounds like dumb advice, and I would have agreed, except that I've been caught by it more than once -- be sure it's firing on all cylinders. One mis-firing cylinder can dump a lot of gas. I had an engine that was all tuned up except that one cylinder way in the back had backed out a spark plug and didn't have compression. So that's just some things that occur to me off the top of my head.
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