Newbie w/ 79 XSF

[crazy steve]

How is the reliability of these things? If I take care of it now and keep it going, run seafoam occasionally and change the filters before each season, etc, how many miles, on average, should I be able to expect? (assuming the 24k is a legit reading - which btw, is there a way to check outside of tearing the engine down?)
I've heard people say they can get 100 - 150k on these things if cared for, is that true?

The reliability of these bikes is excellent if cared for; 100K+ is no problem with proper maintainance. They do have their issues; you've already noticed how 'clunky' the transmission shifts and that's a trait of these. Always shift the bike very 'positively' (being gentle is bad; shift firmly), don't shift without the clutch, and you can probably avoid the dreaded 'dremel fix' if you're not having any problems. Broken ignition pick-up wires at the crankshaft is another known problem. Clean/maintain all electrical connections (another trouble point if neglected), and keep the valves adjusted. You should at least check your valves to see how they are. The factory recommendation is to check every 6K miles (which is overkill IMO), so you may be overdue. They get tighter as they wear, and if they get too tight you can end up with burned valves. Somewhat noisy valvetrain is usually a good thing...

As far as estimating mileage, even a engine teardown may not tell anything. Run a compression test, that will give you a pretty good indication of motor health. High readings, it may need to be 'decarbonized', low, check the valve adjustment. Oh, yeah; as far as fuel requirements, these were designed to run on regular gas. Run it on the lowest grade that prevents knocking (ignoring the occasional knock you'll get when 'transitioning' throttle openings) as higher grades will promote carbon build-up. Making sure the carbs are properly adjusted will help with this too.