How likely is a burn down due to heat if the the air / fuel ratios are correct? Pretty much nil? Seems 2 me that EGT gauges are insurance against burning down while the afr is more of a tuning tool. I'm almost thinking that having both would be smart.
Absolutely. I have a race-only vehicle, and work things a bit differently. For starters, I use only the precise same fuel always, from a metal can, pre-mixed the morning of an event, drained
(edit: and discarded) between events. Out of fuel? Push/tow vehicle to the trailer. If you run pump gas, even pump race gas, you have to jet richer, for safety, because each fillup will have a different mix in it.
EGT's really don't help you tune. At best, they do two things: First, they help you see if one cylinder is burning hotter than the other. Of course, swap the two probes to confirm this. Some folks need to jet their carbs differently for each cylinder. Second, it can be an indicator that something is going wrong. Record EGT's after each run, noting the values (paper, of course), and if the numbers start to change, figure out why. My EGT's are outrageous, but I know why.
An O2 sensor is, technically, an "oxygen ratio sensor." What percent of the gas that it is exposed to is oxygen. You're not measuring fuel, or air, so it doesn't really tell you A/F ratio, or Lamda. It outputs a value in millivolts. Most units on the market use math to approximate one of these more common values. That's fine, though not very precise. For example, I might be shooting for (a very wrong number) a value of 234 millivolts. What would 254 show as on an A/F readout? The same, I would guess.
Also, I have a separate switch to turn on the O2 sensor's heater, and warm it up for at least 60 seconds before I start the engine. This will dramatically increase the lifespan of the sensor. It would of course be a pain in the rear for a casual rider.
I tune for millivolts. The proper way to set this value is to put the engine on a dyno, then jet for max HP/widest torque. See what the O2 sensor is saying, and that is the value to tune for. You can approximate this with about 400 pounds of math and a dash of understanding, though. No, my number is classified, sorry.
And of course you need to know the weather conditions during this test. Temp, humidity, and barometric pressure. I use a small hand-held weather station for this, that makes barometric pressure predictions. You can enter these numbers (for the day you are about to drive) into a spreadsheet, and jet accordingly. This is complicated for snowmobiles that change elevations. . .
My O2 sensor spends most of its life on a shelf in the shop. I used it to find the perfect tune, and now jet according to my spreadsheet. I install it occasionally just to check state of tune.
Lots of spreadsheets available for free download. Most have a silly element, and ask for both altitude and barometric pressure. I don't understand why. Barometric pressure is all that you need to know. Though I suppose that it will change with altitude, and you could enter the altitudes you expect to see that day, and jet accordingly. . . Hmmm. I guess that these spreadsheets are useful for snowmobilers that change altitude during the day's ride.
And this is only TRULY useful if you have data acquisition, to tie throttle position to O2 level. To jet your carbs precisely. When I first bought my vehicle, anyone but an expert driver would likely burn the engine down. Poorly jetted in the mid-range. I suppose that you could run at 1/8, then 1/4, then 3/4, then full throttle for a period. And glance at the O2 readout, then re-jet and test again. Get close and usually only the main jet needs changing for the day. But again, I don't change altitudes much. My engine can idle for hours, and still have a crisp instant run up to full throttle - it's happy at any throttle position, because I have jetted my carbs correctly.
As you may have realized, I run near burn-down jetting. But then, 1 HP is important to me. Out for fun? Jet rich, be safe, and have fun!
Enjoy!