Backfires are usually indicative of a timing issue: if it's timed too far in advance it will backfire through the intake, of the timing is retarded too far it will backfire through the exhaust.
It doesn't usually just come on suddenly, and will exhibit power loss and possible overheating as timing generally tends to go to the retard side over time through component wear. Unless the distributor got loose and was moving a bit on it's own.
If you suspect the timing may be off, determine if it's fast or slow by the backfire and adjust the distributor a few degrees accordingly (note the original position) and try it.
It's also possible that it could have a stuck float, especially if it smells exceptionally gassy after turning it over for a while. Of course, excess fuel will cause the backfires, but if it's getting way too much it might not be able to fire enough to run. Tap on the side of the float bowl with a ratchet handle a few times, that will often free up a sticky float/needle valve. Once it gets running the vibration should keep it from sticking, at least until it sets for a while again.
There is a good 741-742-743 book in the manual thread that has a section on Mitsubishi engines and a good troubleshooting guide.
