My '84 Westy has been driving fine (for a car that won't hold tune
particularly well) lately.  Yesterday I went for a short drive and about
5 mins into it, it sounded like the engine was banging around when I
would give it juice.  When I would let off a bit it would subside but
the engine still sounded bad.  I limped home and started it today,
taking it down the street for a test drive.  When it got warm it started
making the noise again when I would try to give any sort of
accelleration.  On further listening I found that it was rapid
backfiring, not engine noise (my engine is pretty noisey anyway).

Pertinant details:
    Engine backfires rapidly under load when warm
    Engine fairly quiet (not backfiring) when cool.
    Engine has new heads (AMC)
    Engine will not hold tune particularly well
    Valves are clearanced per boston bob
    Engine likes to turn rich a while after being tuned (hence won't
hold tune)
    Engine has some minor exhaust leaks (but I doubt they could cause
anything this serious)
    Lifters are pretty noisey (even for a clearanced top end).

I suspect:
    Cat gone bad from running rich
    Temp II gone to hell and doing something wierd to the mix when hot.
    Collapsed lifter(s) as valves won't hold adjustment very well.

Please god don't tell me it's:
    Burned valves, cause I'm just NOT going to go back into that engine
again (Subaru here I come).

Need help soon as I've got a pretty good commute to work and my westy's
my only transportation (hello Hertz).