I'm finishing up a long (3K miles) road trip and the van has been
running like a champ (for the first time ever, wahoo!).   However,
I've noticed a problem with pinging (aka knock aka detonation).
It's very particular -- it only happens at 1/2 throttle or higher at
3200 thru 3500 rpm, even on 91 or 92 octane gas.   Happens most of
the time -- only exception seems to be at temperatures of 30F and
lower at 3000F altitude or higher, which seems very consistent with
it being ping (heat & pressure related).

As far as I know, the timing of the engine is good at idle, but I
doubt that the advance has ever been tested.

This is a 85 westy automatic with a newly rebuilt 2.1L block
installed (about 5k miles post rebuild).  It has the stock 1.9FI
system, including a rebuilt computer & distributor & afm.

It seems weird to me that the RPM range is so specific -- it really
only does it at that particular RPM range.  Any ideas?   Bad
distributor advance?   Flat spot in the AFM?   1.9L FI system not
well matched to a 2.1L block?