Badly need help figuring out what is wrong. All this started after a
valve adjustment which was done improperly, and fixed later by me.

Here is what I know till date. The car runs okay for the first 15
minutes or so and then the performance and mileage drop drastically.
It seems to limp around even if I floor the pedal.

No CEL codes

The ignition timing advance is 10 degrees during idle ( must be
between 11 and 13 according to the honda diagnostics unit).

The spark plugs are blackened (sooty, so rich mixture)

The o2 sensor is shifting between .1 and .89 volts, but quickly enough
or not - I dont know..

The car revs smoothly, too smoothly than it ever did earlier in fact
(maybe sign of retarded timing)

I think the exhaust is heating up a lot more than usual (again sign of
retarded timing?)

Drives okay for around first 15 minutes, then acceleration and
response declines drastically as I drive longer, mileage also suffers.

What could be wrong??

thanks,

es