Engine stumbles. . .

[Jim Davis <jamdav@woodward.com>]

Listers, 
This winter I installed a Petronix Electronic ignition, high voltage coil
and new plugs.  I also adjusted the valves, balanced the carbs, set the idle
to 800 rpm and adjusted the timing to 4 deg. ATDC.  The car now seems to
idle fine, but when I rev the engine she stumbles and runs rough.
Did I miss something?  Is the new ignition system the problem?
Thanks for any ideas,
Jim Davis
1974 TR6