Missing at high RPM only

["Dave" <dgfox@uswest.net>]

Hi List,
 I need some help with diagnosing this one please.
1275cc w/ SU's, fairly fresh rebuild, plenty good oil pressure ......
It idles smoothly and has good pull when accelerating through the gears
 but when I try maintaining a constant 4200 (or so) RMP it starts breaking
up
and missing like mad. I can bring the rpm's down and
it's fine again, back up and it does it again.
 Should I be looking into fuel supply/carb. stuff
or dizzy problems ? I have an Ignitor installed.

any and all help GREATLY appreciated

thanks,
Dave
'67 Midget