The problem you are experiencing could be many possibilities. First thing after checking the electrical side of the engine, would be the carbies. SU type carbies are fiddly little animals. Being a constant velocity type carb, they love operating flat out. From my experience, they are not so crash hot at low rpm, not as responsive as a pair of webers.
1. I would try and identify what profile needles your carbs have. The taper of the needle influences the feul delivery.
The second link i quoted to a SU carb site has a needle calculator from memory.
2. Check that the dampening plunger is not restricted in its movement. Is there to much or not enough oil in the dampeners.
3. Are both carbs synchronised?
4. Is there air leaking through the throttle butterfly shaft. this is a common wear prob, with this type of carb.
5. Float levels?
Try these articles, they may help
http://www.sonic.net/~kyle/su.htmlhttp://www.jetlink.net/~okayfine/su/sutech.htmlShort of all that maybe the cam you are using is too big. The cam may be better suited to higher compression and high octane fuel.
Good luck. and im sure you will let us know the outcome.