Hi Franko,
My Cag style used to do the very same thing and it was a really simple fix.
I'll bet London to a brick that the bike is running out of fuel top end as the fuel flow through the tap on these things is very restricted to say the least. The piddling amount of fuel comming through the tap doesn't replenish the fuel level in the float bowl fast enough at peek revs and you end up with a surging affect at peek revs on the straight.
After bending the float arms to the max to raise the float level I still had the same trouble. Darren from Pocket Racing Australia put me on the right track when he suggested that I remove the tap assembly from the carb to drill the inlet and outlet of the fuel tap out to 3mm as well as the two holes behind the tap. I then also used the 3mm drill as a router the make the C shaped groove on the inside face of the plastic tap larger.
The bike screams now with no probs, however with the future addition of a full circle crank and higher rpm I may need to remove the inner part of the 4 holed rubber disc behind the tap to increase the flow even more and run a remote tap elsewhere.
Give the 3mm drill a go and let us know how it turns out. It worked a charm for me
