I've been wrestling with the firing order, paired cylinder thing (my head hurts).
That explains why the Mini can use a header: the cyinder pairing has 360 separation.
What pairing of exhaust pipes have you tried?
I suspect that the zoomies are either:
1. too big, and you're getting reversion because of low vel in the primary, and it's backing up into the port
2. the length is creating a wave that arrives at a bad moment and stifles something else
As Smith suggests: the easiest, most trouble-free exhaust system is 8" stubs. No wave can arrive late enough to affect anything, no backpressure, no length at all (they can differ if you have chassis clearance probs). Probs: sounds like gunfire, flames under the hood. Even this can be fixed (as you'll see on really old racers), the stubs empty into a very large can - as long as the cam area immediately inside the stub entrance is 10X the stub area there's no collector effect. If the pipes are 1.75" OD (1.652" ID based on 18 ga.) the area is 2.14", so if the can is at least 5" in all directions from the stub it's the same as exiting in free air (except for the noise, heat, flames). You only need a single exhaust, any size will do, and pointed per rules (down and back).
