The balance tube (or box) or crossover pipe (or chamber or plenum) does exactly that - serve to balance the exhaust pulses from the two cylinders. It effectively doubles the muffler volume for each individual exhaust pulse. Supposedly, it also quiets the machine down, too.
But, no, not necessary at all on any bike. And usually, no rejetting would be required by blanking it off.
My downpipes have already been blanked off, badly, with weld and I don't have a box. Downpipes are however u/s so the question is more what do I need to replace them with and did I therefore need a box or not.
If the down pipe still has the "T" branching out, any piece of chrome tubing the right size would do the trick, plus it would look like the cross over pipes you see on many old Britt bikes. Thought about doing this myself.
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