Is your 'non-stock' exhaust / chambers brand new or have they been used before?

The reason I ask is because if the pipes have been used before, Excess oil can build up over time in the chambers, which if not drained or removed, will gradually emerge from the cans (or the manifold joint) when the engine is running.

Even on a perfectly set up 2 stroke, the oil coming in with the fuel (or injected separately) is never burnt 100% completely during each combustion cycle and this oil will move into the pipes.

It then has to come out somewhere.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30