If by zinc you mean ZDDP (zinc dialyldithiophosphate) the chemical is said to be an enemy of catalysts.
Is the S&S engine really designed to need high ZDDP levels and thus not be catalysed? I'd be rather dubious since the USA has been fitting catalysts longer than the UK has. If I had a five speeder, I'd be consulting with both S&S and an oil company chemist I know. He's spent quite a lot of time debunking myths.
