I'd be more inclinded to think it's a problem with the top cylinder itself not firing properlyIF there is a problem at all, a massive amount of smoke is common if it's not run for a while. I was that concerned mine was smoking like crazy on the top (untouched) cylinder and not at all on the front when it was first fired up after it's latest drama we took the carbs off to check the front carb had an oil pipe on at all.
And some oil does come out unburnt anyway but not really an amount you can see (unless the ground it wet and you can see the colours of the oil reflecting in the rain puddles).

But if you're concerned it's the pump why not switch the oil pipes round so the problem should move to the front cylinder? That'll either eliminate or confirm the problem is the pump. I've never heard of one going wrong though.

I don't see how the carbs could allow the pump to pass too much oil through.
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