Monobloc tickler not flowing fuel

This seems an odd issue to me, I've found nothing similar anywhere as yet.

Having an issue with the Monobloc 389 mounted on my Dad's JAPton special.

When you tickle a Monobloc, the fuel is supposed to "overflow" through the tickler, yes?

Under what circumstances would it "not" overflow through the tickler (which is simply a tube open at both ends) ?

I've removed the tickler and it's completely free of obstructions. (and the entire carb has recently been ultra-sonically cleaned.)

When filling the float bowl with the tickler - the fuel eventually seems to fill the main body of the carb and overflow through whichever passages exit at the larger of the two holes at the carb mouth/entrance.  It then flows away somewhere further "back" down the underside of the manifold flange.

I initially thought it might be the paper gasket that sits under the jet block, so I removed that yesterday and reassembled the carb with a small amount of blue-hylomar on both sides of that paper gasket to create a better seal, and it's made no difference.

The carb was cleaned and rebuilt recently with Burlen's repair kit, including the new viton-tipped float needle and "stay up" float.  As far as I can make out, everything is "working", apart from the tickler, which is odd and I'm concerned that whatever the issue is, it might affect the normal running: ie making it harder to "tune" the carb for good running.

For reference the carb is sitting at about a 10 degree "up" angle making it a slight "downdraught" entrance to the engine in practise.