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I got an 18mm manifold but its the wrong one, it fits the barrel with a screw on front pipe and mounting studs are closer together, don`t want to change the barrel as mine is fine, still not running properly so i think i will just buy a new standard carb...

I have a Gilera 50TS, an original UK registration.
It is fitted with the 18mm manifold from Bob Wright, but readers will be aware that the corresponding SHA18 carburettor is rare as hens teeth.
What I did was sleeve the 18mm manifold down with 19mm outer diameter, 16mm inner diameter steel tube. That enables you to fit a new SHA16 carburettor which are £45.00 new. The size 68 main jet seems perfect on mine. Yes, it’s not 18mm, but it’s a lot better than the original 12/14, and a cheap and easy modification.
Wanted to share this information in case it helps anyone else with a 50cc model.