Lister D Fuel Problem

[Mitch1966]

FiI finally purchased a Lister D and have given it a full service with new oil, a new spark plug, and the usual checks. I can get it running occasionally, but it runs very rich and sometimes, when it won't start, it blows fuel back out through the top of the carburettor.
I've also noticed what appears to be a missing part where the carburettor joins the engine. I can't find it shown in the engine diagram in the manual. I've attached a photo of the area. Fuel seems to be blowing out of this hole as well, so I'm wondering if it's supposed to have a fuel/air mixture screw or some other fitting installed.
I'm completely new to these engines, so I'd really appreciate any advice or guidance. Thanks in advance!