Totally agree with all the comments above, the bolt in the pre-heat tube is where someone has drilled to clean it out and unblock which they do over time - it stops your carb from freezing, but sticking that bolt in may restrict the flow if it has not been cut to the thickness of the wall probably best to make sure it's unblocked and get it welded up. Nothing to do with the oil leak mind just another of many jobs to check out.
Ooh listen to me it almost sounds like i know what I'm talking about.
Ooh listen to me it almost sounds like i know what I'm talking about.