Sorry but I've only owned fuel injected diesels so I'm not familiar with Mercedes carburetors. I am very familiar with certain types of 2 barrel carbs for bikes and ski's and their needle is stationary so as the piston is raised more fuel is delivered. If the needle is loose I would think the fuel delivery might be less precise? Is the taper on the needle consistent from top to bottom? Worn needles will actually loose some surface thickness and will no longer have a straight line look. I've torn down lots of old dirt bikes to find needles like that and after you replace it the bike runs so much better..... but I digress, you should take nutz advice and find someone with MB carburetor experience.