Hi Twin's, Leo.
Boy, have you guy's got patience, even after all these years. R.G. too. Richard, it can be hard to get your mind around what these guy's are saying to you, but it isn';t all that difficult. Think simple. Carb's are a very basic, simple way to provide fuel to the engine. On the BS 34 carb's, as CDN stated, are extreamly lean running. The mixture screw's are on the top of the carb body, under the brass cap's. A small hole is in them most of the time. Just really a tiny pilot if you will. Use an 1/8" drill bit to open that tiny hole just enough to get a sheet metal screw into the brass. Don't go in too far with the sheet metal screw. Use either a real good set of diagonal cutter plier's, to pull the plug out, or needle nose Vice grip's. Pull straight up and out. It help's to mount the carb LIGHTLY in a vice so it is steady. The plug's can be quite stuck into the carb body, but a steady, even force will get them out. Using the guide that Leo gave you a link to, get all the passage'sreally clean. Some say to NOT use wire because it will damage the passage. Yea, that can happen, but just use descretion. After all it is aluminum. On the 34's, because they are lean to start with, as RG stated, the pilot circut is very small and the #1 area of problem's. It can look clean, even have carb cleaner go through it, and still be dirty. Carb cleaner is under pressure, where as the fuel mix is under vacuum. Big difference. Here is where there is no such thing as to clean.
Ok, now for part's. Mike's has a very good selection of the thing's you need to rebuild the carb, from floats to gasket's to jet's, ect. I think Merc has them too. 650 Central.
On another note, once you do these, you will be giving advice on what worked for you.Try remembering that they are simple, allbeit small, device's. It can be hard to not overthink about the problem's. Could it be this, or could it be that type of thing. Get one area done up correctly, then move to the next, by doing what your doing, asking question's.
One more thing, sorry. Make sure your filter's are new. If you put on a filter that used the wrong way, guess what, you just put all that crud into your nice, freshly cleaned carb';s. Same goes for the line's.