Since I haven't seen it written about yet in this thread there are a few points worth mentioning. Copper starts out very soft but suffers "work hardening" because even small amounts of bending causes the formation of dislocations and grain boundaries. Although the original softness can be easily recovered by heating with a torch to anneal it, until that is done the copper is susceptible to cracking. If, for example, you ran a copper fuel line the entire length from the tank to the carburetor, the relative motion between the two ends would cause work hardening and eventual failure of the line (very likely what caused the failure on BigEd's Sunbeam).
Next is ethanol in fuel. For some years I've been conducting an extended test of brass jets and Monobloc body material in gasoline, E10 gasoline, and pure ethanol ("E100"). Briefly, every once in a while I take the pieces out and reweigh them on my microbalance. Thus far any weight loss due to dissolving of these two metals in any of these fluids has been undetectable. So, I'm not so worried about the metals (although, that said, pure copper is known to have long-term issues with ethanol). However, the rubber fuel line is a real problem.
When left immersed in E10 fuel for several years the rubber of "common," black fuel line material turns to something of the consistency and stickiness of silicone sealant freshly squeezed from the tube. Since this happens inside your fuel line you very well may never notice, although if/when some of that goop detaches itself and is carried downstream the tiny jets in your carburetor will. Although ethanol-rated fuel line is expensive (~$8/ft.), it is well worth the money to replace all the segments of rubber fuel line currently on your bike with it. There are two types of such fuel line, expensive stuff for our bikes, and very expensive stuff intended to be submersed in the fuel tank itself (i.e. so both the inside and outside of the line are exposed to the E10). Both types will work fine, but there's no need to pay for the submersible kind for use on a Vincent.
Very much related to this is I recently bought a BB Gold Star that had the clear fuel line with a diamond pattern of thread for support. The bike had been restored in 2010 and not run for the past year (due to death of the previous owner), so at most the line had been filled with gasoline for 3 years (but, certainly much less than that since the owner would have drained it, as evidenced by the fact the pilot jet wasn't clogged). When I pulled that line off to replace with E10-resistant line it was hard almost to the point of brittleness. The point being, if you have any "inappropriate" fuel line on your Vincent now there won't be any visual sign there is a problem. You won't know there's a problem until the inside turns to goo and clogs a jet, or the entire material becomes brittle enough and a crack develops at the most inopportune time for you.
