I have just installed one of Hitchcocks oversized, high quality 43 mm exhaust valves in my 500 classic. This is made of higher quality material and has a better gas flowing profile (flatter backed, not so tulip shaped - it should generally block up the exhaust port less). It also features a waisted lower stem.
Along the way, I did a proper blend and smooth of the head casting where it meets the hardened valve seats. There was a massive step in the exhaust port where these areas met, and a smaller but significant one on the inlet side. I just had to get rid of these ghastly flow blockages, given that the port area close in by the valve seat is said to be the bit that matters most when porting a head, I smoothed away other sundry lumps and bumps in the exhaust port but left the inlet side well alone for fear of doing more harm than good. Though I did blend the carb. stub manifold and heat proof gasket into the inlet port, since there were pronounced steps between these three components.
The valve itself is larger than the stock one, which Hitchocks reckon to be undersized and therefore liable to poor sealing. The new valve is noticeably bigger. In fact, I was slightly nervous about piston clearance so I relieved and profiled the piston striking edge of it a tad, until it gave the same drop to contact with the piston distance as the stock valve. Probably unnecessary but once I get an idea in my head...
I gave the valve a light lapping, The contact ring is of good width and noticeably larger diameter than the one on the stock valve.
What was the outcome of all this activity? Well, I've got good compression. I can stand on the kickstart now. There was actually more of an exhaust "blat" when I kicked the engine over, ignition off - which I took to be a promising sign. On the road, the exhaust note is a good bit louder and it has a snarl to it that wasn't there before. It also breathes noticeably better at high rpm - such that there is now some point to holding onto third gear. Before it was better to get into top early and use torque rather than revs to get a move on.
I am quite pleased with the outcome, given the 50 GPB spent on parts and gaskets. This was probably a better place to start than bunging a bigger, flat slide carb on such a rough hewn cylinder head as mine.