S'funny, 'cos I bought a valve for my CR500 and according to the website and other info they were supposed to run without rectifiers. The site shows flow numbers compared with other valves etc and there's seemed to flow more.

When I fitted them I had great fun trying to jet it. Eventually went from around a #165 main on the standard valve up to #210. I then epoxied up the inside of the valve and cut it into what I thought was a better flow path and again needed to rejet. This time settling on a #200. The new 'flowed' valve offered the same top end power but with better low end throttle response. Proof enough I think that the rectifier does it's job, which is to keep intake velocity high.

I've also found that if you reduce the actual cross sectional area of the valve (i.e. add epoxy to decrease the hole size) it again increases bottom end power, but at the expense of extreme top end. Probably due to becoming a restriction. Gave a very similar effect to fitting a smaller carb.