Well, yes... Previous comments about overall engine set-up apply. I have three Jawa engines with varying combinations of set-up and significantly different characteristics. I know that for about £2k I can build an engine that would be significantly faster than any if them, and have a substantially higher wear rate on various components. Or, I can have a reliable engine which is consistently fast enough for most purposes, for a great deal less. We usually take two bikes and ride whichever one seems better on the day - air temperature and humidity have a noticeable effect, for one thing. We changed the jetting on one bike mid-season with good resukts; but we've run the same settings before to no effect in the past, some minor changes as the engine wears, I suppose. One of the three heads is definitely fastest, for no readily apparent reason - I attribute this to some subtle dufference in the porting, standard of finish in the ports, or both.
Changing the valve profile on the JAP has immediate and obvious effects. It's all a matter of where I want to be on the speed/reliability/durability/ride ability scale.
I'm sure the WR and especially, the KR are just the same, for the same reasons
Shoot, a man could have a good weekend in Dallas with all that stuff...