I've found out that these are time adjustable, there is a lot of content online saying otherwise. If you loosen the 7mm nut with a long spanner, the stud is sprung towards the valve body in a slot, you can just about get it without too much hassle. I've set mine away from the valve, as this is reputed to extend the time open. Getting the valve off to bench test means taking the inlet manifold off as one bolt is totally inaccessible, so I'm going to have to see what happens.

Yes, further away from the body opens it further at stone cold, thus increasing the time it takes to close fully. RPM at this setting is >2000 when you fire it up, and most of the usual splutter goes away. Worth knowing, and thanks to Tim at Rottele Racing in DE for raising this to my attention.