My RH500 has a racing pedigree. Sadly, I don't have the original engine or many of the parts from it which were spread to the four winds when we were young kids and failed completely to recognise what we had in front of us.
But, I do recall that the inlet tract was extended by means of longer carb studs and a series of tufnol spacers between carb and head. What the total thickness of the spacers was I don't know and my guess after almost half a century would be unreliable, to say the least, but they were about 1/4" or possibly 5/16" thick. Variation of the number of spacers would give you some control of the variables described by David Anderson.
If anyone happens to own EGR 351 here in the UK, the long studs and spacers may still be fitted. But don't get too excited - they are (as far as I remember) the only bits from the racing engine on that bike.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.