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[Winkie]

Hi Alan. Good points, as always, but I can put a slight twist on this. My bike does have a blanking screw where the PAV has been removed, but underneath this it also has an auxiliary air screw as well. I have never really bothered about how this works and I simply leave it fully screwed in as recommended on our hosts technical notes. You have set me thinking, though. How the heck does it work when it is on the same spacer as the PAV? Presumably with the PAV removed it will no effect? My head hurts trying to work this out...
Sometimes agnorance is bliss....

[Alan R]

Hello WINKIE -------- now there's a good one to mull over !! My guess is it would work like this }--- At sea level the bikes' carb would be jetted "rich" such that it would be correct for , say 10,000 ft ?/ and the aux. air valve fully opened.......... this would give the required air/fuel ratio for the engine at sea level ... As you progressed up to different altitudes the screw would be progressively shut in stages until 10,000 feet is reached, by which time the valve will be fully shut.....As I say, it's only a guess but that would explain having the extra air screw....As the change in oxygen % is almost proportional to height above sea level, the air-screw's position can be generally calculated and given on a pocket chart, for instance...You'd need an altimeter as well to be accurate....The reverse happens if you take an engine down a deep mineshaft when the air becomes more compressed than at zero feet....

[Winkie]

Hi Alan. That probably sounds about right. Luckily we don't have to worry about high altitude around here - even the Wrekin is not that high and I don't think you can drive up it anyway! Mines might be a different matter -lots of old ones near me and probably even more near you = so if we fall down an old mineshaft when out on our Bullets the engines might misbehave!!! I seem to remember that a few years ago part of the Birmingham New Road collapsed into an old mineshaft- don't think anything fell down it though!