I have a 250s, and noticed that the vacuum diaphram on the distributor is adjustable if you remove the hex nut on top of the adjustment screw. I tested and proved that the vacuum diaphram is air tight, but can't find any reference of how it should be set. I think that it needs to be adjusted relative to the engines vacuum at idle, but again, I have not seen any documentation as to the purpose/ adjustment. I am hoping someone can shed some light on this topic for me. Thank you in advance.