hello; sorry this is not strictly Austin 7 but could you help please. My interest is in how the SU down draught [ or draft ] works .. I am familiar with the side entry SU workings as Mini or Oxford and have tuned them etc. Understand the fuel level and vertical variable venturi / needle valve /jet/ vac piston.
However with the horizontal vac piston / needle / jet how does the fuel level relate to this ? also is there oil in the dashpot and if so, how does it not drain out being in the horizontal position ?.... This particularly relates to a Wolesley 3.5 L straight six [ sorry ! ] I can't find a site nor forum for this.
any help would be most appreciated ...... best regards Keith
I have a 1 1/4" DD SU fitted on one of my A7s. It was originally one of a pair fitted to a MG SA.
Float level is set to just below the jet in the same way as on a side draft SU. The main external difference to the side draft unit is that there is a flip top oiler on the top (which would be the side on a conventionally mounted carb) of the skinny section of the dashpot -i.e. the flip top oiler faces upwards. The DD SU has a very strong spring as the piston doesn't get any help from gravity. The other peculiarity is that there is a helix cut into the smooth surface inside of the dashpot where the piston goes up and down -at first glance this looks a bit like a machining error, but isn't because I have seen the same detail on other DD SU's. Tuning is the same as any other SU really.
Without a laborious hunt, I vaguely recall that it was established that the term dashpot referred only to the oil damper device when so fitted, and that downdraught models were not. Whether or not the rod part is then strictly still a dashpot I dunno! The oiler is apparently just to lubricate.
I guess the spiral is to prevent a hydraulic lock in the cap due too much oil .
(My father used to recount how in the Air Force Kitty Hawk guns were found to block due oil finding its way into the firing pin. A flat was ground to cure. He and another armourer fitter were modifying a plane with scaffolds around when they took a break. In the meantime another plane, fully prepared for the Pacific, against the rules, was bought inside and set up in the same place. The six half inch guns blasted a hole in the reinforced hangar roof. Apparently the pilots were impressed. Whenuapi airfield.)
Or, on second thoughts, and more likely, with the constant weight to suport, it was considered necessary to distribute an oil film!
It is unwise to post first thoughts, but then I am told they are all wrong anyway!