Subject: Time:3:14 PM
OFFICE MEMO Vertical (Downdraft) SU's Date:11/17/93
> My tuppence: I'm looking at a cross-section picture of a downdraft SU
> There is an oil cap but no damper in the piston shaft.
> Maybe they found they needed the damper but it wouldn't hold oil in the
> horizontal position.
>Nope- this is the SU design of the 30's. My '34 MG has aluminium SU's >with
*brass* pistons, and a cap marked something like "Few drops oil >weekly". The
oil damper and return spring were introduced in the late >30's or 40's.
This is facinating, Roger. I've never seen a pre-war MG with vertical SU's.
I thought they were all horizontal. A friend of mine needs one for his '36 SS
Jaguar. All he has is the picture in the manual. If anybody knows of one for
sale, E-mail me and I will pass it on to him.
The picture I have for the SS Jag shows the usual bell-shaped suction chamber
(but horizontal), and there is a part screwed into the end labeled "Oil Cap",
which appears to have a small through hole where you would expect the oil
damper rod to be, but there is no oil damper. I wonder if this hole is little
because it acts as a damper, allowing air to move in and out when the piston
moves, but preventing a quick movement of the piston. There is the usual coil
spring on the suction disc.
Then there is also a little oil fill cup with a spring-loaded toilet seat type
lid, screwed into the base of the neck of the bell, which is drilled and tapped
at a 45 deg angle into the bore where the piston rod slides. The instructions
say this is where you are supposed to put in the few drops of sewing machine
oil, and under no circumstances to use a heavy oil like engine oil.
There is also a thing labeled "Tickler Pin" on the float chamber lid, with
which you can push down the float to flood the carb. Quaint, huh?
Anyway, my theory on the demise of the downdraft SU is that it wouldn't work
with the oil damper, so it became outdated, passed up by advancing SU
technology. Now there's a phrase we can all smile at in our 1993 hindsight.
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