There has been quite a lot of discussion regarding SU carburetors lately on
the list. At least one submission mentioned HS6 and HS8 carbs and others
have talked about the HD8 model. As the HS and HD series carbs are
different, I thought I'd provide a little explanation. BTW, Triumph
sportscars (TR2, 3, and 4) used H4, H6 and HS6 carburetors at various times
during their manufacture. The earliest of those carbs are the "H" or
"Horizontal" carbs with the H4 being a 1.5" size and the H6 the 1.75" size.
The "HD" carbs that were seen on other British cars were an improvement in
that the jet had a little diaphragm attached to it to seal the fuel entrance
to the jet while allowing the jet to move up and down for adjustment. The H
carbs used "glands" to seal the jet that worked sort of like the packing on
an outside household faucet, and they could develop leaks at this point.
Also, the HD carbs had a separate passage way for the idle mixture that was
controlled by a big needle valve. At idle, the throttle is totally closed
on a HD carb and idle speed is adjusted at the needle valve, which makes
balancing multiple HD carbs a snap. The popular HS carbs also were
succesors to the H carbs. They also solve the jet leakage problem, but with
a simple setup where the bottom of the jet has a flexible hose attached to
it that connects to the float chamber. Setting the idle speed on a HS carb
is done with the normal setscrew on the throttle bell crank, and the carb
mixture is adjusted via a nut at the jet on the bottom of the carb. This is
similar to the H carb, but not the HD carb that has a lever and setscrew on
the carb side to set the mixture. In short, the HD and HS carbs both solve
the leakage problems of the earlier H carbs, but the HD carbs have some
additional refinements. I think that the same needle range is available for
both series of carbs, so availability of needles shouldn't be an issue.
However, jets and other replacement parts might be easier to find for the HS
carbs, as they were so popular. If you are considering switching to SUs,
read the various sections on SUs in the Haynes carburetor workshop manual.
Rex Townsend
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