The first thing that's bad about Z-s is that they are apparently less
efficient than the equivalent SU -- Ted Schumacher claims that switching
from Z-S to SU is good for 3 to 4 HP.
The second is that the rubber diaphragms deteriorate and there's no
easy way to notice short of disassembly.
The third is that Z-S spares are really getting hard to come by, but
SU spares are ubiquitous.
I, like you, like the design of the Z-S, and will probably stay with them
on my TR-4A, but if I found a pair of HS6s, I might switch.
You can probably find a richer needle (yes, going to 87mm liners will
require that). The TR4A carbs should have a 2H needle; the next richer is
a 2G needle, which has the same base diameter but steps up from there.
The Z-S part number is B17005Z. I don't know where you'll find it, though.
Interestingly enough, TriumphTune says that a 2A needle is stock (this
is listed as stock for the TR4, but not TR4A), and that the slightly
rich needle is the 2E. The 2E is richer than either 2A or 2H, and 2H
seems slightly richer than 2E.
One trick you can try is to bend a straight pin to have approx 1/8"
protrusion. Use this as a guide to push the needle up into the damper;
make sure that both carbs are the same amoutn. Presto! Richer needle.
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