Pat...
A variation of only 400' isn't gonna make
any measurable difference. From sea level to 7,000'msl is another story
altogether.
Now, just following tuning instructions
for normally setting mixture at sea level
while actually at 7,000'msl is going to
automatically take the less dense air
at that altitude into account.
In other words, relative to sea level,
you will need less "flats out" at 7,000'msl
from full lean than you would at seal level
to get that "momentary" increase in rpm prior to the engine returning to
the set idle
speed, subsequent to the air valve having
been lifted 1/32nd of an inch or so to check for mixture strength.
You see, the SU tuning procedure, in and of itself is pretty much
self-correcting for
variations in air density, to great extent One will notice in my area of
the country
(Connecticut) where summer temps can
be 90F and winter 32F that the difference
in correct seasonal settings amount to
1 flat difference between summer and winter, once mixture strength has
been
properly set for the temp. average of either season.
This having been said, the "correct" thing
to do would be to switch to the recommended "weak" needle for the
standard carb. set-up which would, in
the case of a fixed-needle stock 1275
SU set-up, would be "GG" according to
"the book".
Cap'n. Bob
'60 :{)
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