Noble Scions,
I have observed a consistent surge of power when I just start to
let up on the gas. This is quite repeatable, and I'm sure you know how
I feel by wanting to get out the maximum performance. Here is some
background:
MGB with HS6's (totally rebuilt/rebushed), BUT because of engine
modifications (Piper Blueprint 1/4 race cam) I had to make an assumption
on the SU needles. I installed new needles for a Triumph TR4 (or 4A, is
there a difference in needles?) and new blue springs. I know that the
chances that these are the perfect combo are not in my favor, and I supspect
I am observing the mismatch. Here is my humble theory. Please have pity
on me old wise ones (Roger, Randy...), I am but a student of SU. When I
just let up on the gas the throttle plates close up a bit, and this raises
the manifold vacuum just for an instant, and the carb pistons come up a hair
for a second or so. Then the vacuum drops and the pistons drop to approach
the steady state condition. So, based on this theory I am running too lean,
and letting up on the gas for an instant enriches the mixture and gives me
a surge. I did inspect the plugs after running several hundred miles, and
they are a light gray, which supports the lean theory.
Please help me all you carb masters!! I am prepared to take some emery cloth
to the needles to customize the profile. But I don't think color-tune will
help. If I can set the mixture to be optimal at idle, and the profile is
not matched to the engine requirements, then the mixuture everywhere but
idle will not be optimal.
Patiently awaiting the wisdom of the world...
- Skip
'72 B roadster (Daily driver)
'74 BGT
'85 BMW 524td turbo diesel
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