Hi Rich,
That's a complicated question - are you going for really accurate
tuning, or just close enough? If the former, you either need to have the
car tuned on a chassis dyno ("rolling road" in Brit-speak), or get one
of the AFR meters that are available. The cost can be high for the
logging types, but they are worth it because you can download real-time
data to a computer to show A-F mixtures at different RPMs, etc.
In general, using tube headers requires adjusting your carb(s) for a
richer mixture; headers are designed to be better at scavenging exhaust
gas, which tends to increase vacuum, which can lean out the mixture
through the carbs. More specifically however, unless you know that your
carb was tuned correctly for your stock exhaust, the lack of baseline
data will make further adjustments hit-and-miss, which is the "close
enough" approach. Plug cuts can help but aren't really that accurate,
particularly because cuts should be done at high revs, preferably under
load, which is when these engines sometimes tend to run-over, defeating
the test.
I'm not an expert on DGVs - I've only tuned a couple - but there is some
good info online. Try Googling "tuning Weber DGV", and you'll come up
with several. The good news is that these are easier and cheaper to dial
in than sidedraft carbs, wither Weber or Dellorto.
Here's one site just for Spitfires and GT6s -
http://www.triumphspitfire.com/weber.html
I think you'll find that you can do a serviceable job of getting your
carb close (fair gas economy, limited stumbling or flat spots, etc.),
but it will take some reading and fiddling (suggest lots or reading
first). Obviously, either an amateur or pro who has tuned for this carb
on this engine with headers will have a better idea of where to start...
Cheers,
Greg
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