On 21 Nov 2002 at 18:15, Randall Young wrote:
> Well, I'd sure rather hear about GT6 carbs than about those dingbats !
Don't know about dingbats. Don't know that anyone does! But about
the carbs...
The GT6 has SU carbs. At the suggestion of various people here and
abroad I tried springs and needles spec'ed by SU for a non-US '74
Triumph 2000 engine. The result was terrible! These springs are
firmer, so the pistons wouldn't rise as much. Plus, the needles were
quite a bit thicker over most of their range. Both factors would
make it leaner, but it always seemed too lean already. At lower
rpm's it ran well but 3000 rpm's and up was impossible. Dropping the
jets (a lot!) would have compensate somewhat but I couldn't find any
good reasonable setting.
So I reinstalled the original MGB parts, being very careful to
position the needles properly in the pistons. Before installing them
I considered thinning them, but I had no idea which stations (i.e.
which positions) on the needles were pertinent. I did try sanding
them lightly in the regions between idle and midpoint, but I gave up
after deciding that I didn't know what I was doing and wasn't making
any real difference anyway.
Then I spent an hour or more twiddling various screws. Eventually I
satisfied myself that the mixtues were close (erring slightly on the
rich side, especially with the cold weather), and that raising the
piston pins on each made the rpm's do what they was supposed to (rise
a bit then fall off significantly) to the best of my imagination. It
was tedious work, and the results were sensitive in ways I've never
seen described "in the books". I ended up nauseous but the engine is
now much smoother, less buzzy at speed and with less apparent missing
while cruising.
The unexpected behaviors were the following. 1. It seems that one
carb being too rich will make the other insensitive to its mixture
setting, allowing it to run okay even when set way too lean. The
mechanism for this to happen is probably the crossfeeder tube. The
implication is that I had to get both nearly right before either
could be dialed in well. 2. The idle speed has to be run as low as
possible (as described in all the manuals). But even more important,
the idle rpm's have to be dominated by the carb one is trying to
tweak. Backing out both idle screws and hand-pushing the butterflies
closed helped. But though one carb could be set well, I had to raise
the throttle slightly on the other one ever so slightly before trying
to set it. Otherwise changes in its mixture would have minimal
effect on idle speed.
The entire process was interative and very subtle. It was no wonder
that I had never had confidence in what I was doing. When I somehow
stumbled on a good mixture for both carbs, they both began to behave
as expected. I had suspected the "which carb is dominating the idle
speed" phenomenon, but I was able to work through it only after
getting both to work properly individually. The resulting engine
smoothness was a pleasant surprise. Further taeaking might be
possible but I'd be foolish to try without first writing down the
current settings!
On the subject of manuals, I have an old Glenn's Foreign Car Repair
Manual from, like, 1968 or something, purchased originally to provide
insight into my first car, a BMW 700. It has a section on tweaking
SU's which turned out to be quite useful.
I wonder what would happen if I used the stiffer springs. The
needles would be running on thicker (leaner) portions, but the
venturis would be smaller too. This would me faster airflow rates,
which should result in a richer mixture. What would final result
be??? Swapping a spring is easy but I don't want to go through the
tuning procedure again!
--
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire, '70 GT6+
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