Hi. You wrote:
> Thanks. That makes sense.
Y'er welcome. Glad to help out and even more glad that it makes sense!
> BUT I have a TR7 with vacuum retard. Actually, iy does not
> have it active at this moment.
I've considered disabling my Spitfire's vacuum retard every so often, but
whenever I did before it failed emissions tests. It doesn't have to pass
them anymore so maybe I'll re-disconnect it.
> The vacuum tap in the carb is close to the edge of the throttle valve.
> Pretty much under the edge when it is closed.
> It LOOKS like it would have little vacuum when closed, and then the
> hole is uncovered with cracking the throttle open.
> This would appear to be a vacuum advance, right?
I guess I don't know what you mean by "under the edge". If you mean on the
upstream side (visible from the air filter), then it should see little or no
vacuum when the throttle is closed. If opening the throttle then moves the
throttle plate so that the hole is now on the manifold side, then it would
seem to pick up vacuum when the throttle is opened. Whether it is intended
to be an advance or retard still isn't obvious. I don't know the TR7. It
is possible that they worked the spring in the diaphram to go either way,
like this: The spring could apply advance that the presence of vacuum then
takes away, or the spring could apply retard that the presence of vacuum
then takes away. The way to tell is to note on which side of the diaphram
the vacuum line comes in. Since you don't have the Euro distributor you are
kinda' sunk on that one. The close proximity of the vacuum hole to the
throttle plate suggests that whatever effect it was supposed to do they
wanted it to happen early in the throttle movement, suggesting that it was
an idle-retard device. But I'm just guessing here. We don't know how
progressive that vacuum would be with increasing throttle opening.
> I also suppose that if the distributor is designed with vacuum retard,
> then it essentially uses mechanical advance only.
That would make sense.
> And further, as you say, that you need less advance at
> wide-open-throttle, then there is a little loss of
> efficiency at part throttle with this design.
Yeah, any no-vacuum-advance ditributor would seem mean less efficiency. The
pertinent question is whether that really matters much.
> Therefore I think I should look for an advance-type distributor....
If the vacuum hole in the carb is as it seems, then yes. What you don't
know is how well calibrated its effect would be unless you get the exact
match for that carb. On the other hand, since technically you don't need
it, you can maybe get by with anything. If it causes not enough advance and
you set your timing for full-throttle, then you'll simply be running with
not quite enough advance at part-throttle, which is okay. If it causes too
much advance and you set your timing for full-throttle, then you could get
pinging at part-throttle that goes away under acceleration. The only fix
would be to back the timing off, causing you to lose full-throttle power.
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire (Percy)
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