Hi Per,
That's correct. Sort of. Healeys generally get all their advance at idle
mechanically, not by vacuum. Is your vacuum connected to the Su or manifold??
Not sure specifically about a tri carb setup.
Perhaps I should have said the difference is "opposite" at idle.
At the throttle plate, at idle, there is hardly any vacuum. That's "ported
vacuum" - and that's what a Healey distributor is generally set up for.
At the manifold however, at idle, there is pretty much full vacuum.
As I said before, the most common 6 Healey vacuum profile is 5 - 12 - 8
When mercury is raised 5 inches by vacuum, the vacuum advance starts, and by
the time mercury has been raised 12 inches, an additional 8 degrees of advance
will have been delivered.
So if you connect your distributor vacuum pipe to the manifold, instead of to
the SU, what will happen at idle??
You'll get an extra 8 degrees of advance. At idle. And what happens when you
give your engine an extra 8 degrees advance at idle? It doesn't idle very low,
and may even ping.
When you are cruising at 3,500 Rpm or whatever on light throttle, then there
is high vacuum at the SU port, the vacuum advance provides more advance to
fully burn the fuel = economy and smog reduction.
At full throttle, vacuum is almost equal at manifold and SU vacuum port i.e
low vacuum = no vacuum advance.
Part of the clue that a Healey doesn't want more ignition advance at idle is
that it doesn't even operate below 5.
I only used 5 - 12 - 8 as an example, as its common on Healeys Look on your
vacuum advance to see what you have.
Hope that makes sense.
Best
Chris
Sent from my iPhone
On 19/04/2012, at 8:33 AM, Per Schoerner <healeyguy at bredband.net> wrote:
> Chris
> I checked on my car, a standard 3000 mk2 with three HS4 carbs, and there is
very little vacuum at idle, and when I give it some throttle it seems the
vacuum goes away. As I recall, vacuum has been stronger on other cars I have
checked.
> You say that these vacuums are opposites of each other, I fail to understand
this. Can you elaborate this a little, please.
>
>
> Best regards, Per in Sweden
>
>
> Chris Dimmock skrev 2012-04-16 18:18:
>> Fair call John.
>> A Lucas healey distributor uses ported vacuum. Which is an SU thing. Vacuum
is
>> taken at the throttle plate. Not the inlet manifold.
>> These - ported vacuum and manifold vacuum - are exact opposite vacuums.
>> Opposite.
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