Subject: [Healeys] Vacuum advance (was Which electronic ignition?)

From: Per Schoerner <[email-masked]>
Date: Thu, 19 Apr 2012 00:33:24 +0200

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.

As I understand it we need some ignition advance at idle, because the 
mix is weak and takes a long time to burn, this is accomplished with the 
vacuum advance. And we need a some advance at high revs because the 
pistons travel very fast and the spark must ignite the load some time 
(degrees) before TDC to make useful work, this is accomplished with the 
weights and springs. What we don't need is ignition advance at low 
engine speed and open throttle, because that will cause detonation and 
nasty things will eventually happen.
As the vacuum in my car is quite weak I wonder if the Healey distributor 
vacuum thing is more sensitive than on "normal" cars?

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.
> That's why you never connect a std Healey Lucas distributor vacuum advance to
> a manifold vacuum port on eg a triple weber Healey.
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---

From: Chris Dimmock <[email-masked]>
Date: Thu, 19 Apr 2012 09:28:56 +1000

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 <[email-masked]> 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.

[email-masked]

---

From: Per Schoerner <[email-masked]>
Date: Sat, 21 Apr 2012 01:47:39 +0200

Chris
It makes a lot of sense. I checked mine, and it's 5-12-8 as you thought. 
And the vacuum pipe is connected to the the throttle plate at the rear carb.
So, at idle we have very little vacuum, and we have very little force on 
the weights in the distributor, so we shouldn't have any advance really. 
Or should we, you say that the advance at idle is mechanic?
As engine speed rise we get more advance from the weights and and more 
advance from the vacuum. But when does it stop?
Is the advance from the weights proportional to the engine speed or 
something else? At what engine speed do the weights start to produce 
advance, and at what speed do we have to most advance.
And when do we have the highest vacuum, at half throttle perhaps?

I'll tell you the reason I'm investigating this. Last summer I was asked 
by a friend to take a look at his overdrive. It didn't work as it 
should. It turned out to be an electrical problem, so that was easily 
fixed. But while test driving his car I noticed that it didn't really 
work above 3000 RPM, it just misfired and was generally miserable. I 
began to suspect the ignition timing, and he told me that he some time 
in the past had had some problem with the vacuum unit. So he replaced it 
with a unit from what we think a Sprite. I don't know yet what profile a 
Sprite has, and I haven't checked if I can see any numbers on his unit 
yet. Since last summer I have replaced points, capacitor and spark plugs 
on his car. Rotor and cap are in good condition. I have also changed his 
carb linkage to a standard set up from what looked to be a complicated 
home made construction that didn't really work, why someone had changed 
it in the first place is beyond my understanding. So now the carbs can 
at least be adjusted in the correct way.

I haven't yet tested the car on the road after these repairs, it's still 
winter (sort of) here, but I'll make a try next week. It will be very 
interesting. It runs fine on idle at least.

Today I also checked on a BJ8, and it's also 5-12-8, and the pipe is 
connected at the throttle plate. That car has a leak in the vacuum pipe, 
so I doubt it works as it should. But that will be a later problem.

Perhaps I should get myself a dyno!! Have never seen one.

Chris Dimmock skrev 2012-04-19 01:28:
> 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.

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