Weber 32/36 vacuum advance question

[thehackmechanic]

Yeah, yeah, it's been asked dozens of times before, but bear with me. The actual question is at the end.

 

Background: I'm trying to get "Hampton," my 49,000-mile survivor 2002, to start and run as well as possible. The car came to me basically stock except for a Weber 32/36. It had the original vacuum retard distributor, and the EGR valve, vacuum dashpots, and control circuitry on the firewall were all still present, though not all of them were connected. I eventually removed the EGR valve, as I was concerned that it was a source of vacuum leaks. I assumed that the thing to do was replace the vacuum retard dizzy with an earlier vacuum advance unit, so I found one, made sure the mechanical advance and vacuum advance both worked, and installed it.

 

I thought that the question was then "Which port do I hook the vacuum advance diaphragm to," but the more I read, the more I think that there's not really an answer.

 

The short-but-seems-wrong answer seems to be "you want ported vacuum, not manifold vacuum, so you want the port on the Weber above the throttle plate."

 

The longer-but-unhelpful answer gets subsumed into the question of "How do I connect the Weber to a smog-compliant system," which isn't what I'm asking.

 

I always thought that you DON'T want extra vacuum advance when you mash the throttle, as that'll cause knocking, but you DO want it at idle and at even throttle. Neither the port on the Weber nor the port just below it on the base of the intake manifold have those exact characteristics. Measured with a vacuum gauge, the port on the Weber ("ported vacuum") is zero at idle and increases linearly with throttle opening. It jumps upward when I snap open the throttle. This seems to be the last thing I'd want to connect the vacuum diaphragm to. Manifold vacuum seems a better choice, as it's about the same at idle as it is at even throttle, drops to almost zero when I snap open the throttle, and rises only when I release it.

But—and here's the real question, and forgive me for sounding like Perry Mason:

• Isn't it true that only pre-'72 2002s with the single-barrel Solex ever had the vacuum advance diaphragm directly connected to a port on the carb?
• Isn't it true that by the time two-barrel Solexes were on the cars, so were EGR and vacuum dashpots, and there was never a direct vacuum line between the carb and the advance diaphragm?
• And thus, isn't it true that trying to connect a Weber 32/36 this way only moves the whole question further into "neither the carb nor the dizzy are really meant to be connected like this," and that if you do, it's likely to result in an advance profile with characteristics that are more complicated than the simple "just set the total advance" many of us have gotten used to?

 

I spoke with my brother-from-another-mother Paul Wegweiser about this last at some length night, and he basically agreed and said that he only uses the vacuum advance diaphragm if there's not enough advance at idle, but I'd love to crowd-source a larger set of opinions.

 

I've had tiis and Bertha with the dual Weber 40s, both of which have straight mechanical advance distributors, for long enough that I realized that it's been decades since I directly fiddled with a 32/36 and vacuum advance.

 

Thanks.

 

--Rob

[Mike Self]

1 hour ago, thehackmechanic said:

Isn't it true that only pre-'72 2002s with the single-barrel Solex ever had the vacuum advance diaphragm directly connected to a port on the carb?

Admittedly from an example of one--but one very original car:  my '69 has the dizzy vacuum advance connected to a port high up on the carb body, above the throttle plate, and a second vacuum hose connected to another port at the very bottom of the carb body.  It runs to the valve cover breather hose connecting valve cover to air cleaner.  There are no vacuum ports in the base of the 1 barrel manifold like there are on the two barrel manifolds.  

 

Oh, and on my '73 (2 barrel Weber, 9.5 pistons, 284 Schrick, so not stock) when I had my dizzy (vacuum advance only) rebuilt, it was recurved to take advantage of the cam, eliminating the vacuum advance altogether.  It's there for visual purposes, but plugged at the carb end.  The advance curve is different from a tii dizzy and it seems to work fine.

 

mike

[rstclark]

Someone with more knowledge than me will reply but for now I vote for a vacuum advance distributor connected to the manifold under the throttle plates

The theory here is the a lean mixture takes longer to burn than a rich mixture so you have to light the match sooner

In other word at idle and cruise when you have a lean mixture and vacuum ,you want more advance . When you floor it and have no vacuum  you can rely on mechanical advance alone

This of course requires the right combination of mechanical and vacuum advance

 

Edited March 25 by rstclark

['76mintgrün'02]

29 minutes ago, thehackmechanic said:

isn't it true that trying to connect a Weber 32/36 this way only moves the whole question further into "neither the carb nor the dizzy are really meant to be connected like this," and that if you do, it's likely to result in an advance profile with characteristics that are more complicated than the simple "just set the total advance" many of us have gotten used to?

 

It's more complicated, but you're still "setting the total advance" and seeing what that leaves for idle advance.

 

The main difference you will see in terms of advance (between m&p) is at idle. 

 

Using manifold vacuum will bring in advance at idle, but ported will not.

 

Both ported and manifold vacuum drop out at full-throttle.

Both add advance at light-throttle-cruise.

 

I ran two vacuum gauges in my console for a while, so I could watch both the ported and manifold vacuum at the same time.  It's an interesting comparison. 

 

Now, I have a ported-vacuum gauge in the console, so I can see when the pod is adding advance.  I also have a switch hooked up to one of those cool little "emissions" solenoids, so I can turn-off the vacuum to the carb and note the difference it makes.

 

It invariably stumbles and runs worse when I turn off that added advance during light-throttle-cruise.  (full-advance for the pod ~ 10-12 degrees)  I've had co-pilots confirm the difference it makes, to rule out wishful-thinking. 

 

There's more to it than just idle-concerns when it comes to assessing the benefits of vacuum advance, but choosing manifold vs. ported is based on idle-advance needs.

 

Tom

 

[jimk]

43 minutes ago, thehackmechanic said:

Measured with a vacuum gauge, the port on the Weber ("ported vacuum") is zero at idle and increases linearly with throttle opening. It jumps upward when I snap open the throttle.

What you saw is as it should be.  The depressed pressure (partial vacuum) occurs on slight blade opening because the blade edge rises above the port as would be on cruise light load.  Further blade opening the pressure at the port rises again (less vacuum) when there is a demand for more power, thus more reliance on centrifugal and less additional vacuum advance. 

 

This characteristic is explained in the Weber manual. 

[John76]

3 hours ago, '76mintgrün'02 said:

Using manifold vacuum will bring in advance at idle, but ported will not.

 

Both ported and manifold vacuum drop out at full-throttle.

Both add advance at light-throttle-cruise.

Well said Tom.

Ported vacuum on the later 2-barrel carbs is used to control the EGR valve. No vacuum at idle = No EGR.

I like to use manifold vacuum for distributor advance. Engine will crank and start easily with just 3-degrees of static advance.

At idle, the throttle plate is almost closed, and thus no ported vacuum to the EGR. Manifold vacuum is high, which adds 10-12 degrees of advance to the static setting, resulting in 10 - 15 degrees at no-load idle speed.

Crack the throttle and the manifold vacuum starts to decrease while the ported vacuum kicks in (activating the EGR).

Cruising speed (high rpm, light load, throttle opening small) = static + centrifugal + vacuum advance. Result: great gas mileage.

Stomp on the throttle = NO vacuum advance and max advance is limited to static + centrifugal.

The vacuum port on the intake manifold directly below the carb is for the secondary crankcase ventilation system. This sucks gas fumes from the charcoal canister and oil/gas fumes from the crankcase. This port is designed to distribute these "nasty" fumes equally to the 4 cylinders (so they all suffer the same mixture dilution).

A vacuum retard distributor can be plumbed to provide the same as above but starting with very different static and mechanical settings. Must have something to do with satisfying the early emission requirements mandating retarded timing along with EGR.

Some of the later cars had dual vac advance and retard pods that turned on and off advance based on operating temperatures.

[HBChris]

I stick with what BMW did, ported advance and I have the Weber 32/36 on my NK.  I was told that retard on the manifold helps with idle on Automatics, I don’t know if this is true though.  On my e3 and e9 with Zeniths I also use the ported advance as designed.

['76mintgrün'02]

3 minutes ago, HBChris said:

... I don’t know if this is true ...

 

... this tool tells the trooth! ...

[TobyB]

It's been a while since I messed with this, but what I recall

with a healthy stock vacuum advance distributor

 

is that the all- in advance at idle with manifold vacuum is awfully high.

Ported vacuum lets you set idle in the 8-12 degree range, 

which is a little soft, but quite smooth.

You then do hard pulls from off- idle with appropriate (15 rising to 3oish) advance,

but the vacuum pod then lets you cruise at 2500+  with 35- 40+ degrees of 

advance, which gives a nice, early spark for popping off a really lean light cruise mixture.

 

There are a million ways to do it, but that's what I recall.  Timing and  mixture interact,

so running richer means you probably don't want that much advance anyway, for maximum power.

 

t

 

[Dudeland]

12 minutes ago, TobyB said:

It's been a while since I messed with this, but what I recall

with a healthy stock vacuum advance distributor

 

is that the all- in advance at idle with manifold vacuum is awfully high.

Ported vacuum lets you set idle in the 8-12 degree range, 

which is a little soft, but quite smooth.

You then do hard pulls from off- idle with appropriate (15 rising to 3oish) advance,

but the vacuum pod then lets you cruise at 2500+  with 35- 40+ degrees of 

advance, which gives a nice, early spark for popping off a really lean light cruise mixture.

 

There are a million ways to do it, but that's what I recall.  Timing and  mixture interact,

so running richer means you probably don't want that much advance anyway, for maximum power.

 

t

 

I alway thought that a richer mixture( I.E full throttle) had a slower flame front, so you wanted all the advance.  My ignition is controlled by the ECU and is dynamic. Otherwise my head is like this. 

 

 

 

 

[John76]

23 minutes ago, Dudeland said:

Otherwise my head is like this. 

Now that's retarded!

[jimk]

40 minutes ago, Dudeland said:

I alway thought that a richer mixture( I.E full throttle) had a slower flame front, so you wanted all the advance.

Just the opposite.  WOT and rich mixture results in the highest compression pressure.  High pressure is more dense and the flame speed is the greatest.

 

And so at light loads, more advance and high loads less advance (relative to the light load advance.  Confirmed by lots of reading and the ignition table extracted from the stock M3 chip.

[rstclark]

When drag racing was the thing, all you needed was a mechanical advance curve because you only ran flat out That's one way to remember about a rich mixture. 

I recall the simple weight and spring kits sold for Delco distributors I used in small block Chevys. Put in the kit, bump the timing up a few degrees and you had a snappy car

Now Dudeland when you get that turbo going you will have a different challenge, but that's another story

[Dick R]

The attached article regarding vacuum advance written by Lars Grimsrud, was published on a hot rod site.  While big block Chevys and Fords are not 2-liter BMW engines, I would expect that fundamentals apply at least to some extent.  In addition to his discussion of why manifold vacuum should be used, he also included is the reason ported vacuum advance came into use in the early days of emission control systems.  

 

I recall reading a similar article about vacuum advance that was in FAQ some years ago that was written by a retired Ford engineer.  (I couldn't find the article now.)  That article also said that manifold vacuum was the way vacuum advance should be controlled.  I also seem to recall that it included a discussion of why non-vacuum advance distributors are used on fuel-injected engines and that they were not meant for carbureted engines.   After reading this article maybe 5 or 6 years ago, I replaced the Tii distributor that was on my 2002 when I got it with vacuum advance distributor connected to the intake manifold.  (By the way, that distributor was donated to me by rstclark, the above poster.) 

 

No doubt, the highly skilled and experienced members of this forum can make most any configuration work quite well, so there's not just one solution. 

 

Timing & Vacuum Advance 101 -good article by GM engineer | 460 Ford Forum

WWW.460FORD.COM

 

['76mintgrün'02]

25 minutes ago, rstclark said:

I recall the simple weight and spring kits sold for Delco distributors I used in small block Chevys.

 

Big Block Chevies too and they put them right on top, under the rotor, where it's easy to get at them.  Big block Delco distributors also have adjustable points.  (the 5568 also has a dwell meter).

           

 

Distributors are built to be adjustable, but the techniques are being forgotten;  as tech-No-Logic-al de-vices do the work for us (so-called up-grades). 

 

Long live https://www.advanceddistributors.com/ !

 

 

I shortened the curve in my distributor, so it works with ported vacuum.  I am content with how it is now, aside from some pinging if I give it too much throttle between 2-3k rpm. 

 

I'm wondering if stretching the curve and switching to manifold vacuum would eliminate that problem, by providing less advance in that rpm range.

 

I'd rather not modify the one I'm using now, but might look back at my #164 and set it up for manifold vacuum.

 

Starting with 3 degrees at idle, plus 12 for the pod would put me at 15.  Then it'll drop back as throttle's applied and start increasing from there. (like John's)

 

With an all-in advance target of 35, minus the three I'll see at idle, I'd want to achieve 32 degrees at the crank; so the distributor would put out 16.

 

@thehackmechanic , do you know how much advance your vacuumed distributor is putting out?  Would you mind sharing the part number on that one?

Tom