Hello all,
So while I wait to finish all the metal work on this car, I thought I would begin to get the carbs and distributor straight so that I can start the thing up and go when its ready. I've got a set of Italian Weber 40 IDFs that I'll probably just go ahead and rebuild while I'm at it. Also, a Bosch 034 SVDA dizzy with vacuum advance. So, the dilemma I have is that these carbs don't have a vacuum advance port already drilled. I know this has been covered ad nauseam in other threads, but I hope that this will be the final one and potentially a "how-to" for others.

By following the Weber diagram that was posted awhile back on the Samba, we can see where the later models included a boss for the vacuum advance port, directly above the brake/manifold vacuum tube. That diagram is below, but unfortunately I do not have any photos of a later Weber 40 with the actual boss from the factory.
Click to view attachment

If you look at the Webers I have, there is no existing boss to be drilled. The best you can do is look at the diagram and try to use it to mark a hole to drill. The problem is, all references that I have read say that the hole should be drilled just above the throttle plate (butterflies) so that your source of vacuum is throttle-controlled, not the brake/manifold signal, which is on ALL of the time. I have confirmed this with three different carburetor shops in the area, but all of whom did not want to drill the port for me. I was told I could do this myself. Click to view attachment

So, I am going to give it a shot. I'm looking for final confirmation from anyone here before I do, however. In the image I posted above, the hole I marked to be drilled is just above the throttle plate, but it differs from the later factory location. Did Weber put their vacuum advance boss in the right place, or am I missing something?

Thanks!
-George

The other option would be to drill into the other side of the carburetor, as the throttle plate is at a 76 degree angle in the throttle body. On the other side of the carb, the plate is much lower and I'd have more room to drill.

I actually just purchased the same distributor from Hot Spark for my '74 VW Bus.

I'm going to have to check my Webers to see if they have that vacuum advance port. For some reason I was thinking I could just plum it right into the brake booster vacuum lines and be fine, but clearly that isn't the case. Do you have to plum the distributor into BOTH carbs if you run dual, or just one?

Back in 1973 I put twin sidedraft webers on my Toyota Corolla. And as you are dealing with there was no provision for vacuum. So I drilled and tapped each manifold runner and connected all four with tubing to run to the distributor.

Here is one more picture, showing where the throttle plate intersects the throttle body. Again, the potential drilling sites are marked.

I have used a micrometer to verify exact locations of specific components. The bottom of the throttle plate is about 5/8 of an inch from the bottom of the carb, and the plate itself is 1/16th in thick. I have mic'd the hole for the existing manifold vacuum and its a 1/8th inch hole and the tube is 5/32 OD. That means if I were to drill the hole above the throttle plate that it would need to be a minimum of 0.75 or 3/4 inches above the base (5/8 + 1/16+ 1/16th - half the diameter of the hole). I'd probably add a little for comfort.

I'd like to hear what everyone thinks. From what I read, this should be correct. My biggest concern is diverting from what the factory did, but since I cannot find a picture to verify that they drilled below the throttle plate, this is my best estimate.

Thanks!
-George

Hi Adam,
I know that you cannot use the brake/manifold vacuum since that signal is always "on". Essentially you would be supplying vacuum to the distributor at all times, defeating its purpose. Another point that bulitt brought up in his post below is drilling each carb and each throttle body. From what you can see in the Weber diagram, they only supplied one vacuum advance port per carb. Apparently this doesn't interrupt the flow of each throttle body enough to warrant tapping both bodies on each carb, but I would probably tap both carbs and "tee" them together to ensure an even signal from both.

Maybe I didn't articulate my thoughts correctly. I drilled into the "intake manifold runner" directly under the carb mounting flange. Drilling a carb would have made me too nervous.

You have obviously done research on this (far more that I did). I am not questioning what you are doing, just passing on my experience. When I bought my car it had a poorly built “high performance” 1.7 that was punched out to over 2.0. It had stupid high compression and was running an 009 POS dizzy. I tried for 2 years to get it to run right. It finally dropped a valve seat.
I replaced it with a factory 2.0, but kept the Webber 40’s on the car. The shop that did the final set up used the stock 2.0 distributor and ran the vacume line to the existing factory port you already have (next to the mixture screw). That car ran fantastic. It started with no effort, It stayed cool, had zero hesitation and pulled great. Only really cold weather or a once in a blue moon clogged idle jet gave away that that it was not fuel injected.

For vacuum advance you want unported vacuum. This means that the vacuum must come from a source prior to the throttle plate.

Make sure that you drill above the throttle plate.

Vacuum from the manifold is not good.
Vacuum from the air filter chamber is fine but not best. You could use this to check the general performance improvement before you drill the carb.

One port form any of the carbs is sufficient and recommended. Don't connect the vacuum ports from different carburetors as there is no advantage in doing so.

Scott,
It is entirely possible you were getting some kind of vac. advance signal even though you were using the manifold ports. However, as valy said in the next post, the signal should be coming from above the throttle plate. I'm glad to hear that some have had good experiences with carbs, even though everyone recommends the FI. I wish I could afford a megasquirt or to find all of the original FI - this car came with none of it. Thanks for the input!

Valy,
I knew to drill above the throttle plate - thanks for affirming this. I was also told I only needed to drill one carb, since the vacuum is so small it won't really change the airflow much. I'll try it with one first, and if for some weird reason it won't run right, I'll tee them together.
I'll be posting how I did all of this and show pictures at the end, in case anyone else needs to do it. Hey, maybe I'll even get a sticky!

Scott's motor and carbs weren't using manifiold vacuum, but a port on one of the carbs. I have that motor sitting in my garage, almost ready to go in my car. I can get you a pic of the carb vacuum port if you'd like.

Ohh, I must have misread his post then! Sorry. Yeah, a picture would be great. Was this carb one of the older ones without the port already drilled?

I believe you are all wrong, please read,

"Distributor Advance Explained

This was written by a former GM engineer as a response to a similar question on a Camaro board:


As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren't fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don"t work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will 'dither' in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15" Hg. of vacuum at idle need a vacuum advance can that's fully-deployed at least 1", preferably 2" of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8" of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively."

A friend sent this to me, hope this helps. By the way. the early D-Jet Throttle body that used both advance and retard used the small port below throttle port for the advance and the larger port above the throttle plate for the retard. Use a stock FI distributor and ported vacuum!

That is great info, John, thanks for posting!

Here is the link to Hot Spark for the SVDA distributor w/ electronic ignition I bought. It even lists the advantages over the 009. $144 shipped domestically is a good deal!

http://www.thesamba.com/vw/classifieds/detail.php?id=641377

+1 manifold vacuum



+914!!!! ported vacuum is for emissions purposes

Well, I drilled it about an hour ago, so I'm not sure what to think now. The car won't be running for some time anyway, but I'll be sure to update this thread when it does, along with my results. I'll post some pictures of the drilling as well, in case anyone else wants to give it a shot. Not sure if I mentioned, it, but I'm running a SVDA with points. That other one looks sweet however. Never thought about running the stock FI dizzy.... might have to check that out...

Here you go. Weber 40 IDF 70 stamped on both.

On the left carb note the vacuum tube with the small screw plug. On the right carb in the second pic, you'll see a vacuum tube in the same location that is not plugged. That's where the dizzy pulls vacuum from on this motor.

Left carb:

Click to view attachment

Right carb:

Click to view attachment

ummm...... where is the blue? Is that oversrpay on those maifolds?!!





(totally kidding...... )

Sorry Scotty. The blue had to go. I might hit the overspray with a die grinder and wire cup. Just for you. And Ferg.