SVDA Distributor Vacuum Question

[GTman]

I have dual Kaddie 40s on a 1600, and am running with a Pertronix Flamthrower SVDA with electronic module.

I've checked during driving conditions and see that each carb is putting out between 4 to 8 in.Hg ported vacuum. And I can see with my vacuum guage that when revving the engine from idle I am also able to hit 5 in.Hg ported vacuum.

I am running my timing at 30 degrees BTDC (vacuum disconnected) at 3,000 rpm.

When I pull a vauum on the distributor (by sucking on the vacuum hose) I am also able to see my timing change about 10 degrees.

The problem I have is that when I connect the ported vacuum to the distributor, and rev the engine, I don't see the advance going beyond the 30 degrees mechanical (e.g., when reving to 3,000 or 3,500 rpms).

This makes me wonder whether the say 5 in.Hg on average that I am able to produce is enough for the vacuum advance mechanism of the distributor to work.

When driving the car I have the feeling that the vacuum advance is not working as the engine seems alittle unresponsive, and the engine has been running alittle warmer since converting from the 34-Pict carb to the Kardons.

Thanks in advance, Grant

[udidwht]

Where did you get the SVDA?

If it's new it may need some time to loosen up. With a screwdriver how easy/difficult is it to move the vac advance?

Try revving to 3400 rpm's and start feathering the throttle at that RPM and watch the timing. I've found that with the Pertronix SVDA dizzies they need a bit of a load to see more vac advance and as time goes by they end up giving more.
_________________
1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires

[GTman]

Its a Pertronix about a year old. it worked fin giving me 40 degrees total advance when I was running it with my 34PICT3 carb. The Kadrons just dont seem to have a strong enough vacuum signal to get the vacuum mechanism to work. Which sort of makes sense cuz by moving from a single intake manfold system to a dual one I've effectively reduced the ported vacuum signal in 1/2.

But now what ? Has anyone measured to see what amount of ported vacuum they are getting at their Kadron or other type of dual carb and are they able to get a good strong 8 to 10 inHg signal?

All help and input appreciated!

[udidwht]

It's hit & miss with aftermarket carb set especially duals. I'm in a similar situation with my IDF duals 40mm. I get about 5 degrees of advance at 3400 or so RPM's using the Pertronix SVDA as well. I have approximately 1000 miles on the dizzy.

My Webers have the vac ports off of 1&4 barrels. I'm also using a 73 Web cam which is contributing to the lack of advance. I'm going to put an anti pulse valve in and see if I get an improvement.
_________________
1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires

[special-k]

I've run both drla36 and 40's and neither gives me the vacuum everyone says they get. Maybe our gauge is junk but we checked it with a mityvac when it was installed. I might get 5 or so going downhill feathering the gas or tiny jump when shifting gears. We changed the diaphram to one from an old large dia bus vacuum only distributor to get it to move with so little vacuum. Its been said on here that most svda's have a similar curve to an 009 so even if it doesn't advance it should feel similar to an 009. Good luck.

[udidwht]

[Danwvw]

I just Put two 36mm Dellorto DRLA's on my bus engine and experimented with 3 different distributors two of which were Combination Vacuum Centrifugal distributors, One would produce 10-12 degrees vacuum advance in operation and the other only about 4 or 5 degrees. When I checked the specifications for these distributors I found they were doing just what they were supposed to do. The thing about vacuum advance is, it falls completely off under a load. What vacuum advance seems to do is make an engine a lot more free revving out on the open highway and that is good because you wind up using a little less throttle. Try a different Vacuum diaphragm.
_________________
1960 Beetle And 1679cc DP W-100 & Dual Zeniths!

[E4ODnut]

This is my understanding of vacuum advance, and I stand to be corrected.

First of all, above idle speed, that is just after the throttle plate starts to open, there is no difference between ported and manifold vacuum because the port is now exposed to true manifold vacuum.

The purpose of vacuum advance is to optimize ignition timing at light and part load conditions where the fuel mixture is, or should be leaner. Without getting too technical, a leaner mix burns slower, so more advance is required to get peak cylinder pressure where it will produce the most torque in the power stroke. Intake manifold vacuum is probably the best indication of engine load that is available in a spark ignition engine. It works very well for engines with 4 or more cylinders with a single carburetor or throttle body. But, it is not perfect. If you have a cam with long duration, it affects low speed vacuum and the reading is no longer representative of true engine load. If you add venturis or throttle bodies it has the same effect. The result of all of this is that even though you may have a healthy engine, if your cam profile, or induction system will not provide you with a "typical" vacuum curve, the vacuum advance unit on your distributor cannot do what it was designed to do for a typical mild cam and single carb or throttle body.

If you have the specifications for the vacuum can on your distributor, and if you know what your manifold vacuum is under all speed and load conditions, you will be able to tell if that particular can will give you the desired advance or not.

If not, there is a way around this, sort of. If your distributor has an adjustable vacuum can available, or several different "curves" for vacuum advance available, you are in luck, then it's just a matter of finding the best combination for your engine. I say "just" tongue in cheek, because it's not all that easy to find the best combination.

If you don't have either of these options then there is not much you can do to optimize your part load timing with a conventional mechanical/vacuum advance distributor. Your only other option is a programmable ignition system that can use a combination of inputs from engine revs and throttle position. It's not as good as manifold vacuum, but if you have little or no vacuum under light and part load, it's your only option, or at least, the only one I am aware of.

In your case I suspect that increasing your venturi size has decreased your manifold vacuum at light and part loads. Are you sure you need these larger venturis?

[udidwht]

Well, the vents I'm running are 28mm which are stock for the 40 IDF's. That said if one is getting only 5 degrees vac advance you can look at it this way...it's better than none at all.

I set mine with the hose off & plugged to 28 degrees. When ran up to 3400 RPM's if I feather the throttle up/down slightly I can see it advance to 33 degrees. Like I stated earlier I'm going to stick an anti-pulse valve in the vac line and see what results if any I get.
_________________
1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires

[Danwvw]

My carbs and manifolds are producing full specified vacuum advance and I have 30 mm venturi's!
_________________
1960 Beetle And 1679cc DP W-100 & Dual Zeniths!

[RailBoy]

Something to be aware of is that after seeing this site for a while it is said that cam choice can have an impact on vacuum distributors... Not 100% on why, but that is what they say... RB
_________________
[quote="smitty24"]"HELP, I ported my own heads and now have fire coming out my ass" .[/quote]

[special-k]

udidwht we took the diaphram of an old cast iron large diameter bus vacuum only dist. The radius is wrong for our 205P but we just put a small spacer under one end and tweaked it to make it fit.

[Danwvw]

The Large Can vacuum diaphragms should be a lot more sensitive to vacuum than the smaller later ones, There must be a mechanical problem with the matching of the diaphragm with the distributor. Something must be in a bind!
_________________
1960 Beetle And 1679cc DP W-100 & Dual Zeniths!

[special-k]

That's why we put the old one on our dist. It moved with less vacuum whereas the original one needed around 7 inches and we weren't getting that much vacuum at any time consistently.

[Danwvw]

I don't Know what the problem is but if it is free to move the advance which should be loose and move freely with only the spring pressure offering any resistance the distributor should advance the spark. Not sure what the vacuum should be on your carbs, Played with the Stock carbs and they have various vacuum ports with reduced vacuum one port has close to full vacuum at idle. Think it is for the retard side of a dual vacuum advance set up. My Dell's on these dual manifolds and pretty much stock heads and a mild cam make plenty of vacuum to move the advance as well as modulate it, I see advance drop off on the timing light under just accelerating when free revving the engine then it climbs back up when the engine is at rpm. One of my distributors had so much vacuum advance it would push the timing up to 43' with the vacuum line connected under no load with it set to 30' without the vacuum. Never the less I elected to go with the 009 as it seemed to be a distributor in better condition than the SVDA's I had. It is just set to about 32' full. I do get a bit of a lag when giving it throttle on the highway after cruising down hill a bit with the throttle closed. The SVDA did not do that!
_________________
1960 Beetle And 1679cc DP W-100 & Dual Zeniths!

[udidwht]

My Pertronix SVDA's vac advance is stiff. It will move (by screwdriver) if I use some effort (not a lot) but not as freely as I anticipated. If I use the hose to check the movement it will also move but requires some ump . I was told it require some miles to free up some.

I looked it over carefully and it's not hanging up on anything but I believe it's just a combination of stiff spring/diaphragm and new hardware.
_________________
1972 Westy Hardtop/Type-4 2056cc
96mm Biral AA P/C's~7.8:1CR
Headflow Masters New AMC 42x36mm heads w/Porsche swivel adjusters
71mm Stroke
73 Web Cam w/Web solids
Dual 40mm IDF Webers - LM-2 - 47.5 idles/125 mains/190 air corr./F11 tubes/28mm Vents - Float height 10.45mm/Drop 32mm
Bosch SVDA w/Pertronix module (7.5 initial 28 total @ 3400rpm)
Bosch W8CC plugs
Pertronix Flamethrower 40K coil
S&S 4-1 w/Walker QP 17862
3 rib 002 Trans
185R14 Hankook tires

[[email protected]]

Take the vac pot off and see if it's still stiff. If it is, remove the plate and clean/grease it. One piece of grit can bind it up so it no longer moves, and this is not uncommon even when new.

http://vwparts.aircooled.net/Anti-Pulse-Valve-for-Vacuum-Advance-Distributors-p/anti-pulse-valve.htm

can also help get more vacuum to the distributor, if the signal is weak.

[Axitech]

Quick question : I'm setting up the new engine in my rail, getting the final bits done. Where do I draw vacuum from on the carbs (dual 40 IDF's)? I do not see a vacuum port anywhere. Also, am I correct in the assumption that the SVDA is using vacuum at part throttle for advance and therefore I only need to grab vacuum from one carb, or do I need to grab it from all four and somehow 'balance it' in a chamber somewhere? Thanks. Bob
_________________
He said to them, “But now if you have a purse, take it, and also a bag; and if you don’t have a sword, sell your cloak and buy one. Luke 22:36


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[Juanito84]

A lot of carbs don't come with ported vacuum. You have to have them drilled. Yes, it's best to combine all four carbs. Reading vacuum off one carb will probably pulsate too much.

[Bugzilla]