Dual carb and distributor setup?, Looking for your feedback on this design Options

[NJ914Guy]

Car is headed into the shop this Friday. Pulling off the single Weber progressive carb and 009 distributor. My plan is to switch to Dual Weber 40's (on sale this week at AA) and upgrade the distributor to an SVDA 034 with an electronic ignition module (no points).

Engine is a stock 1973 2.0, no plans to convert back to factory FI.

How would this setup work on a 914 driver? Looking for solid performance across the RPM band with improved off idle response to get rid of the flat spot.

[hot_shoe914]

I have always been told the 050 dizzy works better with carbs than the 009. I know when I swapped from the 009 to the 050 I had a lot better power and response. I vote for a Mallory but if you can't get one I suggest the 050.

[rhodyguy]

i would leave the current distributor in the car during the dual carb intstallation. you want the car to fire up so you or your mechanic can get the carbs sync'd to a approx operational setting. if you sart throwing a bunch of variables in to the mix you might encounter a nightmare. some point replacement units don't react well to excesive no start cranking. remember, your valves will need to be spot on to fine tune the carbs. get a FIRM estimate from your mech on the labor costs to do the switch.

k

[sean_v8_914]

Dave: I did not understand your comment about Bosch VW. could you explain that for me?

I felt that I understood the GM engineers explanation as it related to the various versions of type 4 d jet and l jet dizzy configs

[NJ914Guy]

Dropped her off at the shop yesterday. They are going to start with a quick check of the basics, and then we will work from there. I think the idea of changing the carbs only first, and then moving to the ignition. Too many variables makes for confusion, and we don't want that. I will keep this thread up to date as we progress. Thanks for all the feedback!

[sean_v8_914]

here is how Porsche solved teh vacuum pulse issue

[sean_v8_914]

I bought this engine to put it in the 10jreen but Herb wouldnt let me cut any more metal off the chassis so I put the 2056 back in and sold this one at teh spring valley swap meet. all that chrome makes it run too hot anyway

[Dave_Darling]

Dave: I did not understand your comment about Bosch VW. could you explain that for me?

The article said that the ported vacuum was (and the implication was solely) due to the air recirculation system on the GM engines. Almost all of the 914s that ran ported-vacuum advance were made before they started putting smog pumps or air injection on 914s, so it could not have been for that reason. So my note was a slightly snotty, slightly sarcastic way of saying that. (Sorry; some days I feel more obnoxious than others.)

--DD

[gothspeed]

From this site:

http://www.netwelding.com/ignition_timing.htm

However and old Peterson publication entitled "Basic Ignition and Electrical Systems" reviews the history of "Manifold" versus "Ported" vacuum. As they indicate, "Ported Vacuum" came about during the start of pollution controlled vehicles to reduce nitrous oxide emissions with lean air/fuel idle mixtures. They state that by using "Manifold Vacuum" the spark at idle will occur sooner and "less cylinder wall is exposed to the heat (of combustion), thereby lowering coolant temperature." Interpreting their statement, with more advance at idle, the combustion starts on the upper portion of the cylinder on the compression stroke. Flame front travels very slowly at the low cylinder pressure that occur when idling. Therefore the cylinder head and upper cylinder walls will absorb most of the combustion heat and the lower portion of the cylinder on the power stroke will be cooler. This transfers less total heat to the cooling system. In fact, on this 502/502 motor an additional ~ 10 degrees advance occurs at idle when the "Manifold Vacuum" port is connected to the distributor vacuum advance. This provides about 25 degrees advance in total when idling or when cranking! The engine idles considerably faster compared to when the "Ported Vacuum" port is employed. The throttle butterfly's have to be closed about 1/2 turn on the idle adjustment screw to maintain the ~900 RPM idle speed. All else being equal, less butterfly opening means less fuel consumed which equates to less total heat to transfer to the coolant.

Quoting another source; "Ported vacuum showed up when emissions became a factor - before that time, full manifold vacuum was provided to the distributor. Many emissions engines use a thermal vacuum control switch to deliver ported vacuum to the distributor until the engine reached operating temperature, then manifold vacuum to keep the engine cooler at idle."

From another site (on page 2):

http://www.lbfun.com/warehouse/tech_info/t...vance_Specs.pdf

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the
distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times. Ported vacuum
was used as an emissions method control to retard timing at idle (by eliminated vacuum advance) in order to reduce
hydrocarbon emissions.

More info here (page 704):

http://books.google.com/books?id=6VqT6EutH...p;q&f=false

[mrbubblehead]

if you use ported vacuum, as the throttle plate opens you will gradually advance. until you get to the point that you lose vacuum (anything above 1/3 throttle) then the vacuum adv will drop and the centrifugal adv will take over.
which is what most of us are after.

if you use manifold vacuum, with the throttle plate closed, vacuum will be super high and you will have full vacuum adv. then as the throttle plate is opened. vacuum is decreased as well as vacuum adv. which is not desirable.

what most of us want, is that lean cruise and low temp, at up to 1/3 throttle. that is how you get mpg and driveability.

.

[gothspeed]

That works for a lot of cruising/driving conditions, though some cars will spend some of their life in city driving/traffic and that 'cooler idle' with manifold vacuum along with even more fuel economy .... would be quite welcome.

Just putting more info out there, so owners can make a more informed decision (IMG:style_emoticons/default/beerchug.gif) .

[audio_file]

(IMG:style_emoticons/default/icon_bump.gif)

Alright, bumping this because I have a question...
I bought a 205aa distributor for my 2.0 with dual webers (it has an 009 now), and I'm not sure about the vacuum connections. The vacuum can on the 205 has two vac ports, as I understand it, one for advance and one for retard ( (IMG:style_emoticons/default/drooley.gif) (IMG:style_emoticons/default/biggrin.gif) ). What do I do with the retard side, use it or plug it? Where would the retard side plug into? Will this operate as well as an SVDA (assuming both have petronix installed), or should I re-sell this 205 and pick one of those up instead? Do I need to use the vacuum from both carbs, or will one suffice?

TIA
Chris

[markyb]

...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...

Interesting read. But not necessarily germane to the stock 914 engine--very few of these (and only the later ones) had A.I.R. systems ("smog pumps"), while the earlier engines did have ported-vacuum spark advance.

I suppose the Bosch and/or VW engineers may have just been kidding themselves, but it could also be that an SBC engine is not a VW Type IV engine.

--DD

But we're not talking about a "stock" engine after you put dual Webers on it. It seems to me that it calls into question the common wisdom of using a mechanical only advance distributor when switching to carbs. It also seems to point to the distributor as the source of alot of the driveability problems blamed on the Webers.

I doubt that Bosch and VW engineers were fooling themselves, they were doing exactly what the author said. That is engineering for emissions compliance not smooth or efficient running. Also remember the early (pre 75) 1.7, 1.8 and 2.0 engines had dual advance (manifold) and retard (ported) vacumm cans. The 75+ only had the ported retard connected.

Any way, I think I will use the FI distributor with Webers and manifold vaccumm on my soon to be built 2056 and see what happens.

.

later '74 through'75 had both advance and retard, early '74 had retard only

http://bowlsby.net/914/Classic/zTN_FI_Hose...Late1974-75.jpg

[audio_file]

(IMG:style_emoticons/default/sad2.gif) (IMG:style_emoticons/default/help.gif)

(IMG:style_emoticons/default/icon_bump.gif)

Alright, bumping this because I have a question...
I bought a 205aa distributor for my 2.0 with dual webers (it has an 009 now), and I'm not sure about the vacuum connections. The vacuum can on the 205 has two vac ports, as I understand it, one for advance and one for retard ( (IMG:style_emoticons/default/drooley.gif) (IMG:style_emoticons/default/biggrin.gif) ). What do I do with the retard side, use it or plug it? Where would the retard side plug into? Will this operate as well as an SVDA (assuming both have petronix installed), or should I re-sell this 205 and pick one of those up instead? Do I need to use the vacuum from both carbs, or will one suffice?

TIA
Chris

[markyb]

as I recall, the retard port is hooked up to a vac port that is below the throttle plate, having an effect on fuel burn while the throttle plate is closed. There is a thread here regarding svda that may help you, do a search. John at aircooled.net helped me sort out my svda installation with L jet, worth a call or email to him...

[audio_file]

as I recall, the retard port is hooked up to a vac port that is below the throttle plate, having an effect on fuel burn while the throttle plate is closed. There is a thread here regarding svda that may help you, do a search. John at aircooled.net helped me sort out my svda installation with L jet, worth a call or email to him...

Thanks, the question on the SVDA is just for comparison, i.e., will the 205 work as well as an SVDA, based on experiences...
Any more input out there?

[72hardtop]

if you use ported vacuum, as the throttle plate opens you will gradually advance. until you get to the point that you lose vacuum (anything above 1/3 throttle) then the vacuum adv will drop and the centrifugal adv will take over.
which is what most of us are after.

if you use manifold vacuum, with the throttle plate closed, vacuum will be super high and you will have full vacuum adv. then as the throttle plate is opened. vacuum is decreased as well as vacuum adv. which is not desirable.

what most of us want, is that lean cruise and low temp, at up to 1/3 throttle. that is how you get mpg and driveability.

.

Bingo....

Correct.

With an SVDA you do NOT want vacuum with the throttle closed. It (SVDA) is meant to be pulling vacuum above the throttle plate.

One should have 0in vacuum at idle (with SVDA) and 0in vac at WOT or more than roughly 1/4 - 1/3 throttle.

[72hardtop]

That works for a lot of cruising/driving conditions, though some cars will spend some of their life in city driving/traffic and that 'cooler idle' with manifold vacuum along with even more fuel economy .... would be quite welcome.

Just putting more info out there, so owners can make a more informed decision (IMG:style_emoticons/default/beerchug.gif) .

Only issue is....

People don't spend their time driving at idle. We don't tune for idle. It's meaningless. One doesn't attain best MPG's by focusing their tuning on an idling engine.

The only time I can see manifold vacuum being used is possibly for an EFI system that calls for it. But for a single vacuum dual advance distributor? No.

[JFG]

i have twin spanish made brand new 40's on my 1.7 motor and the set up is like this


carb jet sizing

chokes 28mm
a vents 4.5
mains 115
airs 200
em tubes f11
idles 55
pumps 50
n valves 175

I also have an svda 034 with electric points distributor. The carbs do have a vac take off built in to them. I have a line of each which i join just before the distributor with a t piece

The only running issue i have is that sometimes it will stall on approach to a junction after a fast run, like 50+mph but i overcome that with a blip of the throttle first.

I haven't noticed a flat spot, in fact it launches pretty quick of the mark.

[72hardtop]

i have twin spanish made brand new 40's on my 1.7 motor and the set up is like this


carb jet sizing

chokes 28mm
a vents 4.5
mains 115
airs 200
em tubes f11
idles 55
pumps 50
n valves 175

I also have an svda 034 with electric points distributor. The carbs do have a vac take off built in to them. I have a line of each which i join just before the distributor with a t piece

The only running issue i have is that sometimes it will stall on approach to a junction after a fast run, like 50+mph but i overcome that with a blip of the throttle first.

I haven't noticed a flat spot, in fact it launches pretty quick of the mark.

Better to use no T. Pull from the cylinder closest to the distributor (#4). Pulling from more than 1 will likely cause the signal to fall due to them fighting (pulsing) each other.

You're also likely rich with 50 idle jets. Get a set of 47.5's in them and the engine will wake up. If the engine starts rather easy when cold that another sure sign you're rich.

For 1.7:

28mm vents
F11 tubes
47.5 idle jets
200 air corrector jets
115 main (maybe even 120)
Fuel pressure 3.5lbs max
Float height 10mm (gasket in place ball not depressed)
Float drop 28-32mm

[porschetub]

Read on the Samba or maybe somewhere else the SVDA isn't very happy with a high lift cam on carbs,something to do with vacuum issues,I don't know but it kinda makes sense,the latter Dellorto's only came with a vacuum pickup on one of the two carbs,not sure about the ones CB imported a while back,but with further research vacuum off one carb will do the job however have no experience of that.
Anyone on here have their svda running off one vacuum point??.
Issues with the latter Bosch distributers are about 80% of used ones have gummed up mechanical advance plates,the end result is normally flats spots and a slow return to the correct idle ,or in some cases little or no advance.
Interesting thread seeing what people have experienced....keep it going (IMG:style_emoticons/default/beer.gif)