TheSamba.com :: View topic - Solex Dual 34 carbs, Is there a vaccum port?

Fri Dec 22, 2023 11:58 am

Hi,

I've got a dual Solext 34 PCI set up on my 65 1200 bug.

Runs well, but it does have a flat spot at low throttle. Currently running a 010 distributor. I've checked the advance and it works OK, going from 10degrees at idle to 32 degrees at 3000rpm.

I think I need to try a SVDA type distributor which will hopefully fix the issue.

I took the carbs off today, and can't find an obvious vacuum port. I've heard on other threads that using the vaccum from the manifold won't work. (There is a port for the balance tube on the manifold)

I wonder if it's possible with these carbs. Any advice welcome. Some pictures of the LH carb (RH looks the same).

Fri Dec 22, 2023 2:19 pm

Hello.
Well, you have made the classic mistake in trying to make a time typical tuning.
34´s are too large for a 1200. On the other hand 32 PCI´s´s are sumbitches to dial in, so I guess thats achoice between the plague or the cholera.
Second, you have chosen the classic 010 dizzy which is about the stupidest of the old distributors, unless you have a high compression dual carb engine with not too much cam in it. When will people learn...
Your 34´s do rightly not have provisions for vacum outlet, but they have the casting for it. Look at your third picture beside- and a hair above the throttle shaft. If you have the heart for it you can drill and tap the carb for an outlet there. Or you have to live without.

Now, how can we correct it (?)
If we assume that you do not want to drill and tap the carb with the risk of messing up., here are a few things that can be done.
For your own sake and blood pressure levels you should probably replace the venturis with a set of 23 or 24 mm at the most.
Then we need to adress the intake scavenging at lower rpms. One easy way to reduce that is by doing the - in my book - classic split valve lift. You keep the 1200 1-1 rockers on the exh. and use the later 13-1600 1,1-1 rockers on the intake. This helps the intake charge to over shoot the exhaust valve right at tdc.
The easier route would be to get a 123 BT distributor and then drill & tap the manifold for vacum. Then it is reasonably simple to set up a timing- and advance curve for such an engine.
If you WANT to stay old school you can replace the 010 with a 019. Then you can get the added timing at idle and lower rpms. This will help a good deal. - Enough(?) maybe. You probably need to jet it extra rich to mask the issue best possible.
If you decide to drill & tap the carb,- and get it right, you can use just about any SVDA except the 205 AL and a couple more that I don´t recall the numbers on. You will still have to modify the timing curve to suit that engine. But its doable.

Fri Dec 22, 2023 3:11 pm

Alstrup wrote: Hello.
Well, you have made the classic mistake in trying to make a time typical tuning.
34´s are too large for a 1200. On the other hand 32 PCI´s´s are sumbitches to dial in, so I guess thats achoice between the plague or the cholera.
Second, you have chosen the classic 010 dizzy which is about the stupidest of the old distributors, unless you have a high compression dual carb engine with not too much cam in it. When will people learn...
Your 34´s do rightly not have provisions for vacum outlet, but they have the casting for it. Look at your third picture beside- and a hair above the throttle shaft. If you have the heart for it you can drill and tap the carb for an outlet there. Or you have to live without.

Now, how can we correct it (?)
If we assume that you do not want to drill and tap the carb with the risk of messing up., here are a few things that can be done.
For your own sake and blood pressure levels you should probably replace the venturis with a set of 23 or 24 mm at the most.
Then we need to adress the intake scavenging at lower rpms. One easy way to reduce that is by doing the - in my book - classic split valve lift. You keep the 1200 1-1 rockers on the exh. and use the later 13-1600 1,1-1 rockers on the intake. This helps the intake charge to over shoot the exhaust valve right at tdc.
The easier route would be to get a 123 BT distributor and then drill & tap the manifold for vacum. Then it is reasonably simple to set up a timing- and advance curve for such an engine.
If you WANT to stay old school you can replace the 010 with a 019. Then you can get the added timing at idle and lower rpms. This will help a good deal. - Enough(?) maybe. You probably need to jet it extra rich to mask the issue best possible.
If you decide to drill & tap the carb,- and get it right, you can use just about any SVDA except the 205 AL and a couple more that I don´t recall the numbers on. You will still have to modify the timing curve to suit that engine. But its doable.

Thanks for the reply, All this came on the car. It's not my first choice!

I like the idea of the 123 BT, I have that on my 1776 Ghia and that goes really well without any hesitation, but it's a very different motor. But I can take it out of the Ghia easy enough to try.

It seems quite powerful for a 1200 (Maybe it has bigger pistons? Feels just as torquey as my old 1500) , but it's either on the gas and it goes fine, at light throttle is likes to stumble. I think it's running a bit rich too given the black plugs I just got out.

I'm mostly interested in making it drive nice, at present it's hard to enjoy around town, but fine on the open road.

So if I tried the 123 BT, would I be able to get the vaccum signal by putting in a T on the balance tube? Or need another drilling? That sounds like an easy starting point.

My other option in take it back to a stock carb / exhaust and distribtor and sell the Reichert Kit and the 010.

I'll measure the venturi's, no idea what it has.

Thanks so much for the advice.

Fri Dec 22, 2023 4:10 pm

Fair enough.
In fact, I would try and block the balance tube for starters. It may give you a slightly rougher idle, but we can adress that later. (Reason being that the balance tube effectively makes the engine thing the tthrottle body´s are larger than they really are at idle and low speed) But you can easily take the vacum off of that. One carb is fine.
But no matter what you should still do the split lift thing. While it sounds like a minor thing it will help you quite a bit.

1200´s with dual carbs tend to do just that, - pull "a lot" of lower rpm torque.

Fri Dec 22, 2023 4:29 pm

Hi,

OK that sounds like a resonable line of attack.

I'll borrow the 123 from the Ghia for now.
I'm guessing I need the vaccum signal not to do much until RPM goes above idle?

Run the vacuum from the current balance pipe fitting on one manifold to the 123, block the other.

Switch the inlet rockers to the 1300/1600 style.
So I could buy a used set of stock 1300/1600 rocker arms and replace just the inlet ones on my current set. The rest of the rocker assembly stays the same?

I'll give that a try and let you know how it pans out.

Oh I forgot to mention, it has a stock exhaust with TDE tailpipes.

Thanks :thumbsup:

Fri Dec 22, 2023 4:34 pm

jaket3 wrote: Hi,

OK that sounds like a resonable line of attack.

I'll borrow the 123 from the Ghia for now.
I'm guessing I need the vaccum signal not to do much until RPM goes above idle?
Yes. Set it to be active from 1300 for starters. You will also likely have to increase the drop off point to something like 87- 90 Kpa due to the relatively large butterfly for the displacement.

Run the vacuum from the current balance pipe fitting on one manifold to the 123, block the other.
Yes. Just remember to advance idle timing for the dual sgl. carb set up.

Switch the inlet rockers to the 1300/1600 style.
So I could buy a used set of stock 1300/1600 rocker arms and replace just the inlet ones on my current set. The rest of the rocker assembly stays the same?
Yes

I'll give that a try and let you know how it pans out.

Thanks :thumbsup:

Sat Dec 23, 2023 12:56 pm

Regarding Venturis

I measured the venturis and I think they are around 28mm internal diameter.

See picture below.

If I was to change to a smaller Venturi where could I get them from? I did a search but couldn't find any suppliers.

Thanks

Sat Dec 23, 2023 2:27 pm

Look for NSU enthusiasts. there are stock 25 mm venturis for the NSU 1000 and the TT.
Or you can buy a set of 24 mm venturis for Solex 32 PBIC. Dependant on the exact model of 34 PCI you may need to modify it everso slightly.
https://www.csp-shop.com/en/brand-shops/main-venturi-129-337-3224-31764b.html

Sun Dec 24, 2023 12:31 pm

I'd start with the smaller venturies before you throw any other money at it. As Alstrup has pointed out, your carbs are for the 1300/1500/1600s. Get your airspeed up with smaller vents first.

I bought my Reichert complete carb set directly from Herr Reichert 10 years ago, and have run it with an 010 (rebuilt by Glenn Ring) on both a 1300 and 1600. It runs as smoooooooth as a fuel injected car. And that's with an 8 lb (!) flywheel, which will not forgive poor tuning. A great carb kit with a great linkage - zero fuss carbs in my experience.

Look at Reichert's site, as I believe the 1200 carb specs are listed there. For example, for a 1300 singleport, he recommends the following:

28 mm venturi
140 main jet
180 air correction
17mm air nozzle
G50 idle jet
Injection quantity = .4ml per stroke (check this!)

The only differences for a 1600 singleport are as follows:

142.5 main jet
175 air correction

Yours will be different with a smaller vent, but you should be able to find the 1200 specs to get you closer.

FYI, Reichert PCI carbs were the NSU Prinz version, while TDE's PCIs were the BMW 700 version.

A few other minor tweaks that helped:

1. Phenolic spacers between the manifolds and carbs for hot weather (I put those on all dual carbs)
2. Added 2 inch rubber stacks to raise the air cleaners, acting as a velocity stack (you shouldn't need to do that because you're running VW oil baths that already have that built in)
3. Pressurized heat from the driver's side heaterbox vent to the air filters in winter to help with fuel atomization

My 1.5 cents. Good luck.

John

Sun Dec 24, 2023 8:31 pm

Guys- my only input is the correct spelling (V-Dub Guy gallery)-
German pronunciation sounds like REE-shirt, not RYE-shirt as it would if spelled your way.

:wink:

Mon Dec 25, 2023 7:20 am

Check with Alfa1750 on Ebay for parts. He had everything I needed for my PBIC's.

Merry Christmas!
d

Mon Feb 12, 2024 1:29 pm

Thought I'd do a quick update.

As suggested by Alstrup, I have so far:

Removed / blanked off the manifold balance pipe. Ran a vaccum line from one manifold to the new distributor.

Fitted a 123 BT distributor. Picked a map which idles at 5 btdc, and at 3000rpm is at 30 degrees.

What a difference. Idles super smooth. Flat spot vanished, it's a joy to drive.

I still need to:
Get some smaller venturis (Still can't find any, but on the lookout)
Fit the 1.1 inlet rockers (Got them, just need to find the time)
Fine tune the manifold vaccum signal. Any suggestions here welcomed as I'm using manifold vaccum and I've really no idea what to do here, just got the default values.

FYI on the 123 I timed it to 5degrees at 850rpm

Then the Centrifugal Curve is:
500 rpm. 0 (So total timing is 0 plus 5, 5 degrees)
1000rpm 0 (So total timing is 0 plus 5, 5 degrees)
3000rpm 25 (So total timing is 0 plus 25, 30 degrees)

MAP Curve
1 0Kp 0 degrees
2 29Kpa 0 degrees
3 30Kpa 10 degrees
4 50Kpa 10 degrees
5 87Kpa 0 degrees
6 200Kpa 0
7 300Kpa 0

Thanks