jetting help, 35mm pdsit

[platatomi]

Hello everyone, I have a set of scat 35mm pdsit carbs that I bought new, and I can't help but think they should be running much better than they are.

Here are my observations:

Poor MPG, 16-18 on a good day.
Sluggish acceleration when hot.
Lag/Stumble around 2000-2500 rpm or so, especially after overrun, sometimes a cough here two.

The carbs came set up for my engine, 1600 dual port with single quiet pack, 1000ft elevation.

When I received them, the carbs had these specs:

*27mm venturi
*50 idle
*140 mains
*then the two jets under in between the bowl and throat, I think one is air, not sure which is which, are marked 80, and 100 on each carb.

Other checks:

*Fuel pressure is a little over 2 pounds, but I am confident that the needle valves are not leaking because the fuel line will hold pressure for a long time after shutting off engine.

*No vacuum leaks as far as I can tell.
*Timing tried setting from 28* to 32* btdc, no difference.

As these carbs seem to be a little unusual, I haven't been able to find any examples of the jetting people are running in these with similar engine specs, so I just want your input on whether things are fine as is, or what I should do to make improvements.

Today I took a first step by installing 135 mains to see what happens. I did this by reasoning that these carbs are close in size to kadrons, (27mm vent vs 28 ), so the jetting may be close too. I hear kads usually run around 130 mains. Is this sound reasoning?

I went for a test run with the 135 mains and nothing seemed worse, perhaps a little better in the top end, so maybe I'm going in the right direction.
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'69 Beetle daily driver, fixing things as I go...

Update -

No longer daily driver but still fixing things...

[NUG637]

I to am running these carbs on a 1699cc (reasonably stock 1600 top end with a 74mm stroker crank) same jets were supplied - 50 idles, 140 main on a 27mm venturi. i had a really awful crackle on take off which i put down to a lean idle circuit. i went to 55 idles and it went away…. tried 60 idles and there was no noticeable difference so i went back to 55 idles and it seems happy… I'm getting horrible fuel econy to…. but I'm pushing a lot of weight and windage

i only just finished building the car and the engine so I'm still learning whats normal but i feel the miles per gallon are a little extreme and was gonna drop down in main jet sizes as these are very similar to a type3 carb and they came with 132 mains from memory? so 140's could be on the big side?

I'm running a german 009 and have a killer flat spot during take off…. if not enough gas is given it will pop and stutter and act like a real pig which I'm trying to tune out…. hoping to go to an SVDA dizzy soon… hoping that might help the stumble off idle…

keep me informed if you make any progress and ill do the same…. i haven't given up on these carbs yet but not raving about them either

[platatomi]

[smitty24]

You guys should not be getting that type of crap mileage with those! They are basically smaller Kadrons. I used to have 2 sets of them...the ones Scat produced with their own linkage/manifolds. They do awesome when setup correctly. The SVDA is imperative...and a good one, not a Pertronix or Hot spark, etc. Ive used most of the ones out there. With a 1641 with headwork and Engle 110, I was getting 32-34 mpg freeway and 27-30 in town with the svda.

The 35s do better with the balance tube. Scat didn't do that back years ago, so I had to drill and tap intakes. Jetting is critical. I can't recall what I used at sea level. Fuel pressure MUST be 1.5= NO MORE.

The good about these carbs= similar to Kads. Easy to tune, small, good mpg, good performance gain.

The bad= one 1 or 2 people sell parts for them anymore. Aluminum linkage changes with heat. Parts are not cheap. Carbs cost too much. You can buy something better for same price. The 35s also like to idle on 2 cylinders. Some people chase this for weeks, thinking something is wrong.

I have bad throttle shafts after 2 years of use. Got some new ones from John Connolly. They are good for economy small motors. I wouldn't use them on anything built. Kads are even a better option with the parts availability and lower prices out there. I use Webers now.

[smitty24]

Platatomi,

While changing jetting, I would also check timing, idle speed, carb sync. You have to check and adjust accel squirts on these carbs as well. They need a lot of playing to get them right. Both sets of mine were way off from Solex/Scat. Also check and adjust chokes...there are many adjustments on these carbs. I wouldn't use them again. There are many many write ups on these carbs in the forums on how to do this stuff. When these first came out, tons of people bought them up and had issues.

Chico used to love these carbs and put them on his "mileage" 1776s or whatever the hell they were. They all had Engle 110s and 35 Pdsits...due to better economy and tunability to Kadrons or something. Problem is, Kads are easier to work on, cheaper, more common, and many more people know how to work on them.

[PumaVW79]

[platatomi]

Sounds like I have my work cut out for me, I will start by putting in a fuel pressure regulator and check again for vacuum leaks.
_________________
'69 Beetle daily driver, fixing things as I go...

Update -

No longer daily driver but still fixing things...

[NUG637]

They come with a lot bigger jets from scat, I bought my carbs online but got my jets that john from ACN recommend at my sea level and went one size bigger because of the crackle.

I drilled and tapped mine for a balance tube to witch I "T" pieced for my brake booster vac line. Timing is set to 30Deg full advance and about 9-10 at idle. Both carbs synced to 9 on the snail guage. It's not pulling vac at the carbs at idle and my fuel pressure is set to 1.9 max! As they say under 2psi. I checked it all looking to fix this horrid stumble off take off. The stumble you speak of is exactly the same as mine... If you ride the clutch or ease the throttle into it, it's fine but defenly no launching off the lights otherwise it bogs and pops and farts... I backed my accel pumps right out... Have pulled both carbs down twice to make sure there was no blockages... And both times clean as a whistle... Not the first time I've played with dual single throat carbs and am getting 32miles per gallon on my other car running dual single throats but in a beetle. As I said a lot less weight and a lot less windage to the bus.

I had the rubber "o" ring on the idle screw tare on one of mine which made for tuning the idles mixture difficult until I noticed and replaced both, they then became really responsive to tune....

As mentioned earlier I want to try a svda from ACN to see if the bigger advance from vac fixes the stumble off take off... Other then that I'm all out of ideas as to why it stumbles and why my mileage is so bad.

[smitty24]

repost

[smitty24]

[platatomi]

As far as the distributor goes, first tried running my svda that I had already been using with the old 34pict3 for months. After I installed the new carbs, connected the vac line, and started the engine, no more advance. Something siezed up inside the distributor and it would not advance at all. I have tried taking it apart and don't know what I'm doing so haven't been able to figure out what happened. It was a pertronix which may be part of it, but if I were to spend $200 on a good svda, how do I know the same thing won't happen?
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'69 Beetle daily driver, fixing things as I go...

Update -

No longer daily driver but still fixing things...

[[email protected]]

You are driving a 40-50yo car. If you have to pay someone else to work on it you better be a millionaire.

VWs are high maintenance cars, you need to learn to do this stuff yourself. You even have to do this to NEW parts, to make sure they are operating properly out of the box.
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[platatomi]

Of course if something is not right or setup correctly, it is up to me to fix it, replace it, or live with it. That is why I bought this car, because I can save a bunch of money doing my own work. I have daily driven this for more than two years, and the only garage it's been in is mine.


In the mean time, anyone want to go though the svda with me? Sounds like that would be a huge help to get working.

When I suck on the vacuum can hose fitting, I should see a response in the diaphragm/advance mechanism, right? That thing is stiff as a board!
_________________
'69 Beetle daily driver, fixing things as I go...

Update -

No longer daily driver but still fixing things...

[NUG637]

[smitty24]

You never mentioned what the needle and seat are doing. These carbs had issues with these. Many of us had to trim the gasket to allow the float to also rise and not get caught. I had to also add a copper shim to the seat base. Im not talking out my ass here, this is all stuff that has to be checked. That is one of the reasons I moved on to real carbs. I hated all the crap you have to do with these.

One thing a lot of people never figured out was that the Scat linkage was aluminum and would expand when hot. You could set the carbs and have everything perfect...connect linkage and suddenly have them off, due to temps.

The Svda does help, but it doesn't solve everything. I had an 009 with mine for 3 years. It was tuned in a manner to where the flat spot off idle was not noticeable, and I had a Engle 110 on there!

[NUG637]

you are correct I'm not 100% on the the needle and seat. and that gives me something else to check along with the float valve but i can't see flooding or simp tons of sticking? Something else i could check.

i agree about moving onto real carbs… thinking that might be in the near future as I've got to to somewhere really drivable and it drives nice but the stumble off "launch" is still there…. appreciate the reply as 2 heads are better then one…. just getting abit fed up with these things…. half temped to go back to a single

[platatomi]

[smitty24]

yeah bro, I got mine to work pretty well, but after WEEKS of tuning and a lot of experimenting, and more money thrown at them. I would never buy these carbs. I had them because they came on a bug I bought. The second pair was a set a friend gave me to fix. They are ok carbs when they are finally right, similar to Kads. They are a nice improvement over a stock Solex! I DO like the fact that they work nicely with Svda and still get good mileage.

When it comes down to it, I won't pay $500-600 for any twin single barrel carb setup. You can have IDF for the same price.

I can't remember what you tried already. These are a dog if the accelerator squirts are not right, or aimed correctly for that matter. Is your choke set correctly? Is it closed when cold and fully open when warmed up? If it were my engine and it was stumbling like that, I would try a larger idle as an experiment, then play with squirt and timing. Svda is nice with these, but not a must. I would fill the carb with gas and blow into line to see if it is closing or sticking. If my memory is right, there was a guy with a write up years ago, stating there was an actual measurement that was vital for that float and needle valve. Damn, all this is making me remember why I wanted to get rid of those so bad!! Sorry for your troubles man, both of you! Perseverance.

[smitty24]

One issue was that the float would occasionally hit the paper gasket and not allow needle to close. Some kits the needle didn't even touch the float until your carbs were already flooding out. (hence the extra spacing that guys would do with washers back then, or different needle/seats altogether). I trimmed quite a few gaskets for these carbs and also made some for other people. Solex did them either wrong or ??

Don't buy another Pertronix Svda. They are NOT how they used to be. The ones now are total crap. I won't use them anymore. I went through 3 last year before I could get one that was right. The German 009s may not be as good as an 034, 010, etc; but they are predictable and don't just take a crap, even with electronics. They are also cheap, due to everyone thinking they need an 010 or 034 now. With timing/jetting/ tuning, you can get the 35 Pdsits to run right with an 009 and be satisfied. The Pertronix Svda is straight from China. At least the old 009s everyone has are German.

Have you tried tuning with and without the balance tube to see any difference?

[platatomi]

smitty -


No balance tube, the manifolds are not tapped for it. I heard that you can just run a line straight from one vacuum fitting to the other on the carbs, like a mini balance tube, but I have no idea if that does anything. Should I give this a try?

I am convinced the needle valves are good, because fuel pressure is maintained after shutting off the engine. If the valves were leaking, it would just drop to 0 instantly after the engine stopped.


So you are saying that if I got a quality mechanical-only distributor, even a German 009, I would still see mileage and performance comparable to an SVDA?
_________________
'69 Beetle daily driver, fixing things as I go...

Update -

No longer daily driver but still fixing things...