Sub-topics -
Dave wrote to Rob regarding the vacuum lines on a dual vacuum distributor -
I've never understood completely which vacuum line went where. I think I've got the vacuum lines hooked up backwards! Let me make sure I have this straight:
Question -
Is there a way to determine whether the retard vacuum line is inoperative?
Rob responded -
Yes -- take the cap off the distributor and suck on each of the lines in turn. You should see the points plate move in opposite directions. The retard line will only retard a little of course, compared to the main vacuum's advance.
Question -
If the retard vacuum line is malfunctioning, what is the fix?
Rob responded -
First see if the hole in the carburetor is clear -- put the vacuum line back on it and blow from the dizzy end to see it you get air through. If it's good, the most likely cause is a broken/leaking vacuum chamber on the dizzy. The retard line only works (when it's working) at idle anyway, so it's less critical than the (working) advance line.
The main vacuum line gets quite a lot of vacuum with the engine running, and may cope with a small leak, but I think the retard vacuum probably operates at a lower pressure difference (engine idling anyway), so would be more susceptible. You can check for a leaky diaphragm by sucking on the main vacuum line, then block the line with your tongue and see if the points plate stays put. If it slowly moves back then you have a leak.
If you do end having to replace the vacuum chamber, they are pricey (here they cost more than a whole 009), and (on the single vacuum units like mine anyway) you have to check for a number on the vacuum operating arm inside the dizzy (the pushrod). This identifies the exact vacuum chamber for that distributor (there are several versions at least for the single vacuum units).
From Rob -
Try this... put the timing light on it and get is up to a steady speed where you see some advance but not all of it say 1500-2000. Now blip the throttle open and let it go so the revs don't change much but the load does. If the vacuum line is operating at all you should see the advance change momentarily then return to whatever it was. If the advance vacuum line is not working, the advance should stay reasonably steady -- only changing with the revs. If this is the case, the dizzy is definitely a dual advance (vac/centrifugal) operating like a 009 (centrifugal only), and this would explain the hesitation you are still getting. These dizzies are supposed to provide the best of both worlds for the VW if you can get it working right.
Dave wrote -
We went out and conducted the test you suggested this evening. Here are the results:
We have already established that this distributor is a combination centrifugal/vacuum advance model.
I hope these results mean something to you that you can then share with us so we can improve the around-town performance of this car.
Rob responded -
You have said that the retard line DOES work (that's the front line - nearest to the distributor body). So since the engine is off when you do it, then contrary to John's reply, it HAS to be retarding from neutral.
And there is only a tiny amount of suction in the advance line at idle, so how is the retard line supposed to be "only working to reduce the main advance". That doesn't make sense to me. Sure the two vacs work against each other as you open the throttle - the retard vac drops away because it works at the edge of the throttle plate which is now moving away (opening), and at the same time the airflow through the main carby venturi is increasing which increases the main vac, so the retard drops out and the main vac pulls in some advance.
Dave wrote -
So it is my understanding that the retard line works to pull in retard first, then the main vacuum takes over as you open the throttle.
Rob responded -
On my vacuum-only dizzy, the plate moves counterclockwise to produce advance (the vac arm pulls outwards). The retard line pushes the arm inwards and rotates the plate clockwise (same direction as the rotor moves). The RETARD pipe on the canister is the FRONT one - nearest the distributor body. The ADVANCE line is on the REAR (outer) side of the canister.
Another test to check it all out -
Try running the engine at 3000-3500 and check the timing. It should be AT LEAST 30 degrees or more (30 degrees if only the centrifugal is working - more if the vac advance is working too). If it's less than 30 degrees, disconnect both vac lines at the carby and plug them, then adjust the timing for 30 degrees at 3000rpm (this means you are setting it using just the centrifugal advance), then reconnect both vac lines.
Now drop the revs back to idle and look at the timing again. Is it 5ATDC or thereabouts? If so, the retard vac line is working OK. And if it is working, as you gently open the throttle with the timing light still on, the timing should change rapidly from 5ATDC to some positive figure as the retard line stops working.
But if its somewhere from 0 to 10BTDC at idle, my guess is that it's the RETARD line which is not working. I say 0-10 because I don't know exactly what the "neutral" plate position is on these dizzies.
This is just a test set-up, and I haven't tried it myself as I don't have the same distributor, but it might help determine which vac line is actually working and which isn't.
You'd need to re-time the engine properly after doing this test (5ATDC at idle with vac line properly connected.)
Dave asked, regarding disconnect of the vacuum hose(s) during timing -
The manuals don't clarify which end of the hose is to be disconnected from the distributor and plugged -- the point is to keep the carburetor from sucking air. Right?
Rob responded -
Precisely. The aim is two fold:
It doesn't matter how the carby's plugged ( on the carby on the other end of the vac line) so long as it's the carby which is plugged, not the distributor.
Dave wrote -
The book is not specific -- it just says "plug the line". And so I did -- on the wrong end!
Rob responded -
A nice laugh. That's what we are trying to do with the Joe Shadetree aren't we -- avoid ANY ambiguity.
A final thought from Rob -
If I ever need to replace my distributor, I'll probably be going for one of these SVDA types too. I suspect it will work just a little better than the vac-only unit I have at present. The only concern I have with them for my car is that the rate of centrifugal advance (the 009 part of it) may be slightly different to the vac-only rate of advance at each point in the rev range, which may change the engine acceleration characteristics a little, since it's a single port 1600, with different head/gas dynamics to the tp1600. And the 30PICT/2 carby produces a different flow rate through the venturi than the larger throat 34. It will be interesting to find out if any difference is noticeable.
Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.
We hope you find this information useful, but we don't take any responsibility for anything which happens to you, other people, your VW or any other property or goods resulting from your use of this material.
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Last revised 6 May 2004.
