Troubleshooting a dual-vacuum distributor

[HalHaygood]

Hey there, folks. I was referred to this forum by Bob Ingman, who has provided some very valuable assistance by email. I'm working on something and thought I would ask you folks about your thoughts.

I've been working through some issues with cold stalling (looks like a tired choke element) and overheating (looks like timing). I finally went out a got myself a strobo timing light, and have been, well, enlightened. It looks like my overheating was because my timing was at about 5btdc at idle.

However, when playing around checking the max advance, I noticed two things. First, there's now a bad hesitation off idle, and second, it takes a very long time (10-15 seconds) to return to the idle retard setting. It stays partially advanced for a while, then slowly drops back down to the 5atdc mark. (If it helps, my idle is set about 900, and it idles at about 1200 while this is happening.)

So I see two possibilities. One is that the vacuum retard is not doing its job very well. If this is true, do I need a whole new distributor, or can I just replace the retard component?

The other is that there's something that's causing a sticky idle, and the advance is just going along for the ride. What might cause this? A sticky throttle butterfly, maybe?

Thanks for your help!

Hal

[HalHaygood]

Here's one other weird thing. Nothing happens, timing-wise, if I remove the vacuum retard line. <http://www.geocities.com/motorcity/lane ... tml#timing> says that the mark should move 15-18mm to the left, but it doesn't move at all.

Hal

[Bob Ingman]

Hal there are some very knowlegable guys here at STF that will guide you through this issue. Week-ends are slow though.
One thing you can do yourself is check manually to see if the advance and retard are working properly.
The hose on the inside of the vac canister (pod) is the retard. If you remove the dist cap and rotor you can suck on that hose and see if the plate in the dist moves. It should. Likewise suck on the outer hose and observe that there is movement there and that the plate is advancing. Again it should.
Lack of power/overheating are classic symptoms of timing either advanced or retarded. If your timing light reads total advance set the timing at 28-30 degrees BTDC @ 2500 RPM and let the initial fall where it will. At that point you can begin to dial in the carb adjustment and idle. Good to see you over here. Bob Ingman

[HalHaygood]

Sucking on the line that attaches to the canister port that faces the rear of the car (that's the advance, right?) does move the points plate. Sucking on the other line does nothing. So it does sound like the retard stuff isn't working. I checked out that dual-vacuum pot, and it's like $60 to replace! Ouch.

[Bob Ingman]

Hal, I have contacted Marc V for help on this one. He knows about these things. Maybe the unit can be serviced rather than being replaced.
First of all be sure the plate in the distributor is moving with your fingers before assuming it is the pod. Most likely scenerio is plate sticking rather than BOTH advance and retard falfunctioning concurrently. Main thing is don`t drive car until the dist is functioning correctly. Excessive heat is so damaging to the engine.
I would not give $50 for a pod when you can get a dist for $99
www.CIP1.com Cheers. Bob Ingman

[Marc]

A leaking retard side is the most-common failure with these distributors in my experience. There are several models that have similar characteristics so you may be able to graft on a good used unit if you can find one - typically the mounting screw-hole locations are slightly different and you'll need to redrill one or both to make the "wrong" unit fit.
As Bob has pointed out, you can get a fresh `74-style SVDA distributor for not much more than the cost of a new DVDA canister alone (or search the boneyard for a VW #043 905 205/Bosch#0 231 170 034).
Interestingly, the advance curve of the SVDA is not much different than that of the DVDA when the retard side is inoperable. I'd try plugging off the hose to it to stop the induction leak you have now and setting the timing to ~7° BTDC at ~875RPM with the advance hose connected. Your total timing at high RPM/light load should fall around 42°. You may find that this works well enough and you can't complain about the price

[Bob Ingman]

Thanks Marc. Cheers. Bob Ingman

[HalHaygood]

Thanks a bunch, Marc! I won't get a chance to test this out until tomorrow, but it sure sounds logical.

The only question that I have is, isn't 42 degrees a whole lot of advance? I've only ever heard of 28-32 degrees btdc as a max timing setting.

Hal

[Marc]

The reason you hear 28-32° mentioned as a maximum so often is because it's the safe limit in most cases with an 009 centrifugal-advance distributor. All the advance is in by 2000-2500RPM, and you wouldn't want more at that low of an engine speed where you'd be likely to floor the throttle at any moment. A vacuum advance mechanism can respond to engine load - it'll give more timing at high speed/light load which is good for cruising fuel economy and may help your engine run cooler on the highway, and yet it will cut back automatically to a safe level under load.
Check out the specs for the "SVDA" (49-state 4-speed 1974):
http://www.oldvolkshome.com/ignition.htm#A1974MFD
It's set at 7.5° initial and can have from 28 to 37° in addition to that. Without the vacuum hose it's like an 009 with a stiffer spring (you won't see full centrifugal advance until ~3500 or so); the vacuum-advance component rides on top of that curve and helps to tailor the total advance more closely to what the engine actually needs under varying conditions.

42° at 4000RPM in the driveway is safer than 32° when you floor it up a hill at 50MPH in 4th.

[Bob Ingman]

Hal if you find that you need a dist or pod I think Bobby Riggs will have one. You can contact him at [email protected] . He has a lot of stuff and should have what you need. Good luck. Bob Ingman

[HalHaygood]

Okay, I've finally gotten to this. I timed the engine to about 7 degrees BTDC at idle, with the retard port plugged. Total advance fell somewhere around 35-37 degrees BTDC, if I recall correctly.

It ran great in the driveway. When I drove it to work today, it started overheating pretty quickly (less than 10 minutes) and I had to be very light on the throttle to keep it from pinging while I limped in to work.

So it seems to me that I actually need a new distributor. Will the stock SVDA (043-905-205) that cip1.com has for $99 work well on my engine? (It's an AE case; the PO said it was an 1800, but it feels like it's stock to me. Carb is a 34/PICT 3.)

Hal

[Bob Ingman]

Hal if you back off on the timing to 0 @tdc you will have about 28-30 degs advance. That should eliminate the overheating. The pinging is a sign of advanced timing which also causes overheating.
The single advance dist will work fine on your engine. The vacume retard is an emmissions device to comply with regulations at time of manufacture. Everyone always disconnected that except to be smogged in Ca.
Also check at www.socalautoparts.com for price on distributors. Keep us informed. Bob Ingman

[HalHaygood]

Just wanted to follow up on this. I went with a new SVDA from CIP1. After timing it correctly, it's running great now. Thanks!

[Bob Ingman]

Hal good to hear that it worked out for you. Don`t be such a stranger here. Cheers. Bob Ingman

[scottioc]

OK I have a very similar problem. I have a 75 fuelie with dvda distributor. The canister has gone out, so the VW mechanic in my area set the timing to match an svda but now the thing overheats all the time. So, I'm not sure what to do here. Do I switch to an svda, buy a canister I'm at a loss.