1600dp DVDA odd timing issue

[asiab3]

Greetings all,

Trying to go for a nice drive while the weather is still nice here.

I did 1600 miles with this DVDA (205Q) in my bus this summer with nay an issue. One day the idle timing decided to jump back to 30*ATDC. I was away from home so I put in my 009 and went on my way. I took the DVDA apart and cleaned and lubricated it and inspected for wear, but it looked identical to 6 months ago when I first rebuilt it. Today I managed to get my hands on another vacuum can, even though the old one held vacuum with a tester. Both are the 853 cans with dimple from a stock '71 bus. Same issue. I've posted a video below, but here is the verbose version.

Static timed to TDC engine off, fired up fine. Timed to 28-29* max, hoses off. Idle falls at 8*ATDC. Perfect, it has always idled around 8-10*ATDC, just like the book wants. Several seconds later, the idle waivers for a bit and the engine dogs down to 600RPM or so. I fired up the timing light and see the idle timing has drifted down to 20-30*ATDC! When I rev the engine to engage the vacuum advance, the timing picks up to TDC at 2000rpm, then gains a substantial amount of power as everything returns to normal timing. IE: 35+* at creasing rpm, no load, 29* max still hoses off.

I do not feel any play in any of the point plate components. Anybody ever seen this before? I've got a long drive tonight and I was looking forward to the vacuum distributor low-end torque. This 009 is an old German unit, but still leaves a little to be desired.

What on earth could cause this?

Robbie-Not-So-Advanced Now Are We?


[youtube]http://www.youtube.com/watch?v=hrn94CP1GaI[/youtube]


Update: I have not been able to feel this play before, but I think it may have been the cause of my troubles. Thoughts?

[youtube]http://www.youtube.com/watch?v=716oFXQTNog[/youtube]

[Amskeptic]

What on earth could cause this?
Robbie-Not-So-Advanced Now Are We?

This does not strike me as a vacuum plate issue so much as a centrifugal advance sticking issue.

Experiment:

Take off distributor cap and grab the rotor. It should twist clockwise and *spring back counter-clockwise by itself*.

So twist it clockwise and gently let go. Does it just stay there? Does it go back only a little?

Now force it back counterclockwise. Does it go through a resistance and then stop?

When you did your lubrication, did you separate the two shafts, the drive shaft below and the breaker cam shaft above to clean and lubricate? The one with the diabolical little c-clip retainer under the felt?
Colin

[asiab3]

Ah, I see how that could cause this. That sounds like the perfect cause for exactly what the symptoms are. I went through the distributor and documented it here. I had two other cores for fiber washer donations. Here is what I did:

http://www.thesamba.com/vw/forum/viewto ... light=dvda

I remember the rotor snapping right back 6 months ago, but I didn't check that when the problem first happened, or when I went to reinstall it. Have you ever seen an advance spring fall off in your travels? (Or I guess on a customers?) This is the road that the problem first happened after :)

[asiab3]

Hmmm the mystery continues to perplex me.

[youtube]http://www.youtube.com/watch?v=4wKUIV5sygE[/youtube]

No rotor, hope you don't mind. It's cursing down PCH in a '74 super that I found smoking out the deck lid on the side of the road about an hour ago. ;)

[Amskeptic]

Ah, I see how that could cause this. Have you ever seen an advance spring fall off in your travels?
This is the road .

I did. It's right there. There. Over there. RIGHT THERE, next to the thingy the is that a tree? I was going to tell you.
Colin

[Amskeptic]

Hmmm the mystery continues to perplex me.
No rotor, hope you don't mind. It's cursing down PCH in a '74 super that I found smoking out the deck lid on the side of the road about an hour ago. ;)

Experiment 2 : Verification

Without touching anything, perform a static timing check. Do not even remove the distributor cap, just check where the test lamp turns on as the timing mark comes up to to the crankcase seam. Write it down or remember it. Rotate one full turn and check again.
Is it 3* retarded from the first reading? Is it 3* advanced from the first reading? If so, make a note that your breaker cam is incorrect.

NOW remove the distributor cap and physically rotate the rotor counter clockwise as far as it will let you and recheck your static timing through two rotations.

If you are feeling scientifical-like, do it AGAIN after cranking the rotor clockwise as far as it will go and gently releasing.

You trust the centrifugal unit, it snaps back correctly, the vacuum unit is not gummed up on its post on the breaker plate, the breaker plate rotates smoothly with little resistance . . . next,
what is the crankshaft end play like?
Colin

[asiab3]

Well I don't friggin believe it.

…the vacuum unit is not gummed up on its post on the breaker plate, the breaker plate rotates smoothly with little resistance . . . next,
what is the crankshaft end play like?
Colin

End play was measured at .005 last summer before this trip. About 3k miles on it since, and I've never been able to feel a difference in the pulley since I bought it.

The first vacuum can (months ago) checked out on a suck test, and it failed on that trip. I found another good used one at Kombi Haus in Sacramento, (paid out the a$$ for it,) and that one has held vacuum for as long as I've been troubleshooting this issue. Never got a good drive out of it. When I went to check the post and plate gumming, (which are currently clean and lubricated,) I thought I would check the vacuum can again for laughs, and laugh I did. The retard side is like blowing through a broken harmonica.

It held vacuum three days ago during my test, and now it doesn't. I have a NOS can in the mail, but I want to get to the bottom of this before I fry another one. I checked my vacuum pull on the advance and retard carb ports. Advance measured in at 155mm-hg, in spec. Retard vacuum measure 930mm-hg. Spec is 160-230.

Soooo I personally guestimate that my carb, pulling over four times maximum vacuum, is causing BOTH the destruction of the cans, and the extreme timing.

I'll get it reinstalled for the static timing tests when my family leaves town- what do the mountains care if I have a zamboni distributor. It's sure the right weather for it.

Robbie

[kreemoweet]

I checked my vacuum pull on the advance and retard carb ports. Advance measured in at 155mm-hg, in spec. Retard vacuum measure 930mm-hg. Spec is 160-230.

That is quite remarkable, inasmuch as absolute vacuum is in the neighborhood of 760 mm-Hg. Is your timing-measuring-device made by the same company? That would
explain much.

Also puzzled by your reference to "specs". There are no specs with regard to the vacuum appearing at the various ports. It is quite normal for vacuum measured
at any of them to range from 0 to twenty-something inches mercury column, depending on the momentary engine conditions. The vac cans are used to it, and
take it in stride. Unless the cans have been jimmied somehow, the vac advance is mechanically prevented from exceeding its designated maximum.

[asiab3]

I checked my vacuum pull on the advance and retard carb ports. Advance measured in at 155mm-hg, in spec. Retard vacuum measure 930mm-hg. Spec is 160-230.

That is quite remarkable, inasmuch as absolute vacuum is in the neighborhood of 760 mm-Hg. Is your timing-measuring-device made by the same company? That would
explain much.

Also puzzled by your reference to "specs". There are no specs with regard to the vacuum appearing at the various ports. It is quite normal for vacuum measured
at any of them to range from 0 to twenty-something inches mercury column, depending on the momentary engine conditions. The vac cans are used to it, and
take it in stride. Unless the cans have been jimmied somehow, the vac advance is mechanically prevented from exceeding its designated maximum.

My vacuum retard port on my 34pict3 pulls 930 mmHg on my gauge at idle.

Bentley lists values for degrees advance/retard ranges according to vacuum signals. These are specs:
(205Q)
advance: 2-5* @ 150-170 mm Hg
retard: 11-13* @ 160-230 mm Hg

Engine and Clutch, 19.2, Distributor.


I'm not a professor of fluid dynamics, but I wasn't aware we had to factor the atmosphere in to our pressure readings. Isn't it always accounted for, since our gauges exist, uh, in the atmosphere?

[Amskeptic]

I'm not a professor of fluid dynamics, but I wasn't aware we had to factor the atmosphere in to our pressure readings. Isn't it always accounted for, since our gauges exist, uh, in the atmosphere?

Yes, and that is why we have absolute pressure numbers vs our more usual "relative" pressure numbers. But let's reel this back into relative reality. If you have "vacuum" at the vacuum retard hose at idle, it must slow the engine down when you push it on back on the vacuum retard nipple at idle. If you have vacuum at the advance hose at 3,000 rpm, it must advance the timing when you stick it back on the vacuum advance nipple.

Now then . . . how does the engine run?

[asiab3]

If you have "vacuum" at the vacuum retard hose at idle, it must slow the engine down when you push it on back on the vacuum retard nipple at idle. If you have vacuum at the advance hose at 3,000 rpm, it must advance the timing when you stick it back on the vacuum advance nipple.

Yes and yes. Connecting the retard hose drops timing 10-13* at first, for a few seconds until it dives down 10-20* more like the first video. Connecting the advance hose gives me more advance at cruising RPM without load.

Now then . . . how does the engine run?

Like a bat out of hell when the can isn't busted. The bus dives well enough now (009), but it's an absolute joy to drive around town when the advances are working together. That is, until the idle dogs and it looses all usable power.

Since I don't have a usable vacuum can right now, I installed the DVDA with both hoses plugged at the carb and ran it like a 009. 30* at max advance, and it drove like a 009 for a few seconds until the original timing issue came back. I think it is safe to say the original issue is not a vacuum issue now? I'm all for a nice Occam's Razor solution, but perhaps I have two issues.

My diagnosis: something in the mechanical advance issue is causing the retarded wanderings while the carb is pulling too much vacuum retard for the diaphragm for the can to take over a long period of time.

What say you?

[Amskeptic]

it's an absolute joy to drive around town until the idle dogs and it looses all usable power.

I installed the DVDA with both hoses plugged at the carb and ran it like a 009. 30* at max advance
until the original timing issue came back.

I think it is safe to say the original issue is not a vacuum issue now?

My diagnosis: something in the mechanical advance issue is causing the retarded wanderings while the carb is pulling too much vacuum retard for the diaphragm for the can to take over a long period of time.
What say you?

Try a different distributor?
Colin

[asiab3]

Damn, I had a whole post typed up and I accidentally hit refresh :(

I went full-crazy last night and took the distributor down to every part, and realized that one of the fiber washers was cracked. Replaced with a good donor, and problem still continues. I have narrowed down free mechanical play to the hole in the vacuum can arm. You know when you don't have the right drill bit, so you choose one size under and make a stirring motion while you pray? That's what the hole fits like, and it allows the points plate to rotate back and forth with enough play to bother me. My NOS can is in the mail, and it should be able to eliminate that variable- I'll let you know how it goes.

I want to get to the bottom of this, so I can do more important things during my 2014 Itinerant visit, if there happens to be a tour……
…things like wax the intake plenum and make sure your engine tin screw heads are all aligned.

[asiab3]

Try a different distributor?
Colin

In the name of spending a day in the air conditioning, I went back to this project. I don't like to leave threads unsolved.

I had ZERO mechanical play as per the video of the cam snapping back nicely above. Today when I tried it in the car, there was a different story. Out of the engine it behaves fine. There is a bit of extra retarding available when it's installed:

[youtube]http://www.youtube.com/watch?v=oECN7Ncep5M[/youtube]

I think this is my problem. Except the play shown isn't distributor specific. It happens with all of them, but only DVDAs have the retard pulling enough to highlight where the play actually lies:

[youtube]http://www.youtube.com/watch?v=AKf6nVllEV8[/youtube]

[Amskeptic]

Today when I tried it in the car, there was a different story. Out of the engine it behaves fine.

Do a bench test facsimile of the pressure exerted upward by the distributor shaft spring. Does this cause a loss of return? P.S. even though you are plagued by mysteries right now as far as temps, please know that there are legions of air-cooled VWs running fine with all kinds of sloppy or sticky distributors. So while you are trying to nail down the variables, understand that the root cause of your issues are generally more glaring.
Colin