'hiccup' at 3000 rpm only with vacuum advance connected

[onovakind67]

Set the timing with the VA disconnected. I then use a Mighty-Vac to suck down the VA can to see what the total advance is. IIRC, it's about 12°, for a total of 48° under full vacuum. You also want to assure that your idle vacuum is more than the vacuum required to pull the VA can all in, otherwise you'll have odd rpm variations at idle. Here's a good primer on engine timing that was written by a GM engineer:

http://www.camaros.org/pdf/timing101.pdf

[CBradio]

Thanks for that. I looked at that primer, interesting stuff.

I tried using my vacuum advance at idle and indeed I had some funny stuff happen:
at ~1000 RPM (park/neutral) my vacuum advance would work fine.

When I dropped it put it in gear/reverse) my idle went down to like 400
(wow, couldn't believe my car was still running). So I'm assuming my vacuum capsule doesn't work at low idle.

I'm thinking it also may be sticking on occasion. Do I just spray wd40 into the advance port? If its not sticking, then I think I can't be "all in" at idle from the manifold with my model of distributor.

It seems my car runs best with both vacuum tubes connected, when timed to 22 BTDC at around 1250 (timed with all tubes disconnected)

Can somebody tell me if this is plain absurd? Keep in mind, I have a 528i cam and distributor in my 150k mile 530i. At this setting, when I connect vacuum tubes, it idles at about 8-11 BTDC.

I could disconnect the vacuum retard and retard my distributor a bit to get the same idle 8-11 BTDC setting, but I don't think the car takes off as fast. Here I'm timing to like 22 BTDC at 1500 RPM.

Honestly, I would just leave it timed like this, with both tubes connected, but I'm afraid I'll have pinging on the highway and not be able to hear it. can somebody tell me what pinging under load sounds like? I heard "like marbles", but how subtle is it? Is it not subtle at all?

[/b]

[CBradio]

Thanks again for all your help guys.
I wish I could set it and forget it easy peasy T. Hanson style.

I figured out:
My centrifugal advance taps out at about 32-34 degrees BTDC at about 3000 RPM.

I think my distributor is set up for NO vacuum retard-
otherwise my advance curve is too severe.

I also think I have to time a little more advance than 528i spec because lower compression in my engine means I need to advance the timing a bit more:

Also, from what I've read, because the compression on the 530i is SLIGHTLY lower than a 528i, and the engine is old, the compression on my engine is good but not as new, so this explains why my car has little power when I time to 2200 or even 1700 rpm.

Maybe this will be of use to others? I hope so.

Thanks again guys for all the expert input!

[Mike W.]

The vacuum advance is in effect as long as there is vacuum. At 80 mph I have about 14"hg of vacuum, so my vacuum advance is still all in.

No, that's not how it works. It doesn't work off manifold vacuum, it works off of ported throttle body vacuum. The vacuum retard port is just past the butterfly, so at idle it does get full manifold vacuum, but once the throttle is cracked vacuum goes down dramatically, right by where the vacuum port is, so you get some quick advance. The vacuum advance side is the opposite, it's just before the butterfly on the other side so at small throttle openings it get some vacuum, but none at throttle closed or wide open. It doesn't make sense until you take it apart and really look at what is where and exactly where the ports are.

You're assuming a lot here.
My advance-only VA can is controlled by full manifold vacuum from a tube that joins the two manifold plenums together. Teed off this line is my vacuum gauge, so I can monitor the vacuum signal to my VA can on a continuous basis. At idle I have about 19"hg on the can and on the gauge, and normal level-ground cruising is about 14"hg.

Yes, I was assuming it was hooked up correctly which apparently it is not.

Can somebody tell me if this is plain absurd? Keep in mind, I have a 528i cam and distributor in my 150k mile 530i. At this setting, when I connect vacuum tubes, it idles at about 8-11 BTDC.

I could disconnect the vacuum retard and retard my distributor a bit to get the same idle 8-11 BTDC setting, but I don't think the car takes off as fast. Here I'm timing to like 22 BTDC at 1500 RPM.

Honestly, I would just leave it timed like this, with both tubes connected, but I'm afraid I'll have pinging on the highway and not be able to hear it. can somebody tell me what pinging under load sounds like? I heard "like marbles", but how subtle is it? Is it not subtle at all?

You're not nuts, it makes a lot of sense. 2.8 and 3.0 engines are so close to each other excepting emissions they're nearly identical. 8.3 vs 8.2 compression ratio. Less than 10% difference in displacement. Bosch L jet FI. While I'm a big fan of 528i's vs 530i's, it's all in the bolt on stuff.

[onovakind67]

Yes, I was assuming it was hooked up correctly which apparently it is not.

Why would you not want manifold vacuum to operate your vacuum advance? The engine runs cooler and more efficiently with this setup.

[Mike W.]

Yes, I was assuming it was hooked up correctly which apparently it is not.

Why would you not want manifold vacuum to operate your vacuum advance? The engine runs cooler and more efficiently with this setup.

Well, it wasn't designed that way, which I will allow doesn't always mean it's right or wrong, but suggests original is correct. I can see no theoretical benefit to manifold vacuum instead of ported vacuum, but ultimately how it drives is the most important thing. Except when it's time for a smog check.

[onovakind67]

Mine's a 72, carbureted, and no smog required. Ported vacuum retards the timing at idle, raising the exhaust gas temperature and engine temperature. Because the peak cylinder pressure point is retarded as well, you need to have the throttle blade(s) open further to keep the engine running at your idle speed, which requires more fuel. Having to run more throttle opening means greater chance of run-on if you happen to have some lesser quality fuel in the car. Some carburetors have solenoids to either close the throttle blades or shut off the idle fuel to prevent this.

[CBradio]

Thanks for letting me know I'm not nuts.
I can't tell you how much that helps me.

Slightly off topic here,
and forgive my inexperience,
but what exactly does pinging sound like on our engines?

Is this a tough question?

Does it sound like this? (on m20 engine):


If so, my car doesn't have that noise so I must not be too advanced.
My worry is that its so subtle I'm not noticing it-
possible?

[Mike W.]

Not really. Hard to describe, kind of a kkkkgkgggkkgggg sound. If you really want to know crank in another 20 degrees advance and drive it around the block. You won't hurt anything in a few blocks and you can baby it back. It's kind of muted, I wouldn't call it soft, but definitely not a pounding or like the crack of a hammer. If it's real mild, it will just be on one cylinder like aircooled VWs used to do. It occurs under high load, you could putz around in stop and go traffic for a month and never hear it. Romp on it to get on the freeway and it will talk to you. Also, it's more likely when it's hot, both engine and outside but mostly engine.

[Mike W.]

Mine's a 72, carbureted, and no smog required. Ported vacuum retards the timing at idle, raising the exhaust gas temperature and engine temperature. Because the peak cylinder pressure point is retarded as well, you need to have the throttle blade(s) open further to keep the engine running at your idle speed, which requires more fuel. Having to run more throttle opening means greater chance of run-on if you happen to have some lesser quality fuel in the car. Some carburetors have solenoids to either close the throttle blades or shut off the idle fuel to prevent this.

That changes things a bit. I was assuming an L jet E12, probably a 528i. However I did do over 100K in a Bav, well, probably only 80K or so before I pulled off the carbs and put L jet on it, but I know a bit about them and those dual Zeniths. Now I'm struggling to remember, but I think they used ported vacuum also, and much to my surprise I found it ran best when hooked up like the factory did it. Really, I thought it would be better if I left the vacuum retard off entirely, but it wasn't. I think I did have idle solenoids but I never had dieseling problems at all and I don't think the increase of gas at idle would really make much difference overall.

[onovakind67]

Thanks for letting me know I'm not nuts.
I can't tell you how much that helps me.

Slightly off topic here,
and forgive my inexperience,
but what exactly does pinging sound like on our engines?

Is this a tough question?

Does it sound like this? (on m20 engine):


If so, my car doesn't have that noise so I must not be too advanced.
My worry is that its so subtle I'm not noticing it-
possible?

It will make a slight rattling sound, like someone is tapping a hammer. It usually is modulated by load and heat. Here's a real good read on detonation and pre-ignition:
http://www.contactmagazine.com/Issue54/ ... asics.html

One reason why I set my timing at an elevated rpm at the mechanical limit of the distributor is to assure that it never goes above a certain limit. If you are depending on the 40-year old springs in your distributor to hold the timing at some mid-rpm point, you will probably get less than optimum results. For the same reason I test my VA can independently so I know exactly what it does.

That changes things a bit. I was assuming an L jet E12, probably a 528i. However I did do over 100K in a Bav, well, probably only 80K or so before I pulled off the carbs and put L jet on it, but I know a bit about them and those dual Zeniths. Now I'm struggling to remember, but I think they used ported vacuum also, and much to my surprise I found it ran best when hooked up like the factory did it. Really, I thought it would be better if I left the vacuum retard off entirely, but it wasn't. I think I did have idle solenoids but I never had dieseling problems at all and I don't think the increase of gas at idle would really make much difference overall.

I has a 3.5L motor with dual Webers, a 3.0Cs distributor and an MSD6-T ignition. It's been my daily driver for 8 years, it had 150K on it when I bought it and I've put another 150K on it. I replaced the motor about 3 years ago with a $100 Pick-n-Pull long block from an 88 and it has run very well. I poked around the web to find some jetting schemes for the Webers as they were a little stumbly when I got them and ended up using a setup developed by this 2002 guy.
http://www.zeebuck.com/bimmers/tech/dgvjetting.html
It fired on the first crank and has performed flawlessly since.

[CBradio]

It is fun to read you guys.

After some tinkering, I'm thinking my mechanical advance is working fine, if I set it within "spec" everything seems to line up more or less where it should according to some specs I should have come across sooner.

I can't detect any pinging even with a bunch of advance; I don't rev that high with an automatic generally. I am waiting on a TPS; when it gets here I'm going to floor it in second gear and see if it pings.

All this reading has got me wondering why my car needs so much initial advance to move and I'm starting to think some part may have to do with me using the 'premium' 93 octane.

Unless somebody tells me not to, I'm going to fill up with 87 and see if I get a little more power with the timing more retarded.
Thoughts?[/quote]

AND Thank you for the pinging insight!

[onovakind67]

An automatic transmission introduces another variable into the mix - the load of the converter when you are in gear and stopped. Does the 528 cam have the same specs as the 530 cam?

[Mike W.]

An automatic transmission introduces another variable into the mix - the load of the converter when you are in gear and stopped. Does the 528 cam have the same specs as the 530 cam?

No it does not, more lift and less duration than 530i cams.

[CBradio]

They are different, to quote Mike from elsewhere:

Car Cam Lift Duration
528i 7.928 260
530i 7.422 272