'hiccup' at 3000 rpm only with vacuum advance connected

[CBradio]

I just changed my ignition system to the 528i system. All is well but one thing:

With vacuum advance connected, revving in park and holding at 3000 RPM, my car hiccups- like the RPMS Drop to ~2800 RPM then back to 3000 RPM, almost like its working against itself for a split second. It doesn't seem healthy. RPMs above and below seem okay.

I disconnected the vacuum advance {and retard too boot} and car seems to drive great. In fact, better than ever. I didn't take it on the highway yet, but no issues around town. I'm not sure about my gas mileage just yet however

Is my diaphragm shot? Does it just need some lube?
If so, stupid question: how/where do I lube this thing?

As a side note, with these older cars, how advanced is advanced?

I know the answer is "as much as you can as long as it doesn't ping"
But if I say: I can time the car to 22 BTDC at 1000 RPM and don't detect pinging, is that feasible? Or does that sound crazy??? I just don't know!



has anybody else also lost some hair trying to get the timing just right?
Is that normal? LOL Still, I love this car. I hope to put some pics up as soon as I get a back seat in.

[onovakind67]

I set the timing on my carbureted B35 motor to 36° BTDC at 3500 rpm. Being an old Chevy guy I am used to seeing the timing mark on the balancer, much easier than looking in some hole for a BB flying by at 80 mph. The 36° mark is set by measuring 10% of the circumference of the balancer CW from TDC, and the timing tab is a bent piece of metal bolted to the block. You can track the timing from idle to max with a dialback light, or simply set it at 36° with a standard light.
I also use the vacuum advance hooked to manifold vacuum so it's all in at idle. My motor runs very cool, as you can see there is no mechanical fan, only an auxiliary fan that comes on at about 195°, which is about twice a year.

[CBradio]

Thanks for the response and the pics.....

That is so interesting.

So you are using the vacuum advance at idle, which means that the vacuum advance "kicks out" at higher RPMs?

And 35 degrees BTDC is more advanced than 22 BTDC, but you are also timing at 3600 RPM which, if you were sticking to 22 BTDC, would be fare more retarded than timing to 1700 or 2200 RPM...

I wonder what this translates too.

In any event, my car is running so much cooler now.

I just don't have the MPG stats in just yet- if I'm getting good mileage, I will pat myself on the back. The car is definitely more powerful, I'm hoping this translates to fuel efficiency too...

[onovakind67]

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.

I set the timing for maximum advance of the distributor, when the springs in the advance mechanism have reached their limit. Most distributors are all in by 3500 rpm or so, you can check it by increasing the rpm to 4000+ and watching that the timing does not advance beyond 36 degrees.

[CBradio]

I see.

Curious, how is your fuel mileage?

[onovakind67]

I get about 20 mpg on the highway with dual 32/36 Webers.

[CBradio]

I tried to measure my balancer.. i need a re-approach LOL.

But in the meantime I have maybe a silly question, I just want to be sure:
So the white mark on your balancer represents TDC, and the bet piece of metal pointer you made is 36 degrees BTDC?

Ive been thinking about this for few days and setting the timing this way seems to make the most sense to me. Thanks a lot for your help!

[onovakind67]

The first thing you want to do is establish the TDC point, I did this by rotating the engine until the TDC mark came up in the little hole in the bellhousing. I then bolted a tab to the block ( I used one from a Ford rear end ) and bent it so it was close to the balancer. I made a mark on it and continued the mark out onto the balancer, and this is my TDC mark. The next step is to establish a 36° BTDC mark on your balancer. 36° is 1/10 of the circumference, so you can measure the circumference directly, or measure the diameter and calculate the circumference. Divide your circumference measurement by 10 and make a mark 36° clockwise on the balancer, i.e. in its direction of rotation, so that the 36° mark goes by the tab just before the TDC mark.
Now, everything is out in the open and you can see exactly what's going on. If you have a dialback timing light, you can measure the amount of timing that your VA can adds.

[CBradio]

Got it! Ok, its what I thought exactly.
Thank you for taking the time!

I will do this.

[T.Hanson]

Reading the succession of posts, I wish the Star Trek transporter, twinkle dust phone booth were real. Meet in my garage Saturday to teach me the language, acronyms, to have a clue what you are talking about.

I've timed my '79, '81 E12's, E24 by reading the directions. White mark, TDC, insert distributor so rotor points to the notch, engine rpm 2,000,...

Mr. Monkey (Wrench). For all I know the rotor sprays sparks to the wires to fire the plugs in the right order.

The strange thing is, it worked. Cars run fine. Thus, I've never disconnected any vacuum lines, measured 36 degrees being 1/10 the circumference of the interior bell tickle, to advance the balancer franny to the BTDC ping.

[onovakind67]

Whatever floats your boat....

What do the directions say about dual Webers on an '88 long block with a 3.0Cs distributor with the VA all in by 12"hg using the points to fire an MSD6-T ignition?

[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.

[onovakind67]

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.

[wkohler]

I have this issue with my 530i. I could rev it to 3k and it will drop to 2800. It was a noticeable issue while accelerating or if I was cruising at or around 3k with the 4-speed. With the 5-speed, I haven't spent enough time at that RPM to have the issue but I'm sure it will be there. I'm pretty sure though mine is setup for vacuum retard. Need to see where those lines go.

[CBradio]

First- wkohler- the issue was due to a faulty adjuste throttle position sensor.

Second- onavakind, i have a question, regarding timing to 36 degrees at 3600 rpm +, do you time with vacuum advance connected or disconnected?

I marked the marks on my wheel, it seems i was a little too advanced !