Weber 32/36 vacuum advance question

[cda951]

@'76mintgrün'02 don't beat yourself up, your contributions about your experimentations have been great, and as you have just experienced, it is an evolving thing.

 

The ported vs manifold vacuum debate has been a long-running thing in old car circles, and there is no absolute answer about which is better because it entirely depends on the vacuum pod used and the centrifugal advance profile. As mentioned earlier in this thread, the engine vacuum at the pod is the same whether ported or manifold referenced once the throttle plate(s) are opened above/beyond said port.

 

The low-compression, emission-controlled engines that commonly use ported vacuum advance and possibly a dual vacuum pod with timing retard at idle (which is referenced to manifold vacuum) use a distributor that has a centrifugal advance curve with a late and gradual profile. Snapping the throttle open provides a "shot" of ignition timing vacuum advance analogous to a carburetor accelerator pump shot, which helps to move the car off the line. Therefore, if your engine and distributor were designed for ported vacuum advance, keep it that way!

 

If a distributor with a more performance-oriented centrifugal advance curve is used, the manifold vacuum-referenced configuration makes more sense as described in previous posts in this thread.

 

In other words, f**k around and find out. Be careful, but in my experience, pinging/detonation can be easily heard in the M10 engine if the exhaust isn't too loud. I have posted to this forum the link to this excellent article about the difference and relationship between detonation and preignition at least a dozen times, but a baker's dozen won't hurt:

 

Knock/Ping

944ENHANCEMENT.COM

 

Edited March 30 by cda951

[John76]

11 hours ago, '76mintgrün'02 said:

it seems like there's no end to the demands of these pesky old cars!

That's what makes them fun. The new cars are like rolling computers ... no fun in that!

11 hours ago, '76mintgrün'02 said:

P cuts out during deceleration (foot off the gas) while m pulls full advance.

Sure, the throttle plate is closed, so there is no ported vacuum.

Manifold vacuum goes through the roof and tends to suck gas out of the float bowl when you lift off the gas at high rpm.

This is why the dashpot was invented ... it cracks the throttle slightly to lower the m. vacuum and keeps raw gas (HC) from burping out the hot exhaust. The dashpot only cracks the throttle when the speed relay sends +12v to activate (close) the white valve, which cuts off the m vac. and lets the dashpot extend its post. Yes, it is an emissions device, but has absolutely no effect on performance.

John

PS: The dashpot also greatly reduces the tail pipe smoke on deceleration, especially if rings and valve guides are worn. 

[John76]

@cda951 Thank you.  Nice article on Knock vs. Ping.

Love the last sentence..."Start out rich, retarded and with cold plugs". 

I started out poor, smart and hot...where did I go wrong??

 

['76mintgrün'02]

I moved the switch and gauge back over by the distributor. I like it better that way in spite of it being a little crowded. Having the switch in the middle made it hard to see down the timing hole. 

Reconnecting the switch did make it idle rich, so I’ll turn it back up to 12.5. It’s around 11.5 now. 
 

I pulled the on/off switch while coasting to see if it liked the added manifold advance and felt it fall off a bit without it and improve when turned back on. So, that seems like another advantage of manifold vacuum. 

 The old vacuum gauge I found had three Epco Motor Boosters with it, inside a cool little red white and blue box. They’re made by the same company, Harimack Corporation in Chicago. They’re made of wood, with a little glass tube showing through a slit in the side. 

 

 

I am looking forward to the added power, speed and pep!

 

Tom

[thehackmechanic]

Tom, if YOU rather than your car are looking forward to added power, speed, and pep, I shudder to think about what would be cut and where these would be placed!

[John76]

8 hours ago, '76mintgrün'02 said:

Reconnecting the switch did make it idle rich,

Tom,

Try reversing the vacuum hoses on the switch.

On the white electro valve the outside nipple is vented to atmosphere when energized (inside nipple is closed).

John

['76mintgrün'02]

45 minutes ago, John76 said:

Try reversing the vacuum hoses on the switch.

On the white electro valve the outside nipple is vented to atmosphere when energized (inside nipple is closed).

 

Okay, so I am letting air into the manifold when it is turned off.  That's not good.  But, I'd have to rework the plumbing to switch nipples and  I  don't  wanna  hafta  do  that.

 

I've got a black-topped switch and even an elusive California-only red-topped example.  Would either of those happen to be plumbed the other way?  I'd rather swap the switch than the plumbing.

 

I guess I can go blow through them to find out. 

 

I've got it plumbed to the M-port under the carb.  Adding a vacuum leak to runner number one or four would probably make it run crappier than where it's connected now.  It's only been leaking for the test-flips of the switch and when setting the timing.

 

Thank you for the tip, John!

[John76]

4 minutes ago, '76mintgrün'02 said:

Okay, so I am letting air into the manifold when it is turned off. 

 

Yep... but you can control how much it vents by adjusting the screw on top of the valve.

You tuned your car perfectly with the A/F gauge, but with the small vacuum leak, you had to richen the mixture.

When you turned off (energized) the White valve, the "leak" was blocked, and your mixture went rich.

 

The Black valve would be worse! The outside nipple vents rapidly when the valve is energized (inside nipple closed) and vents slowly (controlled by screw) when the valve is open (not energized) This is done to prevent the advance diaphragm from "fighting itself" against vacuum or pressure at either side. If it did not vent, the vac advance would remain "on".

 

The Red valve outside nipple vents quickly too when energized. It is meant to control the EGR control valve, which controls the 2-stage EGR valve. The control valve compares ported vacuum (red hose) to manifold vacuum (white hose) and decides when to open the second stage of the EGR valve (blue hose). With all the "fighting" between the 1st stage and 2nd stage diaphragms, the outside nipple on the Red valve needs to vent quickly (also adjustable with the top screw).

 

I used a manual 3-way pneumatic toggle switch when I was comparing ported/manifold. This confirmed that manifold vac is the correct choice (at least for my car).

I didn't toss this switch into the neighbor's pool, so let me know if you want it. 😎

 

John

 

 

['76mintgrün'02]

14 minutes ago, John76 said:

You tuned your car perfectly with the A/F gauge, but with the small vacuum leak, you had to richen the mixture.

When you turned off (energized) the White valve, the "leak" was blocked, and your mixture went rich.

 

I had enriched the mixture when I found it suddenly running lean (up around 13.5), so I turned it down to 12.5 (before discovering the loose wire).  Then, when I reconnected the wire it went to 11.5; so, now I need to lean it out abit. 

 

I assumed the change was due to the additional 12 degrees of advance from the pod.  It's hard to believe such a tiny vac leak would affect the AFR that much.

 

I went up to the shop and sucked on some nipples, to find that the black and red pods are open when off.  It's interesting to read about how/why they turn(ed) off.  I did not test to see which side might be vented, just whether they were open.

 

I'm not going to change my plumbing, because the switch is always turned on, except when testing.  I don't think that little leak would make a very big difference, would you?

 

I guess it'd be easy to see, by plugging the line with a tee, instead of turning the valve off (after turning my AFR gauge so I can see it).

[John76]

2 hours ago, '76mintgrün'02 said:

leak would make a very big difference, would you?

Well, any unmetered air leak is going to mess with your F/A ratio at unpredictable times.

Did you try tightening the screw on top of the valve? Maybe this will close the vent completely.

['76mintgrün'02]

Just now, John76 said:

Did you try tightening the screw on top of the valve? Maybe this will close the vent completely.

 

Not yet.  I have another white valve I can try that on, using the Sun machine's vacuum pump to test it.

['76mintgrün'02]

On 4/1/2024 at 7:55 AM, John76 said:

Tom,

Try reversing the vacuum hoses on the switch.

On the white electro valve the outside nipple is vented to atmosphere when energized (inside nipple is closed).

 

I should have read this more carefully the first time.  The outside nipple is what feeds the pod and the gauge.  The inside one is connected to the manifold.  So, there's not a leak to the manifold when the switch is turned off. 

 

I tried sucking air through the extra switch and that's the way it works; so, there's no need to change the plumbing.  It wouldn't be a big deal to swap those hoses (tubes), except for having the valve T-ed in.

 

(Trivia-- the difference between hoses and tubes is that hoses have reinforcement. 

 

So, we have fuel / coolant hoses and vacuum tubes.  (Not to be confused electronical components)).

 

 

[TobyB]

I wonder if Elco also made electric turbochargers 'back in the day'...

 

heh.  Carbon.  That 'harmless gas'...

 

t

 

['76mintgrün'02]

On 3/27/2024 at 1:58 PM, thehackmechanic said:

The distributor I'm using is a 0 231 115 081, which according to my archaeology here on the FAQ, is from a '70-'71ish 2002. I'll play around with it more in the garage. I've taken some measurements, but nothing as accurate as Tom's graphs.

The idea of hooking up one of those long-unused dashpots on the firewall to cut the vacuum in and out to see how it actually drives is irresistible.

 

I took an 081 distributor apart and shared what I found here.

 

https://www.bmw2002faq.com/forums/topic/178062-bosch-distributor-mechanical-advance-limiting-screws/page/8/#comment-1160196

 

That model uses little 'hairpin' clips to hold the weights and they wear out where the long leg on the clip rubs against the pin.

 

 

I made up a batch of them and will send you a pair, if you're going to open yours up.

 

   

 

More photos of clip making here.

 

https://www.bmw2002faq.com/forums/topic/178062-bosch-distributor-mechanical-advance-limiting-screws/page/11/#comment-1403655

 

It must be driving season by now!  I'd be interested to hear what you've done/learned.

 

I learned that the pod I was using on the #164 was giving 14 degrees of advance, which gave me more than I wanted at idle, so I swapped it for one that gives 10.  I've got 16 degrees of advance at idle now (thanks to manifold vacuum).

 

What sort of numbers are you seeing with your 081?

 

Hopefully, you have a white-capped vacuum switch to install, so you can witness the difference the vacuum pod makes first hand.  People dismiss vacuum advance as unnecessary, but it allows for more advance during lean conditions, while taking it back out under load; so, what's not to like?

 

Tom