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Weber carb, vacuum advance


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#1 Lockleaf

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Posted 08 October 2015 - 06:53 PM

I'm running a Weber 32/36 on my z24 1985 720 4x4. As far as I can tell, few if any Weber carbs have a provision for ported vacuum to which you can connect vacuum advance. It seems like most guys running Weber carbs run high initial advance instead, which kinda sucks for a street car.

My question is why? Why don't I ever see people mention manifold vacuum for vacuum advance? There is a long history of argument over whether ported or manifold is superior, but both schools would agree that any vacuum is far superior on the street to no vacuum, high initial advance.

I've been running manifold vacuum for a couple years now, and it works great. I just installed a hose barb into one of the ports on my intake manifold.


IMG_20151008_204150994_zpsjsrn3cvn.jpg

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It's hidden behind the power steering pump in this pic, but this gives a little better reference to the location of everything.

IMG_20151008_183936008_zps9isbqe38.jpg

Am I mistaken or could many guys have much more street friendly cars and we just aren't taking proper advantage of our options?

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#2 banzai510(hainz)

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Posted 09 October 2015 - 10:30 AM

all my DGVs have a littlel brass port. So I dont know where you getting this from.

 

When I see people hook to manifold to the dist it dont run as well.Pings on hot days ect... I think it sucks that advance on the distributor just to  hard.

When on the carb I think its right above the butterfly the poert inside. I think. then you have the centrifical advance.

 

So me just wingin this I think you over adavancing the distributor.  And I would NEVER run direct to the manifold.I would run not vacume fisrt than on the manifold

 

 

Napz motor I think is like 5deg BTDC

 

 

so look side of carb and the valve cover. it on that side since it might be hard to see as you Napz has the carb on backwards compared to L motors.  Thats is your main barrel side of the carb that runs this 80% of the time

 

like to see what others say


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#3 datzenmike

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Posted 09 October 2015 - 11:22 AM

Rule #1.... the less fuel and air in a cylinder the more advance you need.

 

This is because even compressed, there is lots of empty space between the molecules and it takes longer for the flame to jump from one to the other. By starting the burn sooner the combustion chamber reaches it's maximum pressure and can push down on the descending piston efficiently.

 

Before the late' 60s all engines were run on vacuum advance from the intake. When is vacuum highest and the cylinder least full??? Yup, it's at idle. Intake vacuum at idle is perfect for proper advance. Enter the EPA. Idling engines make huge amounts of pollution. Carbs were modified and the port for the vacuum advance moved to just above the throttle plate so that at idle there is no vacuum advance but just above idle the port is exposed to intake vacuum. This means that effectively the mixture is ignited too late and is still burning on the way out the exhaust. This causes poor idling and lots of heat dumped into the water around the exhaust ports. But the combustion chambers run cooler and less oxides of nitrogen are produced. If anyone want proof of better idle just set your timing to spec and then manually advance the distributor. What happens is the idle goes way up because as stated earlier a weakly filled cylinder at idle wants lots of advance that todays carbs and distributors don't give them. The engine runs smoother and cooler because the pressure produced is now efficiently pushing down on the piston and not so that it's chasing the rapidly descending piston.

 

Can you switch to intake vacuum for the vacuum advance. Not really because the distributor is set to run on ported vacuum which, while close to intake is not exactly the same. The advance will be too strong at low speeds. You can dial back the static timing to reduce pinging but when revved up won't be near the total advance of 32 degrees? needed. You may be able to alter the vacuum advance can on the side of the distributor to tone it down.


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#4 Lockleaf

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Posted 09 October 2015 - 01:07 PM

Yeah it's like 5 degrees. And..... It's a hemi head so it doesn't exactly have a detonation problem. That's kinda the beauty of the hemi head. The very fact that it is an emissions head also guarantees Nissan was looking for emissions preservation, not perfect per development. Claiming static timing is better than any kind of vacuum advance says you don't understand the system nor its benefits.

Mike, I would need proof that ported vacuum is different from manifold vacuum. And different as in how? Different inches of hg? If that is true, and is significant enough to change things, an inline vacuum restrictor fixes it.

I agree with everything about emissions. My brothers 84 dodge even still ran 4 degrees on its 360, because even with a cat they still wanted the super heated manifolds to act as pre cats. It ran way stronger on 12 degrees, but polluted more.

I'm happy to take some timing measurements through the rpm scale and see what it sets at. If someone with a stock wants to do the same, let's see if you can over advance these distributors and if so, by how much.

Don't tell me it doesn't work right. Show me.

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#5 datzenmike

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Posted 09 October 2015 - 01:25 PM

Setting the static timing on old engines, the directions will always say disconnect the vacuum advance line and plug.

 

Newer ported vacuum advance carbs there is none at idle so the line need not be disconnected. BUT you must be idling normally and the throttle plate more or less closed. If you have problems and the idle is turned up, then the static timing can't be set properly

 

 

 

 

The ported advance slowly comes in as the throttle plate rises above port opening just above idle idle. It can't be exactly the same as a fitting below the plate on the intake but.... close. At least for the first while. As the throttle plate lifts, more and more vacuum is present at the port.


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#6 Lockleaf

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Posted 09 October 2015 - 01:44 PM

Underhood and repair manual both list removing vacuum from distributor for setting ignition timing for the Z24. I have not looked in an fsm.

As far as I understand Mike, youre basically just describing how ported works. I realize that it changes vacuum at idle, usually to zero. I had thought you meant it applied different amounts once activated. I've always understood b ported to be like a switch. Nothing until it dumps on all available vacuum, at which point the two options would match. I might be mistaken on that though.

And I'm curious about the concept of too much advance. Vacuum advances have a maximum amount of advance they can apply, so are we assuming that normally they never use that full amount?

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#7 datzenmike

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Posted 09 October 2015 - 02:34 PM

Rule #2.... Engine vacuum is a good indication of engine load.

 

Heavy throttle means full cylinder filling which means densely packed molecules which means faster burn which means less ignition advance. Light throttle = light cylinder filling = thinly packed molecules = slow burn time = more ignition advance.  Using vacuum to tell the distributor when to send the spark is a perfect method to get the maximum pressure over the piston in perfect time to push down on it efficiently.

 

A ported vacuum carb may have a very small amount but not at all the full 20 inches in the manifold at idle. Inside the carb throat you can see the port and it may have a small vertical slit below it to taper the amount sent to the distributor as the throttle plate lifts off idle. I'm pretty sure the amount of vacuum is not full on or off but varies with the throttle plate position just as the intake vacuum would. In relation to this the distributor gets a weak signal or is set to advance gradually through a range or curve. The problem is that the intake vacuum is a much stronger signal and may over advance in relation to the throttle opening (load) and it may ping. 


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#8 datzenmike

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Posted 09 October 2015 - 02:36 PM

If you have a weber without ported vacuum advance it may be very old.


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#9 Lockleaf

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Posted 09 October 2015 - 02:45 PM

If you have a weber without ported vacuum advance it may be very old.


This is highly possible.

I'm going to take some timing measurements and just see exactly what my timing is doing, with vacuum and without, at the settings I am currently using.

I do know that I ran this carb without any vacuum advance for months and had manually advanced the timing. When I reset it and used manifold vacuum, throttle pickup and slow driveability became much nicer. Ported might be even better, but not currently an option for me.

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#10 datzenmike

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Posted 09 October 2015 - 03:06 PM

There are three sets of timing. Static is just set and forget. On an L20B it's 12 degrees and your timing will never be less than this.

 

The other is vacuum advance and is variable with load. There is virtually no vacuum advance at WOT and no need for it on a race car and up to 10-15 degrees of advance at idle and deceleration.

 

Last is mechanical because, lets face it the fuel in the combustion chamber burns at a set rate determined by physics and as the engine revs up there is less and less time to get the job done. So it needs to be lit earlier and a mechanical advance is used. Mechanical advance is strictly rpm dependent and adds about 22 degrees of advance by about 3K rpms.

 

At or above 3K  and at WOT you will have the 12 static and 22 mechanical for a total of about 34 degrees. This gives just enough lead time for the combustion pressure to push down on the pistons perfectly without damage or detonation. If you should happen to be out on the highway and let off the gas and cruise along at a steady speed there will now be a vacuum signal, perhaps 10 degrees or more of advance added, but because the cylinder is not full there is no pinging from so much advance. When decelerating the advance may top 50 degrees. 


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#11 banzai510(hainz)

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Posted 10 October 2015 - 10:38 AM

on some webers I seen the brass port can fall out ans is open or can have a plastic plug.

 

I don't think we need a scientific explanation to say run the port from carb to the distributor. That's the way it needs to be done

 

But if it feels better than keep it but try the other way and retime it and try that.


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#12 DIY 1985

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Posted 27 November 2017 - 06:54 PM

The 85 factory manual describes 9.5 degrees centrifugal advance at 2100 rpm, and 10 degrees vacuum advance at 9.84 inHg 2wd, 7.5 deg at 7.87 inHg U.S. 4wd. (gas engines).

The Engine and Emissions Control Diagram suggests an upper port source on the Hitachi also works in conjunction with a second, lower-to-the-throttle vacuum source. Between the hosing that connects them are the Vacuum Control Valve and Thermal Vacuum Valve, moderating vacuum advance depending on engine temp and workload.

Possibly that ping is from running 86 octane on glowing summer carbon deposits?

I am skeptical if the Weber port, and a manifold source can replicate the vacuum of Hitachi, but I haven't a means to measure it.

Does the manifold pressure ever drop enough to retard the vacuum advance?

Or, is an engine, prone to blown head gaskets a good candidate for running hot because there is no vacuum or centrifugal spark advance at idle?

My 85 2wd lumber-runner with a 32 36 Weber is set around 14 degrees BTDC and my timing light suggests liberal centrifugal advance. I also take a common trip up to 10,300' for wilderness hiking.
I'm open to suggestions.

#13 datzenmike

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Posted 27 November 2017 - 07:19 PM

The vacuum advance is ported from just above the throttle plate, so at idle it senses basically ambient pressure (zero) As the throttle opens it passes the port and it begins to feel some intake vacuum that is below the throttle plate. Eventually it is completely below the throttle plate but by then the plate is much more open and intake vacuum is beginning to drop off. At full throttle intake vacuum is at it's lowest.

 

When the throttle plate closes everything is the reverse. No there is no negative direction for the vacuum advance. Starts at zero increases and decrease back to the starting point.

 

 DIY1985.... 9.5 degrees... Is this distributor degrees??????? crankshaft degrees would be double this so 19 degrees. What does your timing light say for total timing when revved up above 3K with the vacuum advance unplugged? If you are at 14 static and the mechanical is 19 the total will be about 33 degrees. About right for a full throttle engine.


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#14 distributorguy

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Posted 28 November 2017 - 10:03 AM

Small displacement motors that rev quickly will benefit from ported vacuum, while heavy big block American iron can benefit from manifold vacuum.  The slow-to-accelerate crank and rods benefit from more initial timing.  

Base timing can be determined by how well the engine breathes, the weight of the rotating assembly, and the amount of compression you run.  I might set a Stock L20b at 14 degrees BTDC, but an 11:1 build with DCOEs will be at 20-22 BTDC at idle.  

 

For every 5000' above sea level, you add 2 degrees base timing and lean out your jetting/fuel mixture one step.  

 

Those "book" distributor settings are at distributor advance and rpm.  Both numbers double at the crank.  



#15 DIY 1985

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Posted 28 November 2017 - 07:56 PM

Why does the FSM instruct me to disconnect the distributor vac line to adjust "static" timing? It shouldn't matter at idle? Or, it doesn't change anything when the distributor is connected to the Weber. Could the Hitachi set-up have a more effective venturi that induces vacuum in the distributor line at idle?

#16 distributorguy

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Posted 29 November 2017 - 05:24 AM

Some ports for ported vacuum are drilled wrong and a small amount of vacuum is allowed to pass at idle.  Always remove the vac line when setting timing.  Always.  



#17 Lockleaf

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Posted 29 November 2017 - 08:24 AM

Its just a precaution to guarantee accurate settings. A bit of grunge holding your throttle plate slightly open would change the reading. Imperfect port location. Throttle plate shifted in the bore of the carb. Lots to mess with the reading especially after 30 years.

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#18 DIY 1985

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Posted 30 November 2017 - 11:48 AM

What about the mouse nest in the tube connected to the housing for the air fliter?