Home » Public Forums » GMCnet » Vacuum advance not working?
| Vacuum advance not working? [message #67901] |
Sun, 20 December 2009 20:53  |
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hey guys I have my vac advance on my distributor plugged directly to manifold vacuum, however it does not seem to advance it any, and timing remains same if its plugged in or not. it seems I only get centerflugal advance noticeable when I rev the engine.
any ideas?
73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
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| Re: Vacuum advance not working? [message #67909 is a reply to message #67901] |
Sun, 20 December 2009 22:24  |
Craig Lechowicz
Messages: 541
Registered: October 2006
Location: Waterford, MI
Karma: 0
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Senior Member |
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>hey guys I have my vac advance on my distributor plugged directly to manifold vacuum, however it does not seem to advance it any, and timing remains same if its plugged in or not.
>it seems I only get centerflugal advance noticeable when I rev the engine.
It sounds like it's not working, although it takes a pretty big hole in the diaphragm to not work at all, so maybe the hose has degraded enough to be plugged? I don't think I've ever seen the plate that turns with the vacuum advance stuck, but that is another possibility. (GMC's are famous for the mechanical advance weights sticking, though, so at least you don't have that . . .)
If you have the vacuum line hooked to the manifold, and the advance is working, it will have maximum vacuum advance at idle, as the throttle plate is almost fully closed and hence, high vacuum. When you open the throttle, vacuum drops roughly with throttle opening (also rpm and load, but dominated by throttle opening) and the vacuum advance will be reducing, while the centrifugal (mechanical) advance will be increasing directly with rpm, so probably hard to sort out which is doing what. Easiest way to check is just unplug and rehook the vacuum hose at idle, and the timing mark should move a fair amount (10+ degrees.)
Craig Lechowicz
'77 Kingsley, Waterford, MI
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| Re: Vacuum advance not working? [message #67918 is a reply to message #67901] |
Sun, 20 December 2009 23:11 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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| Duce Apocalypse wrote on Sun, 20 December 2009 20:53 |
hey guys I have my vac advance on my distributor plugged directly to manifold vacuum, however it does not seem to advance it any, and timing remains same if its plugged in or not. it seems I only get centerflugal advance noticeable when I rev the engine.
any ideas?
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There is not much to it. Vacuum is applied to the vacuum can and it pulls a lever in the distributor. If you are connected to manifold vacuum, start the engine, remove the hose from the vacuum advance. The engine should immediately slow down or quit. If it doesn't, put a vacuum gauge on the hose you removed and check for the presence of vacuum. It should be 15" to 18". If you do not have a gauge, simply feel the end of the hose to see if there is vacuum present. If there is adequate vacuum, then the vacuum source is not the problem.
Remove the distributor cap reach inside and pull or push the vacuum lever towards the vacuum can. If it moves then the vacuum advance is bad. If no movement then figure out why the distributor plate that the advance lever is attached to is stuck.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Vacuum advance not working? [message #67936 is a reply to message #67918] |
Mon, 21 December 2009 06:28 |
Jim Bounds
Messages: 842
Registered: January 2004
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If your vacuum advance is connected to a constant source like your intake manifold, your dist is plumbed backwards. The way you have it, each time you step on the throttle, you motor "retards itself", that is opposite from what the motor is supposed to do. Under load, you want the motor to advance itself. This is a common issue I find in a good 1/2 of the coaches I find. An Oldsmobile 455 and 403 motor dist turns backwards to a Chevy 454 and others. You must plumb the dist to a "ported" output on the carb to the front @ 1/2 way up the body. That port supplies vacuum under load but not at idle. This is the sort of vacuum the dist needs-- what you have will "detune your motor".
Vacuum on our motors is a very important issue, if you need to talk more about this let me know. I have a unique way to tune a ported vacuum motor that seems to work very well. All you need to time your motor is 2 vacuum gasuges-- not timing light needed. Let me know if you want to know more,
Jim Bounds
---------------------------
----- Original Message ----
From: Ken Burton <n9cv@comcast.net>
To: gmclist@temp.gmcnet.org
Sent: Mon, December 21, 2009 12:11:59 AM
Subject: Re: [GMCnet] Vacuum advance not working?
Duce Apocalypse wrote on Sun, 20 December 2009 20:53
> hey guys I have my vac advance on my distributor plugged directly to manifold vacuum, however it does not seem to advance it any, and timing remains same if its plugged in or not. it seems I only get centerflugal advance noticeable when I rev the engine.
>
> any ideas?
There is not much to it. Vacuum is applied to the vacuum can and it pulls a lever in the distributor. If you are connected to manifold vacuum, start the engine, remove the hose from the vacuum advance. The engine should immediately slow down or quit. If it doesn't, put a vacuum gauge on the hose you removed and check for the presence of vacuum. It should be 15" to 18". If you do not have a gauge, simply feel the end of the hose to see if there is vacuum present. If there is adequate vacuum, then the vacuum source is not the problem.
Remove the distributor cap reach inside and pull or push the vacuum lever towards the vacuum can. If it moves then the vacuum advance is bad. If no movement then figure out why the distributor plate that the advance lever is attached to is stuck.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Vacuum advance not working? [message #67949 is a reply to message #67936] |
Mon, 21 December 2009 08:20 |
powerjon
Messages: 2446
Registered: January 2004
Karma: 5
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Senior Member |
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Jim,
I would be interested on your method to tune the engine with vacuum
gauges. You can contact direct if necessary.
J.R. Wright
On Dec 21, 2009, at 7:28 AM, Jim Bounds wrote:
> If your vacuum advance is connected to a constant source like your
> intake manifold, your dist is plumbed backwards. The way you have
> it, each time you step on the throttle, you motor "retards itself",
> that is opposite from what the motor is supposed to do. Under load,
> you want the motor to advance itself. This is a common issue I find
> in a good 1/2 of the coaches I find. An Oldsmobile 455 and 403
> motor dist turns backwards to a Chevy 454 and others. You must
> plumb the dist to a "ported" output on the carb to the front @ 1/2
> way up the body. That port supplies vacuum under load but not at
> idle. This is the sort of vacuum the dist needs-- what you have
> will "detune your motor".
>
> Vacuum on our motors is a very important issue, if you need to talk
> more about this let me know. I have a unique way to tune a ported
> vacuum motor that seems to work very well. All you need to time
> your motor is 2 vacuum gasuges-- not timing light needed. Let me
> know if you want to know more,
>
> Jim Bounds
> ---------------------------
>
>
>
> ----- Original Message ----
> From: Ken Burton <n9cv@comcast.net>
> To: gmclist@temp.gmcnet.org
> Sent: Mon, December 21, 2009 12:11:59 AM
> Subject: Re: [GMCnet] Vacuum advance not working?
>
>
>
> Duce Apocalypse wrote on Sun, 20 December 2009 20:53
>> hey guys I have my vac advance on my distributor plugged directly
>> to manifold vacuum, however it does not seem to advance it any, and
>> timing remains same if its plugged in or not. it seems I only get
>> centerflugal advance noticeable when I rev the engine.
>>
>> any ideas?
>
>
> There is not much to it. Vacuum is applied to the vacuum can and it
> pulls a lever in the distributor. If you are connected to
> manifold vacuum, start the engine, remove the hose from the vacuum
> advance. The engine should immediately slow down or quit. If it
> doesn't, put a vacuum gauge on the hose you removed and check for
> the presence of vacuum. It should be 15" to 18". If you do not
> have a gauge, simply feel the end of the hose to see if there is
> vacuum present. If there is adequate vacuum, then the vacuum source
> is not the problem.
>
> Remove the distributor cap reach inside and pull or push the vacuum
> lever towards the vacuum can. If it moves then the vacuum advance
> is bad. If no movement then figure out why the distributor plate
> that the advance lever is attached to is stuck.
>
>
>
>
>
>
>
>
>
>
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
>
>
>
> _______________________________________________
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> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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J.R. Wright
GMC GreatLaker
GMC Eastern States
GMCMI
78 30' Buskirk Stretch
75 Avion Under Reconstruction
Michigan
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| Re: [GMCnet] Vacuum advance not working? [message #67950 is a reply to message #67949] |
Mon, 21 December 2009 08:28 |
Steven Ferguson
Messages: 3447
Registered: May 2006
Karma: 0
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Senior Member |
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John,
Tuning a carb with a vacuum gauge has always been SOP for me. I
didn't know there was another way. How do you do it?
On Mon, Dec 21, 2009 at 6:20 AM, John Wright <powerjon@chartermi.net> wrote:
> Jim,
> I would be interested on your method to tune the engine with vacuum
> gauges. You can contact direct if necessary.
>
> J.R. Wright
> On Dec 21, 2009, at 7:28 AM, Jim Bounds wrote:
>
>> If your vacuum advance is connected to a constant source like your
>> intake manifold, your dist is plumbed backwards. The way you have
>> it, each time you step on the throttle, you motor "retards itself",
>> that is opposite from what the motor is supposed to do. Under load,
>> you want the motor to advance itself. This is a common issue I find
>> in a good 1/2 of the coaches I find. An Oldsmobile 455 and 403
>> motor dist turns backwards to a Chevy 454 and others. You must
>> plumb the dist to a "ported" output on the carb to the front @ 1/2
>> way up the body. That port supplies vacuum under load but not at
>> idle. This is the sort of vacuum the dist needs-- what you have
>> will "detune your motor".
>>
>> Vacuum on our motors is a very important issue, if you need to talk
>> more about this let me know. I have a unique way to tune a ported
>> vacuum motor that seems to work very well. All you need to time
>> your motor is 2 vacuum gasuges-- not timing light needed. Let me
>> know if you want to know more,
>>
>> Jim Bounds
>> ---------------------------
>>
>>
>>
>> ----- Original Message ----
>> From: Ken Burton <n9cv@comcast.net>
>> To: gmclist@temp.gmcnet.org
>> Sent: Mon, December 21, 2009 12:11:59 AM
>> Subject: Re: [GMCnet] Vacuum advance not working?
>>
>>
>>
>> Duce Apocalypse wrote on Sun, 20 December 2009 20:53
>>> hey guys I have my vac advance on my distributor plugged directly
>>> to manifold vacuum, however it does not seem to advance it any, and
>>> timing remains same if its plugged in or not. it seems I only get
>>> centerflugal advance noticeable when I rev the engine.
>>>
>>> any ideas?
>>
>>
>> There is not much to it. Vacuum is applied to the vacuum can and it
>> pulls a lever in the distributor. If you are connected to
>> manifold vacuum, start the engine, remove the hose from the vacuum
>> advance. The engine should immediately slow down or quit. If it
>> doesn't, put a vacuum gauge on the hose you removed and check for
>> the presence of vacuum. It should be 15" to 18". If you do not
>> have a gauge, simply feel the end of the hose to see if there is
>> vacuum present. If there is adequate vacuum, then the vacuum source
>> is not the problem.
>>
>> Remove the distributor cap reach inside and pull or push the vacuum
>> lever towards the vacuum can. If it moves then the vacuum advance
>> is bad. If no movement then figure out why the distributor plate
>> that the advance lever is attached to is stuck.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> --
>> Ken Burton - N9KB
>> 76 Palm Beach
>> Hebron, Indiana
>> _______________________________________________
>> GMCnet mailing list
>> List Information and Subscription Options:
>> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>>
>>
>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> List Information and Subscription Options:
>> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Steve Ferguson
'76 EII
Sierra Vista, AZ
Urethane bushing source
www.bdub.net/ferguson/
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| Re: [GMCnet] Vacuum advance not working? [message #67956 is a reply to message #67936] |
Mon, 21 December 2009 09:21 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""The way you have it, each time you step on the throttle, you motor "retards itself", that is opposite from what the motor is supposed to do. Under load, you want the motor to advance itself. ""
Sorry to argue that point Jim, but the purpose of the vacuum advance is to retard the timing when you ask for power. That holds true whether you have it connected to ported or to manifold vacuum. On any engine, you want higher advance under light load and less advance under heavy load.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: Vacuum advance not working? [message #67962 is a reply to message #67901] |
Mon, 21 December 2009 10:46 |
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ok, let me clear some things up, I used a timing light and yes I have a vac gauge installed, at idle I get around 18-20" vac, the line is new and there is pleanty of vac at the advance diaphram, but what I see is that the advance doesnt seem to do that, if the line is plugged in or not, this would to me seem to indicae that the advance is stuck closed or not working.
Centerflugal advance works as the timing changed when the engine was reved. however there seems to be no change when vac is applied.
The manual states *8 BTDC with the advance unplugged, whcih tells me the advance should advance when it sees some vac, but it does not, to get decent power im running about 18* BTDC, which seems to work fine, but I would perfer a properly working advance system...
Jim I am also interested to hear what you have to say on this matter as well... I'll also take a look at that link...
73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
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| Re: [GMCnet] Vacuum advance not working? [message #67972 is a reply to message #67962] |
Mon, 21 December 2009 11:44 |
Steven Ferguson
Messages: 3447
Registered: May 2006
Karma: 0
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Senior Member |
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Shan,
Sounds like you've found the problem.
On Mon, Dec 21, 2009 at 8:46 AM, Shan Rose <defconfx@gmail.com> wrote:
>
>
> ok, let me clear some things up, I used a timing light and yes I have a vac gauge installed, at idle I get around 18-20" vac, the line is new and there is pleanty of vac at the advance diaphram, but what I see is that the advance doesnt seem to do that, if the line is plugged in or not, this would to me seem to indicae that the advance is stuck closed or not working.
>
> Centerflugal advance works as the timing changed when the engine was reved. however there seems to be no change when vac is applied.
>
> The manual states *8 BTDC with the advance unplugged, whcih tells me the advance should advance when it sees some vac, but it does not, to get decent power im running about 18* BTDC, which seems to work fine, but I would perfer a properly working advance system...
>
> Jim I am also interested to hear what you have to say on this matter as well... I'll also take a look at that link...
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Steve Ferguson
'76 EII
Sierra Vista, AZ
Urethane bushing source
www.bdub.net/ferguson/
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| Re: [GMCnet] Vacuum advance not working? [message #67981 is a reply to message #67936] |
Mon, 21 December 2009 12:40 |
Rick Denney
Messages: 430
Registered: January 2004
Karma: 0
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Senior Member |
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Jim Bounds writes...
> If your vacuum advance is connected to a constant source like your
> intake manifold, your dist is plumbed backwards. The way you have
> it, each time you step on the throttle, you motor "retards itself",
> that is opposite from what the motor is supposed to do. Under load,
> you want the motor to advance itself. This is a common issue I find
> in a good 1/2 of the coaches I find.
Jim, sorry, that's not how it works. The manifold vacuum differs from
ported vacuum only at low RPMs. Above idle, they should read the same
vacuum.
There is no source for vacuum higher than engine vacuum, so there's no
way ported vacuum can ever be higher than engine vacuum. It can,
however, be lower, as it is when at idle. As has been repeatedly
reported here by ex-GM engineers, the use of ported vacuum was an
emissions-control measure only. They bypass it (using the TVS) and use
straight manifold vacuum when the engine is really hot.
Rick "piling on" Denney
'73 230 Ex-Glacier "Jaws"
Northern Virginia
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| Re: Vacuum advance not working? [message #67999 is a reply to message #67962] |
Mon, 21 December 2009 14:40 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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I just want to make sure the 18" to 20" reading on the vacuum was taken from the unplugged hose that normally went to the advance.
If it was taken there, then you have a stuck distributor plate or a bad diapham in the advance itself.
Also are you sure that your vacuum advance is coming direct from the manifold? What you are describing is correct for the GMC as it was designed. There is no vacuum to the advance at speeds under around 1,000 rpm and no load. Is the vacuum advance really attached directly to the manifold or is it possibly attached to the TVS which is mounted in the manifold?
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Vacuum advance not working? [message #68032 is a reply to message #67901] |
Mon, 21 December 2009 17:23 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Rick is correct in that once on open throttle (past the ported port) the 2 become virtually the same. Where I beg to differ is in that ported was an emissions thing as I remember ported on early 60s GMs and probably before that. The dist vac line always went to the ported port on the carb. What WAS an emission thing with timing was TCS (transmision controled spark) that prevented vac advance until the THM was in high gear. That really 'sucked' to use the vacuum analogy and really made the vehicle a dog. I remember that was the first thing I bypassed on my 70 Buick GS. Big difference. Not affecting us GMCMH guys but one thing you never want to "bypass" is the EGR. Many THM trans have met an early death due to that one hose being disconnected. The car runs 'better' when the EGR is disconnected (talking pre ECM cars here) but the trans was programed to shift nice with the EGR connected. With it disconnected there is way too much clutch slip and the trans dies early. This is because less degrees of throttle are required (at part throttle range where most driving is done) to get the same power when the EGR is disconnected. Ahh the golden age of vacuum controled emissions devices. It just goes to show you what an accurate indicator of engine operating conditions vacuum is to control a number of other functions.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Vacuum advance not working? [message #68033 is a reply to message #67999] |
Mon, 21 December 2009 17:50 |
klassic kampers
Messages: 93
Registered: July 2008
Location: greer,s.c./ellijay,ga
Karma: 0
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being one of the "new guys" on this net, I hope not to make enemies but jim bounds is correct.........the gmc motor home was hosed-up when new with the vacuum advance running primarialy off of port vacuum.........the TVS (thermostatic vacuum switch)switch has three connections.........."MT" on the bottom for manifold vacuum from carburetor(below the throttle plates) or from the intake, "C" in the center for port vacuum from the carburetor(above the throttle plates) and "D" on the top for routing to the distributor advance........the engine idles under normal conditions without any advance except the static timing with the centrifugal and port vacuum blending in for acceleration and fuel economy......when idling as if in heavy summertime traffic and engine coolant temp reaches about 225 degrees F the TVS opens directing manifold vacuum the the distributor advance, advancing the timing speeding up the engine......this increases fan speed cooling the engine and at the same time the timing advance cools the combustion chamber........when the coolant temp gets down to about 210 F the TVS closed and then the engine is on port vacuum only and the static timing figure is restored.......
yes when cruising at moderate speeds the manifold vacuum does "catch up" to the port vacuum but if the speed is increased so as to the motor home is pushing wind , then the manifold vacuum goes down, performance is lost and fuel economy takes a hit.......the article from the magazine is spot on about port vs manifold vacuum but they are dealing with a 35000lb vehicle with a much less mechanical and wind drag than a motorhome...........I do not intend to give the impression of being a "know it all jerk"( which I am absolutley not) but this is what I do in my shop when not working on gmc motorhomes......on older GM trucks, I do Q-jets and solve driveability problems increasing fuel economy while decreasing emissions by curving ignition advance and correcting fuel delivery making vacuum hose routing is correct for the vehicle weight, rear end gear, and tire size............................
thanks for listening to my opinion as that is just what it is and not to suggest that anyone modify their vehicle per my recommendations........
Mike Stewart
1973 GMC 26' Canyonlands /
1973 B.S.A. B50 street tracker-----
Greer,S.C/Ellijay,Ga
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| Re: [GMCnet] Vacuum advance not working? [message #68034 is a reply to message #68033] |
Mon, 21 December 2009 18:05 |
Rick Denney
Messages: 430
Registered: January 2004
Karma: 0
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Senior Member |
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mike stewart writes...
> being one of the "new guys" on this net, I hope not to make enemies
> but jim bounds is correct.........the gmc motor home was hosed-up
> when new with the vacuum advance running primarialy off of port
> vacuum.........the TVS (thermostatic vacuum switch)switch has three
> connections.........."MT" on the bottom for manifold vacuum from
> carburetor(below the throttle plates) or from the intake, "C" in the
> center for port vacuum from the carburetor(above the throttle
> plates) and "D" on the top for routing to the distributor
> advance........
Nobody is arguing that point. I was refuting the notion in Jim's post
that ported vacuum is "reversed" from manifold vacuum, meaning that
when manifold vacuum is high, ported vacuum is low, and when ported
vacuum is high, manifold vacuum is low. That is not the case. The
manifold is the only source of vacuum that can do any work, and
therefore the ported vacuum can never be higher than the manifold
vacuum. It can only be lower, as it is at idle. At large throttle
openings when manifold vacuum is low, ported vacuum will be low as
well. When manifold vacuum is running at 12-15 inches of mercury as
will be the case during highway cruise, ported vacuum will be the
same.
The reason this is important is because some people choose to remove
the TVS and use manifold vacuum directly, including some ex-GM
engineers who have taken a lot of abuse for trying to make the point I
just made, based on the incorrect notion that the two are "reversed".
I was not arguing in favor of using manifold vacuum instead of ported
vacuum (at least not this time). At cruise and wider throttle openings
it won't matter, if the timing is set properly.
> the engine idles under normal conditions without any
> advance except the static timing with the centrifugal and port
> vacuum blending in for acceleration and fuel economy......when
> idling as if in heavy summertime traffic and engine coolant temp
> reaches about 225 degrees F the TVS opens directing manifold vacuum
> the the distributor advance, advancing the timing speeding up the
> engine......this increases fan speed cooling the engine and at the same time
> the timing advance cools the combustion chamber........when the
> coolant temp gets down to about 210 F the TVS closed and then the
> engine is on port vacuum only and the static timing figure is restored.......
Yes, no argument there. On the other hand, my coolant temperature has
never reached even 200 degrees in idling conditions. The only time I
see that temperature is when climbing hills, when vacuum is low in any
case and when the TVS will have no effect.
Rick "the official GMCnet know-it-all jerk, so you have nothing to
worry about" Denney
'73 230 Ex-Glacier "Jaws"
Northern Virginia
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| Re: Vacuum advance not working? [message #68098 is a reply to message #67901] |
Tue, 22 December 2009 10:42 |
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wha I dont get is the notion that the vac advance actually retards timing, that makes no sense or it would be called a vacuum retard rather then vacuum advance. I do understand port versus mani vac, and yes mine is going right to the mani which should see some timing advance at idle 10-14* from what I understand, but I only have centerflugal advance so the unit is obviously defective, which means Im only getting static timing + centerflugal advance when driving, not centerflugal, vac, and static together. from what I understand one should have full advance at 12-15 in Hg is this corect?
73 Canyon Lands, (a.k.a. The Yellow Submarine) West Los Angeles CA
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| Re: [GMCnet] Vacuum advance not working? [message #68099 is a reply to message #68098] |
Tue, 22 December 2009 10:45 |
Steven Ferguson
Messages: 3447
Registered: May 2006
Karma: 0
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Senior Member |
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Correct Shan.
Man, without a properly working vac advance, your mileage must be in
the toilet. The vacuum ad can is easy to replace and can be
accomplished without removing the dist. I prefer to remove it though.
Gives an opportunity to clean up and lube the weights and keeps the
think from freezing in the block.
On Tue, Dec 22, 2009 at 8:42 AM, Shan Rose <defconfx@gmail.com> wrote:
>
>
> wha I dont get is the notion that the vac advance actually retards timing, that makes no sense or it would be called a vacuum retard rather then vacuum advance. I do understand port versus mani vac, and yes mine is going right to the mani which should see some timing advance at idle 10-14* from what I understand, but I only have centerflugal advance so the unit is obviously defective, which means Im only getting static timing + centerflugal advance when driving, not centerflugal, vac, and static together. from what I understand one should have full advance at 12-15 in Hg is this corect?
>
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--
Steve Ferguson
'76 EII
Sierra Vista, AZ
Urethane bushing source
www.bdub.net/ferguson/
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| Re: Vacuum advance not working? [message #68104 is a reply to message #68098] |
Tue, 22 December 2009 11:35 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""wha I dont get is the notion that the vac advance actually retards timing, that makes no sense or it would be called a vacuum retard rather then vacuum advance. I do understand port versus mani vac, and yes mine is going right to the mani which should see some timing advance at idle 10-14* from what I understand, but I only have centerflugal advance so the unit is obviously defective, which means Im only getting static timing + centerflugal advance when driving, not centerflugal, vac, and static together. from what I understand one should have full advance at 12-15 in Hg is this corect?""
We've probably caused a lot of confusion thru the threads. With high vacuum such as at idle (if you are connected to the manifold) and at light load, the timing will be advanced. When you get into the throttle such as when accelerating or going up hill, the vacuum decreases and the timing will start to retard in order to prevent pinging. If you are attached to the manifold, at idle, you should see full advance from the vacuum advance PLUS your static timing, PLUS a slight amount of advance from the centrifugal advance (dependent on your idle speed).
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Vacuum advance not working? [message #68106 is a reply to message #68098] |
Tue, 22 December 2009 12:16 |
GMCWiperMan
Messages: 1248
Registered: December 2007
Karma: 1
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Senior Member |
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Shan,
I'm glad to hear someone finally say that. Ever since I started
trying to understand automobiles, about 60 years ago, I've wondered
the same thing: The reason we use vacuum is so the timing will retard
to correspond with the load, therefore, it's "Vacuum Retard". :-)
Too bad they started 'way back when calling it "Vacuum Advance".
Maybe someday I'll be able to remember it.
How did I start trying to learn about auto stuff 60+ years ago? I was
a bookworm. In my grandparents' attic I found an unused set of auto
mechanics text books -- 4-5 of them, IIRC, from a then-famous
correspondence school. Just as I did the set of Compton's
Encyclopedias my folks bought me, I read every word in them, even
though they were apparently written in the 1920's. Want to know how
to pour a babbitt rod or main bearing -- in the engine? Just ask. :-)
Ken "Full of Worthless Information -- and Shaing It" H.
On Tue, Dec 22, 2009 at 11:42 AM, Shan Rose <defconfx@gmail.com> wrote:
>
> wha I dont get is the notion that the vac advance actually retards timing, that makes no sense or it would be called a vacuum retard rather then vacuum advance...
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