Why did mazda retard timing at idle on carb'd 12A?
I need to worry about this stuff too. What was the final verdict on the rat's nest made easy for PCV valve function? I don't have a purge valve or a PCV valve for this '84 12A I'm working on.
Rather than hijack this thread I'll describe it in detail in a new thread with pics and interesting info. Maybe others will learn something from my efforts.
Rather than hijack this thread I'll describe it in detail in a new thread with pics and interesting info. Maybe others will learn something from my efforts.
Absolute Power is Awesome
Joined: Mar 2001
Posts: 1,973
Likes: 5
From: Issaquah, WA
Originally Posted by Gen1onr
Oh man, now im confused.
Im about to go out and set the timing on my car, so I read over this:
http://www.yawpower.com/pultime.html
Which says that the vacuum advance works off manifold vacuum, and advances the timing at idle.
Is there a straight answer for this???
Im about to go out and set the timing on my car, so I read over this:
http://www.yawpower.com/pultime.html
Which says that the vacuum advance works off manifold vacuum, and advances the timing at idle.
Is there a straight answer for this???
The workshop manual is the best place to find out where to plug the vacuum line and how to do the timing. Yaw's description works too, though.
Too old to act my age
Joined: Feb 2005
Posts: 3,164
Likes: 0
From: Tulsa, Ok.
Originally Posted by Gen1onr
Oh man, now im confused.
Im about to go out and set the timing on my car, so I read over this:
http://www.yawpower.com/pultime.html
Which says that the vacuum advance works off manifold vacuum, and advances the timing at idle.
Is there a straight answer for this???
Im about to go out and set the timing on my car, so I read over this:
http://www.yawpower.com/pultime.html
Which says that the vacuum advance works off manifold vacuum, and advances the timing at idle.
Is there a straight answer for this???
I've been offline for a couple weeks due to a move in my residence, sorry for the delay in adding the correct $.02.....
Absolute Power is Awesome
Joined: Mar 2001
Posts: 1,973
Likes: 5
From: Issaquah, WA
My thesis for my masters ME graduate degree was in engine airflow. I've had alot of experience in engine operation.
Any reference source you look at will tell you the same thing that I'm saying.
Any reference source you look at will tell you the same thing that I'm saying.
I think your car will run ok either of the 2 ways you connect it.
The rat nest made easy drawing is drawn from Carl and Pratch's tutorial.
If you look on page 254 of the Haynes manual, you will see the source of both leading and trailing vacuum is above the throttle plates.
The rat nest made easy drawing is drawn from Carl and Pratch's tutorial.
If you look on page 254 of the Haynes manual, you will see the source of both leading and trailing vacuum is above the throttle plates.
Absolute Power is Awesome
Joined: Mar 2001
Posts: 1,973
Likes: 5
From: Issaquah, WA
Originally Posted by Rogue_Wulff
Engineers and teenagers. Can't tell 'em a dern thing, cause they know everything.......
Hunting Skylines
Joined: Apr 2001
Posts: 3,431
Likes: 4
From: Brisbane, Queensland, Australia.
Originally Posted by Rogue_Wulff
That proves my point. If you hook the vac advance to manifold vac, it becomes a vacuum retard. Manifold vacuum drops as you open the throttle. Venturi vacuum increases, as the airflow thru the venturi's increases, hence the name.
Those diagrams show the total advance of the mechanical and vacuum advance individually, but they are combined to achieve the total advance.
Mechanical is 12.5* on both Leading and trailing, while the vacuum is 4.5* L, and 15* T. Total advance becomes 17* L (12.5 + 4.5) & 27.5* T (12.5 + 15) (12A figures quoted, as that is what the title of the thread is dealing with).
Anyone who say vacuum advance should be hooked to a manifold vacuum source, is wrong. I don't care who they are, or how many books they may have written.
Those diagrams show the total advance of the mechanical and vacuum advance individually, but they are combined to achieve the total advance.
Mechanical is 12.5* on both Leading and trailing, while the vacuum is 4.5* L, and 15* T. Total advance becomes 17* L (12.5 + 4.5) & 27.5* T (12.5 + 15) (12A figures quoted, as that is what the title of the thread is dealing with).
Anyone who say vacuum advance should be hooked to a manifold vacuum source, is wrong. I don't care who they are, or how many books they may have written.
As manifold vacuum increases the amount of advance increases. It's really not that hard to follow.
Vacuum advance is there to provide more timing during light load/low throttle situations. Purple82 is absolutely correct.
I'd say 24 - 26 mpg on a carb'd streetport 12A has probably got it on the correct nipple....for all the folks going with the red diagram, what's the mpg been for y'all? Maybe that isn't the answer after all, despite the smooth idle. Results are more important than belief this time around.
And if we're going to start yakkin' about degrees and letters, post PROOF by posting a pic of your degree...surely that isn't too daunting a task for an ME or even a PhD kind guy...
,...too much 'Net BS sometimes about accomplishments w/o really proving anything goes on everywhere these days, speakin' of BS...BS in Science here...Pre-Med degree pursued in folly...
And if we're going to start yakkin' about degrees and letters, post PROOF by posting a pic of your degree...surely that isn't too daunting a task for an ME or even a PhD kind guy...
,...too much 'Net BS sometimes about accomplishments w/o really proving anything goes on everywhere these days, speakin' of BS...BS in Science here...Pre-Med degree pursued in folly...
Newbie
Joined: Mar 2007
Posts: 1
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From: Cuyahoga Falls, Ohio
so i am new here but i know something about this but i have some questions first i dont have the time right now to read through five pages of posts but has anyone checked the vaccum routing diagrams in a factory service maual
Too old to act my age
Joined: Feb 2005
Posts: 3,164
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From: Tulsa, Ok.
Originally Posted by REVHED
Did you not read the friggin diagram?
As manifold vacuum increases the amount of advance increases. It's really not that hard to follow.
Vacuum advance is there to provide more timing during light load/low throttle situations. Purple82 is absolutely incorrect.
As manifold vacuum increases the amount of advance increases. It's really not that hard to follow.
Vacuum advance is there to provide more timing during light load/low throttle situations. Purple82 is absolutely incorrect.
Full Member
Joined: Oct 2005
Posts: 54
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From: urf
http://members.iinet.net.au/~awcabren/DSC00114.JPG
http://members.iinet.net.au/~awcabren/DSC00113.JPG
I've got mine hooked up to the far left nipple and the far right nipple, should i move the far left one to the second far right one?
http://members.iinet.net.au/~awcabren/DSC00113.JPG
I've got mine hooked up to the far left nipple and the far right nipple, should i move the far left one to the second far right one?
Absolute Power is Awesome
Joined: Mar 2001
Posts: 1,973
Likes: 5
From: Issaquah, WA
Originally Posted by Rogue_Wulff
Engineers and teenagers. Can't tell 'em a dern thing, cause they know everything.......
By the way, I'm not saying that you've got it plugged into the wrong port. I'm just saying you don't have the right reason for plugging it into that port.
Again folks, use the workshop manual for the correct location.
Too old to act my age
Joined: Feb 2005
Posts: 3,164
Likes: 0
From: Tulsa, Ok.
Originally Posted by purple82
LOL, who's the stubborn one? Admit when you're wrong and move on.
By the way, I'm not saying that you've got it plugged into the wrong port. I'm just saying you don't have the right reason for plugging it into that port.
Again folks, use the workshop manual for the correct location.
By the way, I'm not saying that you've got it plugged into the wrong port. I'm just saying you don't have the right reason for plugging it into that port.
Again folks, use the workshop manual for the correct location.
Manifold vacuum drops as the throttle is opened, while venturi vacuum increases as it's opened. Don't take my word on it, though. Use a vacuum guage and test it for yourself.
I don't come on here trying to prove I'm right by backing up what I say with degrees and other stuff that says I be edumacated, instead I stated what every old-school hot rodder has known for years.
The vac advance increases timing under lower RPM's and engine loads by using venturi vacuum, because under those conditions, manifold vacuum is actually decreasing. Why do you think the brake booster has a check valve in it?
Absolute Power is Awesome
Joined: Mar 2001
Posts: 1,973
Likes: 5
From: Issaquah, WA
Originally Posted by Rogue_Wulff
But I ain't wrong. Using the workshop manual to find the right port is excellent advice, as it backs up what I have been trying to tell you. Even the vacuum diagram under the hood shows where the vac line should be hooked up.
Manifold vacuum drops as the throttle is opened, while venturi vacuum increases as it's opened. Don't take my word on it, though. Use a vacuum guage and test it for yourself.
I don't come on here trying to prove I'm right by backing up what I say with degrees and other stuff that says I be edumacated, instead I stated what every old-school hot rodder has known for years.
The vac advance increases timing under lower RPM's and engine loads by using venturi vacuum, because under those conditions, manifold vacuum is actually decreasing. Why do you think the brake booster has a check valve in it?
Manifold vacuum drops as the throttle is opened, while venturi vacuum increases as it's opened. Don't take my word on it, though. Use a vacuum guage and test it for yourself.
I don't come on here trying to prove I'm right by backing up what I say with degrees and other stuff that says I be edumacated, instead I stated what every old-school hot rodder has known for years.
The vac advance increases timing under lower RPM's and engine loads by using venturi vacuum, because under those conditions, manifold vacuum is actually decreasing. Why do you think the brake booster has a check valve in it?
Too old to act my age
Joined: Feb 2005
Posts: 3,164
Likes: 0
From: Tulsa, Ok.
Engineers have no credibility with me. As for Yaw's write up, I already answered that. Yes, it's wrong, and I told him so. He never bothered to respond.
Hook it however you want, I don't care. I know what the correct method is, and I use it.
I'm done with this rediculous thread. You can make a person go to school, but you can't make them learn.
Hook it however you want, I don't care. I know what the correct method is, and I use it.
I'm done with this rediculous thread. You can make a person go to school, but you can't make them learn.
Lives on the Forum
Joined: Jun 2004
Posts: 11,359
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From: Grand Rapids Michigan
Simple; I typed "vacuum advance" into google, and this is what it spat out (top link) http://www.leeric.lsu.edu/bgbb/7/ecep/auto/h/h.htm
In this article, it states that vacuum for the advance mechanism is taken from above the throttle plates so that vacuum increases with throttle opening. This is exactly how I always understood it to be.
Purple82, I believe you are operating under a misconception here...
In this article, it states that vacuum for the advance mechanism is taken from above the throttle plates so that vacuum increases with throttle opening. This is exactly how I always understood it to be.
Purple82, I believe you are operating under a misconception here...
Hunting Skylines
Joined: Apr 2001
Posts: 3,431
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From: Brisbane, Queensland, Australia.
Originally Posted by Kentetsu
Simple; I typed "vacuum advance" into google, and this is what it spat out (top link) http://www.leeric.lsu.edu/bgbb/7/ecep/auto/h/h.htm
In this article, it states that vacuum for the advance mechanism is taken from above the throttle plates so that vacuum increases with throttle opening. This is exactly how I always understood it to be.
Purple82, I believe you are operating under a misconception here...
In this article, it states that vacuum for the advance mechanism is taken from above the throttle plates so that vacuum increases with throttle opening. This is exactly how I always understood it to be.
Purple82, I believe you are operating under a misconception here...
The part I've underlined explains this clear as day. Vacuum advance is for low engine speed/load. The only reason it's above the throttle is to prevent advance at idle in some cases. I hope this puts an end to this nonsense.
Only when a strong enough vacuum signal is present will it advance the ignition timing. The higher the vacuum signal, the greater the timing advance. High vacuum signals are indicative of low engine loads at a closed or nearly closed throttle position. At idle, when the throttle is almost completely closed, vacuum is highest; however, during idle, no vacuum signal is present at the vacuum advance. This is accomplished by tapping the vacuum for the advance system above the throttle plate.
Lives on the Forum
Joined: Jun 2004
Posts: 11,359
Likes: 14
From: Grand Rapids Michigan
But you're saying the same thing I am! No advance at idle, only at low rpms when you put a load on it (open the throttle plates). The two sentences after the part you underlined explains how this works.
Registered Offender
Joined: Jul 2006
Posts: 689
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From: Bishopville, SC
Originally Posted by REVHED
I believe you are the one who is under a misconception. Read the WHOLE article. The vacuum is taken from just above the throttle plates so there is no vacuum at idle. But, as soon as you crack the throttle it uncovers the port which is then exposed to manifold vacuum! Having the port way above the throttle would be redundant as it would advance at virtually the same rate as the centrifugal advance.
The part I've underlined explains this clear as day. Vacuum advance is for low engine speed/load. The only reason it's above the throttle is to prevent advance at idle in some cases. I hope this puts an end to this nonsense.
The part I've underlined explains this clear as day. Vacuum advance is for low engine speed/load. The only reason it's above the throttle is to prevent advance at idle in some cases. I hope this puts an end to this nonsense.
