Why did mazda retard timing at idle on carb'd 12A?
03-14-07, 12:52 PM
#1
Why did mazda retard timing at idle on carb'd 12A?
Ok not technically retarded, but not advanced.
After doing the "rat nest removal" I realized I'm applying vac to the distrib all the time. (advancing timing) With the rat nest removal at idle there was no vac to the distrib.
I like the way the exhaust sounds with no vac. It's more of a deep pinging throaty note, but the idle seems less stable at low rpm like this. I could easily hook up a solenoid to return the car to the factory no vac idle setting. Just wondering why mazda decided this was beneficial?
After doing the "rat nest removal" I realized I'm applying vac to the distrib all the time. (advancing timing) With the rat nest removal at idle there was no vac to the distrib.
I like the way the exhaust sounds with no vac. It's more of a deep pinging throaty note, but the idle seems less stable at low rpm like this. I could easily hook up a solenoid to return the car to the factory no vac idle setting. Just wondering why mazda decided this was beneficial?
03-14-07, 12:56 PM
#2
Lives on the Forum
You have the vacuum advance line running to the wrong nipple on the carb base. Some of them always provide vacuum, and some only when on the throttle. I believe either of the two on the far right will get you where you need to be.
03-14-07, 01:25 PM
#3
Originally Posted by Kentetsu
You have the vacuum advance line running to the wrong nipple on the carb base. Some of them always provide vacuum, and some only when on the throttle. I believe either of the two on the far right will get you where you need to be.
Your saying not like this.....
But like this.....
Last edited by vxturboxv; 03-14-07 at 01:32 PM.
03-14-07, 02:00 PM
#4
Lives on the Forum
Right. You don't want the vac advance running at idle, because then you'll never get it to idle correctly. Also, if you set it up that way, you won't get proper advance when you do need it.
Just going by memory, I think there are five nipples on the base of the carb. Check each one for vacuum at idle, and use one that doesn't have any. I believe either of the two on the far right will work for this. I had this problem for a long time before I read this information and got it corrected. Setting idle speed and mixture is much easier once this is corrected. Idle should also be a lot smoother too. Good luck.
Just going by memory, I think there are five nipples on the base of the carb. Check each one for vacuum at idle, and use one that doesn't have any. I believe either of the two on the far right will work for this. I had this problem for a long time before I read this information and got it corrected. Setting idle speed and mixture is much easier once this is corrected. Idle should also be a lot smoother too. Good luck.
03-14-07, 03:23 PM
#5
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the timing is setup like it is, because they had to pass smog, and the distributor is a little limited. dizzy can adjust the timing like 20-25 degrees, the FC efi does 48 + whatever retards it needs
if you dont have emissions, the idle timing can just be set to where it idles the best
if you dont have emissions, the idle timing can just be set to where it idles the best
03-14-07, 03:48 PM
#6
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Originally Posted by vxturboxv
Good to know, but is there any benefit to doing this? Why not keep it advanced all the time? Is it just to prevent over heating at idle? I don't seem have any problems there.
Your saying not like this.....
But like this.....
Your saying not like this.....
But like this.....
03-14-07, 03:54 PM
#7
hmm... I've got mine connected on the second spot, like in the first diagram. How do you know which of those nipples gives vacuum when? I mean I guess a vacuum gauge could tell you vacuum at idle, but how do you get a vacuum reading under load?
Jon
Jon
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03-14-07, 04:06 PM
#8
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The second from front *should* be a venturi vacuum. Venturi vacuum is drawn from above the throttle plates, only as they are opened, which is how the vac advance is designed to work.
03-14-07, 06:40 PM
#9
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I'm only going by memory here, and I'll admit I'm getting a little old so I may be wrong. I hooked a vac line up to each nipple and felt for vacuum at idle. Once I found one that was static, I plugged my vacuum advance line up to that. I'm pretty sure it was one of the last ones to the right, but the alzheimer's could be kicking in again... Carl or Sterling should know for certain which one you (we) should be using...
03-14-07, 08:17 PM
#10
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Not meaning to thread jack, but I just had a major concern,,, I have a Holley (non-RB) on a RB intake manifold with no emmission and thus, I have my vacuum advance hooked to a constant vacuum source on my Holley Carb... is this why I have to set my idle @ 1300 RPM to get the damn thing to idle? I thought it was vacuum leaks the whole time! What SHOULD I BE doing in this case? I assume the Holley would also have a venturi vacuum source... no?
Last edited by rotaryboy23; 03-14-07 at 08:22 PM. Reason: 'nuther question...
03-14-07, 08:33 PM
#11
I dunno bud, but when I had my vac advance hooked up wrong it didnt change much. Does run a bit smoother now tho...
Try pulling your vac advance tube and plugging it, and see if that helps your idle. If it does you gotta find yourself one of them fancy venturi vac ports if there even is such a thing on the holley. I am not sure if theyre are but its a start!
Try pulling your vac advance tube and plugging it, and see if that helps your idle. If it does you gotta find yourself one of them fancy venturi vac ports if there even is such a thing on the holley. I am not sure if theyre are but its a start!
03-15-07, 01:02 PM
#12
My car with the 2nd gen coils and intake mods I did will idle at 600 rpm with the advance hooked up. It actually won't idle as well with the advance not hooked up. (static) But it does sound cooler! 
I'll play with it and let you all know. As long as your not over heating I don't think it hurts a thing to have it constantly advanced.
I was more interested in the WHY not the HOW. But both are good to know!
thanks...
I'll play with it and let you all know. As long as your not over heating I don't think it hurts a thing to have it constantly advanced.I was more interested in the WHY not the HOW. But both are good to know!
thanks...
03-15-07, 01:36 PM
#13
[QUOTE=vxturboxv]...intake mods...QUOTE]
elaborate please. i assume youre talking about the mods to the inside of the runners for better atomization, correct? if so, i was really wanting to do this but i have yet to have anyone answer me how these need to be. random design? duplicate spiral? ive read the whole other thread about it, but in there my questions were ignored. any info will help. thanks
pm me if youd like to not hi-jack this thread
elaborate please. i assume youre talking about the mods to the inside of the runners for better atomization, correct? if so, i was really wanting to do this but i have yet to have anyone answer me how these need to be. random design? duplicate spiral? ive read the whole other thread about it, but in there my questions were ignored. any info will help. thanks
pm me if youd like to not hi-jack this thread
03-15-07, 01:54 PM
#14
[QUOTE=aws140]
I did a little write up on it... The design isn't all that important. Read up here.
https://www.rx7club.com/1st-generation-specific-1979-1985-18/cutting-grooves-intake-manifold-555308/
Originally Posted by vxturboxv
...intake mods...QUOTE]
elaborate please. i assume youre talking about the mods to the inside of the runners for better atomization, correct? if so, i was really wanting to do this but i have yet to have anyone answer me how these need to be. random design? duplicate spiral? ive read the whole other thread about it, but in there my questions were ignored. any info will help. thanks
pm me if youd like to not hi-jack this thread
elaborate please. i assume youre talking about the mods to the inside of the runners for better atomization, correct? if so, i was really wanting to do this but i have yet to have anyone answer me how these need to be. random design? duplicate spiral? ive read the whole other thread about it, but in there my questions were ignored. any info will help. thanks
pm me if youd like to not hi-jack this thread
https://www.rx7club.com/1st-generation-specific-1979-1985-18/cutting-grooves-intake-manifold-555308/
03-15-07, 08:41 PM
#16
Originally Posted by dantheman
I wanted to chime in here and say that I had mine set up wrong. I connected it up right and it runs quite a bit better now.
Thanks!
Thanks!
which nipple do you have yours hooked up to now? the way of the first diagram or like the red line? if the first diagram that was posted in incorrect, then a lot of forum members have theres hooked up in the wrong location. if it is wrong, then there needs to be changes made in the archieve about removing the rats nest.
03-16-07, 02:54 AM
#17
There is so much mis-information in this thread it isn't funny.
#1 Advancing Timing Causes cooler engine temperature, especially when you are talking about at idle.
#2 The Correct Vacuum nipples do not have vacuum present on them when you are off the throttle in an attempt to keep the idle timing more correct, but still allow for advanced timing under cruising conditions.
#3 The stock distributor does not advance anywhere near 20-25 Degrees. The Distributor will advance by around 13 Degrees from idlish rpm to 2500 or something (I read this in the FSM btw, no making this up). Vacuum advance is sweet **** all anyway, about 3 Degrees on the leading if I remember correctly. Trailing vacuum advance is much more than leading, somewhere in the realm of about 10-15 degrees if I remember correctly. Firing the Plugs very close together under higher rpm and vacuum conditions is a good thing to do.
T2 models with stock ECU's actually fire the trailing plugs before the leading plugs when they are given high enough rpm and a strong enough vacuum.
#4 Correct timing for power WILL make your car idle shithouse, How do I know this, I payed someone to tune my car, realised that 24-28 BTDC at full rpm, means when you subtract about 13 degrees off, between 11 and 15 BTDC at idle, ie way too much timing. I installed computer controlled timing and now I have full range, my timing can vary from 0 to 40 whereever I want it to.
Not narky, just setting you people straight...
#1 Advancing Timing Causes cooler engine temperature, especially when you are talking about at idle.
#2 The Correct Vacuum nipples do not have vacuum present on them when you are off the throttle in an attempt to keep the idle timing more correct, but still allow for advanced timing under cruising conditions.
#3 The stock distributor does not advance anywhere near 20-25 Degrees. The Distributor will advance by around 13 Degrees from idlish rpm to 2500 or something (I read this in the FSM btw, no making this up). Vacuum advance is sweet **** all anyway, about 3 Degrees on the leading if I remember correctly. Trailing vacuum advance is much more than leading, somewhere in the realm of about 10-15 degrees if I remember correctly. Firing the Plugs very close together under higher rpm and vacuum conditions is a good thing to do.
T2 models with stock ECU's actually fire the trailing plugs before the leading plugs when they are given high enough rpm and a strong enough vacuum.
#4 Correct timing for power WILL make your car idle shithouse, How do I know this, I payed someone to tune my car, realised that 24-28 BTDC at full rpm, means when you subtract about 13 degrees off, between 11 and 15 BTDC at idle, ie way too much timing. I installed computer controlled timing and now I have full range, my timing can vary from 0 to 40 whereever I want it to.
Not narky, just setting you people straight...
03-16-07, 03:37 AM
#18
Originally Posted by Jobro
There is so much mis-information in this thread it isn't funny.
#1 Advancing Timing Causes cooler engine temperature, especially when you are talking about at idle.
#2 The Correct Vacuum nipples do not have vacuum present on them when you are off the throttle in an attempt to keep the idle timing more correct, but still allow for advanced timing under cruising conditions.
#3 The stock distributor does not advance anywhere near 20-25 Degrees. The Distributor will advance by around 13 Degrees from idlish rpm to 2500 or something (I read this in the FSM btw, no making this up). Vacuum advance is sweet **** all anyway, about 3 Degrees on the leading if I remember correctly. Trailing vacuum advance is much more than leading, somewhere in the realm of about 10-15 degrees if I remember correctly. Firing the Plugs very close together under higher rpm and vacuum conditions is a good thing to do.
T2 models with stock ECU's actually fire the trailing plugs before the leading plugs when they are given high enough rpm and a strong enough vacuum.
#4 Correct timing for power WILL make your car idle shithouse, How do I know this, I payed someone to tune my car, realised that 24-28 BTDC at full rpm, means when you subtract about 13 degrees off, between 11 and 15 BTDC at idle, ie way too much timing. I installed computer controlled timing and now I have full range, my timing can vary from 0 to 40 whereever I want it to.
Not narky, just setting you people straight...
#1 Advancing Timing Causes cooler engine temperature, especially when you are talking about at idle.
#2 The Correct Vacuum nipples do not have vacuum present on them when you are off the throttle in an attempt to keep the idle timing more correct, but still allow for advanced timing under cruising conditions.
#3 The stock distributor does not advance anywhere near 20-25 Degrees. The Distributor will advance by around 13 Degrees from idlish rpm to 2500 or something (I read this in the FSM btw, no making this up). Vacuum advance is sweet **** all anyway, about 3 Degrees on the leading if I remember correctly. Trailing vacuum advance is much more than leading, somewhere in the realm of about 10-15 degrees if I remember correctly. Firing the Plugs very close together under higher rpm and vacuum conditions is a good thing to do.
T2 models with stock ECU's actually fire the trailing plugs before the leading plugs when they are given high enough rpm and a strong enough vacuum.
#4 Correct timing for power WILL make your car idle shithouse, How do I know this, I payed someone to tune my car, realised that 24-28 BTDC at full rpm, means when you subtract about 13 degrees off, between 11 and 15 BTDC at idle, ie way too much timing. I installed computer controlled timing and now I have full range, my timing can vary from 0 to 40 whereever I want it to.
Not narky, just setting you people straight...
Advancing timing to a degree does make the engine run cooler. But max out your distrib full advance and let it idle and the engine will over heat.
I'm not saying the vac source going to the distrib will cause over heating. But I couldn't think of any other reason.
So I guess the answer is the car is suppose to idle better with out vac. And The vac advance is just for better gas mileage at cruise.
It didn't make sense to me because my car idles better (smoother and lower rpm) with vac constantly going to the distrib. The whole purpose of the thread was to try and find out why mazda thought it was necessary to take out timing at idle.
So the only other thing I could think of was heat issues? (which as you pointed out is probably wrong. )
Appreciate the correct info!
Thanks alot
03-16-07, 10:13 AM
#19
Airflow is my life
Funny this topic came up. I did some yahoo searching a while back and have been running with manifold vacuum on the dizzy (3rd nipple). Mazda had it on port vacuum. From my research this was done for emissions purposes. Try it both ways and post your results. 2nd nipple back is port vacuum.
http://search.yahoo.com/search?p=vac...p=mss&ei=UTF-8
http://search.yahoo.com/search?p=vac...p=mss&ei=UTF-8
03-16-07, 04:59 PM
#20
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Originally Posted by aws140
which nipple do you have yours hooked up to now? the way of the first diagram or like the red line? if the first diagram that was posted in incorrect, then a lot of forum members have theres hooked up in the wrong location. if it is wrong, then there needs to be changes made in the archieve about removing the rats nest.
Red line.
The archives do need to be changed as I followed them exactly.
03-16-07, 05:07 PM
#21
I swapped the lines out today. I really don't notice any driveability changes. I'll keep an eye on gas mileage but as stated above I think it was an emissions deal. So I'd try it both ways and see which your car idles the best. Mine hands down is smoother with a constant VAC source.
Mazda probably wanted the retarded timing at idle for the hotter more complete burn of fuel for emissions testing purposes.
Mazda probably wanted the retarded timing at idle for the hotter more complete burn of fuel for emissions testing purposes.
03-16-07, 05:31 PM
#22
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Originally Posted by vxturboxv
I swapped the lines out today. I really don't notice any driveability changes. I'll keep an eye on gas mileage but as stated above I think it was an emissions deal. So I'd try it both ways and see which your car idles the best. Mine hands down is smoother with a constant VAC source.
Mazda probably wanted the retarded timing at idle for the hotter more complete burn of fuel for emissions testing purposes.
Mazda probably wanted the retarded timing at idle for the hotter more complete burn of fuel for emissions testing purposes.
03-16-07, 05:33 PM
#23
lol "mazda wanted the retarded timing" ... why do I find that incredibly funny?
Well it's one more thing to try when my car comes out for the summer.
My question is: What's the fuel consumption difference between constant vac, under-load-only vac and no vac advance?
As I've heard on here, if you run completely without the vac advance your gas mileage goes to shyte and you don't really gain power or anything so you might as well leave it hooked... Looking at the FSM/Heynes diagrams for the rat's nest does lead you to the fact that the vac advance is run off the second last port on the spacer, so that would be "stock" vac advance. I'm just trying to figure out what's best.
Jon
Well it's one more thing to try when my car comes out for the summer.
My question is: What's the fuel consumption difference between constant vac, under-load-only vac and no vac advance?
As I've heard on here, if you run completely without the vac advance your gas mileage goes to shyte and you don't really gain power or anything so you might as well leave it hooked... Looking at the FSM/Heynes diagrams for the rat's nest does lead you to the fact that the vac advance is run off the second last port on the spacer, so that would be "stock" vac advance. I'm just trying to figure out what's best.
Jon