The Maxton allegedly has a 12A out of a '76 RX3 or similar, but it has the later (81+) RX7 distributor with the two vacuum advance cans. Neither of them is hooked up, which suggests to me that the timing is effectively locked into whatever base timing it is set to. It does not look like the car has any provisions for vacuum connectors to the distributors.
Do I want vacuum advance on a 12A with effectively no emissions gear left?
Usually the vacuum connection is right at the tip of the throttle plates. Doesn't the Maxton uses a sidedraft Weber or Dellorto?
Vacuum advance is for low rpm drivability. The distributors have a couple degrees of mechanical advance.
Yes, sidedraft Dellorto on a Racing Beat upper manifold. The Manifold does have vacuum hookups but they go somewhere else (OMP I think, I'm trying to trace them).
Low speed drivability is OK, but I do vaguely remember my previous 1st gen having better drivability at lower rpm.
Relevant, perhaps.
http://www.racingbeat.com/mazda/performance/rotary-tech-tips/ignition-systems.html
If you wish to install the 1981-85 RX-7 breakerless distributor in a stock 1974-79 engine, there is one potential problem: the centrifugal and vacuum advance characteristics of this distributor are slightly different from the original equipment "breaker point" style distributors. The easiest way to deal with this is to set the timing to: Leading - 24° BTC / Trailing - 14° BTC at 5,000 RPM and use this as a starting point (vacuum disconnected). If you wish to use this distributor for racing purposes, the vacuum advance should not be used, and the timing should be set to the proper total advance setting (Timing Chart - PDF)."
Did someone make a mechanical/centrifugal advance with a faster curve and more degrees for that distributor?
I used those type of kits on my gm stuff but don't know for mazdas.
Vacuum advance is critical for drivability and fuel economy.
I used to run both cans off of the same ported vacuum source. With base timing split to 8 degrees with an '81-85 distributor (the lowest I could get) this will result in negative split under lighter loadings, because the '81-85 unit had a lot more vacuum advance on the trailing than leading. This is not only acceptable at light loads, but beneficial.
I used to get 27mpg highway all the time with a carbureted 12A, and even did 32mpg once.
In reply to Pete. (l33t FS) :
I guess that explains why the engine doesn't seem to really wake up until about 2500rpm and a few people on the Maxton FB group mentioning they're getting around 15mpg.
Let's hope the Dellorto on the car has the vacuum port plugs so I actually have a place to hook up the advance mechanism.
In reply to BoxheadTim (Forum Supporter) :
The 2500 thing might be because gigantic carb. The common Dell is a 48 DHLA which is pretty big for a 13B. I have a Racing Beat set up unit on the 13B in my '81, and it's pretty rich everywhere and there is not much performance to show for it. And you have to baby it through the first part of the tach, and then it's okay in the midrange, and then the ports cut it off before the carb can really sing.
Rip that Italian junk off and stick some Japanese junk (Nikki 4bbl) on it and really wake it up 
Few things drive sweeter than a 12A with a well maintained Nikki. Avoid the modified ones (Sterling's "fat nikki" in particular) unless you enjoy awful drivability. I have Words, but don't want to vent them right here.
Pete. (l33t FS) said:In reply to BoxheadTim (Forum Supporter) :
The 2500 thing might be because gigantic carb. The common Dell is a 48 DHLA which is pretty big for a 13B.
Rip that Italian junk off and stick some Japanese junk (Nikki 4bbl) on it and really wake it up
I think sir misspelled "fuel injection" 
.
There isn't enough space under the hood for the standard carb/manifold/air cleaner assembly, which I why I believe they went with the sidedraft.
If I'm going through the hassle of changing the intake system to that extent I'm dropping in a fuel injected 13B...
A Weber 45 DCOE is a much better match to the engine, if sidedraft must be retained.
Is the intake manifold the type that curls over the engine, or does the carb sit next to the engine?
It's the old, discontinued 'curly' Racing Beat Weber/Dellorto side draft manifold.
Shouldn't be discontinued, I bought one last year The manifold, anyway. Carb stuff is long gone. They do show a price for the air filter element (NOT what you have, it was foam over chickenwire) if you know the part number, but the part number is no longer in the catalog.
I AM deeply curious what filter element that is, in what looks like the Racing Beat housing.
In reply to Pete. (l33t FS) :
It's a K&N filter, The only number I can find on the filter is A770B6. One of the other members of the Maxton Registry on FB seems to have recently made a similar change and used the K&N flter E-1935.
As to discontinued, my understanding was that RB only does the downdraft stuff these days and you can't even get the gasket that goes between the upper and lower manifolds for this manifold, let alone the manifold itself. But I'd be happy to be proven wrong.
In reply to BoxheadTim (Forum Supporter) :
That much of it is a standardized part. The upper on YOUR engine is the same form factor as the mid manifold on a GSL-SE engine ('84-85 13B). The upper will bolt straight on to that engine's lower manifold. So that gasket is a stock Mazda part. ( $2.95 at Rock Auto if you don't want to buy from Mazda ) Racing Beat also made "lower manifolds" for 4 port 12A and 4 port 13B. Those, I do understand, are long gone. (They would allow you to mate a GSL-SE intake manifold to an older engine, which would be useless, as the fuel injectors go into the engine not the manifolding)
I did cheat a little bit. The upper that I bought was to mate to an '86-88 lower, which is much larger and has a different form factor.
And checking their website now, and Mazdatrix's, indeed all the sidedraft stuff is gone, gone, gone. Wow.
In reply to Pete. (l33t FS) :
Thanks, that's good to know. Oh, and in other news, the DellOrto has the correct takeoff ports for distributor vacuum if I can find the correct plugs that go in there.
That's the same setup that is on the Abomination. I'll take a look and see where the vacuum lines are tied in.
Be careful shutting it down hot. The Abomination will boil the fuel in the carb which makes it a everloving bitch to hot start. I usually shut the fuel pump down and let it run dry. I also have a large heat shield between the carb and engine.
Well, never mind. I'm not running vacuum advance. Just mechanical. IIRC I'm set to 8-10 degrees BTDC base timing.
I've never street driven it. I'm not sure I've ever driven it below 2500 RPMs. Probably why I never noticed a flat spot just off idle.
The only lines going to the carb are the OMP lines and the fuel line.
One other thing. If you autocross is and have stumbling problems in the corners it is probably cause by fuel sloshing out of the float bowl vent and into the intake. This gasket will solve 90% of the problem.
If you drive the car at full throttle all the time, there is no need for a vacuum advance. It will greatly improve part throttle power though, and can do wonders for fuel economy.
I always wanted a good explanation as to why they outlawed them in the 70's. I imagine the advance increased some pollutants, but with the reduction in fuel economy, did it wash out?
aircooled said:If you drive the car at full throttle all the time, there is no need for a vacuum advance. It will greatly improve part throttle power though, and can do wonders for fuel economy.
I always wanted a good explanation as to why they outlawed them in the 70's. I imagine the advance increased some pollutants, but with the reduction in fuel economy, did it wash out?
Say what?
Vacuum advance was certainly not outlawed, ever.
Toyman01 (Moderately Supportive Dude) said:Well, never mind. I'm not running vacuum advance. Just mechanical. IIRC I'm set to 8-10 degrees BTDC base timing.
I've never street driven it. I'm not sure I've ever driven it below 2500 RPMs. Probably why I never noticed a flat spot just off idle.
The only lines going to the carb are the OMP lines and the fuel line.
Just in case you're curious, from what I could find the two vacuum take off points one is supposed to use for vacuum advance are at the two little plugs/jet looking things to the right and left of the large cross head screw. IIRC you are/were supposed to take those out and put some appropriate connectors in. Not sure if you can still get those, though.
Toyman01 (Moderately Supportive Dude) said:One other thing. If you autocross is and have stumbling problems in the corners it is probably cause by fuel sloshing out of the float bowl vent and into the intake. This gasket will solve 90% of the problem.
Thanks, added that to my shopping list for this car.
Pete. (l33t FS) said:aircooled said:If you drive the car at full throttle all the time, there is no need for a vacuum advance. It will greatly improve part throttle power though, and can do wonders for fuel economy.
I always wanted a good explanation as to why they outlawed them in the 70's. I imagine the advance increased some pollutants, but with the reduction in fuel economy, did it wash out?
Say what?
Vacuum advance was certainly not outlawed, ever.
Sorry, not trying to turn the point of the thread here. The vacuum advance disappeared (from what I know) in the early-mid 70's when the smog regulations started. I am assuming there where some regulations that did that. Am I wrong? Maybe the emission regs effectively required eliminating it? Where there vacuum advances in any "smog" cars?
In reply to aircooled :
Well, 1st gen RX7s definitely had vacuum advances because that's where this distributor came from. I'm also pretty sure that my CJ7 had a rats nest of vacuum lines including vacuum advance because by hooking those up correctly and setting the timing where it should've been, I got it from 9pm to 12mpg. Yay.
I think vacuum advance went out once fuel injection that would also control the spark became more prevalent.
aircooled said:Pete. (l33t FS) said:aircooled said:If you drive the car at full throttle all the time, there is no need for a vacuum advance. It will greatly improve part throttle power though, and can do wonders for fuel economy.
I always wanted a good explanation as to why they outlawed them in the 70's. I imagine the advance increased some pollutants, but with the reduction in fuel economy, did it wash out?
Say what?
Vacuum advance was certainly not outlawed, ever.
Sorry, not trying to turn the point of the thread here. The vacuum advance disappeared (from what I know) in the early-mid 70's when the smog regulations started. I am assuming there where some regulations that did that. Am I wrong? Maybe the emission regs effectively required eliminating it? Where there vacuum advances in any "smog" cars?
Yes, practically all of them.
Now, with those stone age combustion chambers, they might have eliminated advance so that the engines would be able to burn a leaner mix (leaner mixes are harder to ignite, so firing closer to TDC has more heat and pressure to assist). or to keep the engine more loaded (lightly loaded may need to be rich for stable combustion) But it certainly didn't go away. Heck half the maze of hoses and switches was to deny vacuum advance until after warmup, or only in top gear, or to switch the vacuum source...
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