12a tech info please
 

I am putting a 12a out of a '85 GSL into a '78 MGB for the Gf's father. I have both cars together, engines still in. It's going to be a fair weather car. I'm a diesel mechanic but I do gas now and then, I just don't know anything about these engines.

I'll be running the stock Nikki minus the rats nest and anything else not needed. I have saved all the rats nest and ignition electrical info I could find here.

The questions:

1: There is a water leak from around the intake manifold area. When I do the intake gasket should I block the 2 coolant ports? Why / why not? 20mm frost plugs I've heard?

2: I have a new carter fuel pump here that I'll use, what psi of pressure regulator should I use? I've read it a few times but can't find it now.

3: I've read that I can use a 4cyl tach (stock MGB tach I hope) and simply use the pulse from the neg of the leading coil. Will that work? Any reason why not to do it like that?

4: I don't care about emissions. What are some drawbacks of getting rid of all misc valves and lines including the rats nest? Is there anything I should think of keeping besides the bowl vent solenoid?

5: Do I NEED to run a return line from the carb? I've never had one of these carbs apart and I don't know why it has to spill, or whatever it's doing to need a return.

6: Exhaust. I'm sure I want what I can't get. Low pitched, deep note and no back fires on deceleration. I'm prepared to give up HP to get those things. How do I get it?

I'm planning on the stock exhaust ~manifold and then use 2.5" thicker than regular pipe (I have lots) then 2 or 3 bottles end on end then a muffler at the back. What would that sound like?

Thanks all.

1. yes you can block them (20mm freeze plugs) or get the o'ring if you prefer that. the only downside is that it takes a tad bit longer to warm up if you block the passages, but it's not that big of a deal.

2. fuel psi is 2.8-3.5 i believe.

3. (don't know)

4. no draw backs, i've had mine running like that for 3 years now and there's people been doing it longer than that, and haven't heard any draw backs. it just decreases the number of things that COULD go wrong. one thing i've heard tho is if you take the air pump off you should upgrade your exhaust because your cats will clog. i haven't had this problem then again i believe my exhaust has been modified from exhaust manifold back since i only have 1 cat (previous owner).

5. yes, if you have stock nikki, you need a return line else it will flood all the time.

6. make sure you're vacuum leak free, and you aren't running too rich/lean and you should be fine. mine hardly ever back fires. and also the 2gcdfis (run a search) upgrade reduces the chances of it doing it because it ensures a better burn all the way through, feels like the engine pulls a bit harder, and better gas mileage. plus is a cool upgrade if you don't wanna spend too much money on MSD or other stuff.

and i would really recommend a header to free up some ponies. right now i have a stock manifold to (i'm guessing) aftermarket cat since it's only 1 not sure tho. and at the end is an el cheapo ebay fart can. and it sounds alright, i've got a silencer so it doesn't get extremely loud. but i definitely want some headers.

1. yes because you dont need to heat the intake as it is a fair weather car.
2. 2-4 psi is what your shooting for. i believe a holley will work.
3. im not sure.
4. not really. we mostly remove it to make it simpler maintain, and easier to diagnose vacuum leaks, plus there ugly.
5. yes you do need a return. i cant tell you why, but im sure sterling can chime in, or trochoid or...
6. what kind of muffler are you thinking? the manifold and thick piping will do good to soak up some of that sound. i would run a few magnaflows or borlas and see where that gets you.

Info on the carb: www.sterlingmetalworks.com
Best fuel pressure gauge (which you'll probably need): www.re-speed.com
Info on fuel pump and regulator: https://www.rx7club.com//showthread.php?t=529605


Exhaust; do whatever you can to ditch the stock manifold. It weighs a country ton, and is very restrictive. Even a Pacesetter header would be better than that. Remember all exhaust components have to be able to withstand the extremely high temps put out by the rotary. It will quickly torch standard mufflers and piping. About the only currently available bolt on system is made by www.racingbeat.com but it is very expensive. A lot of others on this site have built their own, or had one custom made out of pieces they selected. Start searching and you'll find a wealth of info on this....

Hope this helps. :)


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3. yep negative coil works fine, thats actually how mazda has it hooked up too

exhaust with a muffler under the car, and one in the rear should be really quiet, pitch kinda depends on the specific muffler you buy.

flowmasters are a no-no. borla reduces db nicely, but they seem to have a nice midrangey racecar note.

enjoy the swap! i remember measuring my MGB for a 12A back in like 94, it looks like it almost bolts in

You've got some good help in here so far.

Post some pics! I want to 12a a mkII sprite at some point.

The coolant passages to the intake warm it to prevent icing in the intake runners. Even in the heat of the summer the intake can feel nearly ice cold due to the fuel going from a liquid to a vapor state. This is even more pronounced on the aftermarket intakes with longer runners such as the wrap around ones. You will need to keep the decel valve if you want to keep backfires down.

That is the valve I was wondering about... I'd like to hear more thoughts on this. People that have it and ones that blocked it. Do I have to keep any solenoids if I keep this valve? How does it work?

Keep up the opinions guys the more the better.

(I'm going to revise my questions above to ask about the secondary vac advance)

Hmm seems I can only edit this one...

My next question was: I've heard it suggested to leave the secondary vac advance diaphram unhooked. Is that a good idea? Why?

Thanks

I removed the shutter valve (decel valve) last year, and with the addition of a Sterling carb and RB streetport exhaust I rarely get backfires. Any reduction in flow impediments I can get out of the manifold I take.

no shutter valve for me neither. stock stripped nikki with mechanical secondaries. hardly ever backfired unless i got on it and it was running extremely rich. then i upgraded to 2gdfis i haven't heard a backfire in months now, running rich or lean.

I haven't run a decelleration valve in years. No problems with backfiring. As long as your exhaust system doesn't have any holes/leaks in it, you shouldn't have issues.

Before you start wondering too much about modifying your carb, do yourself a favor and read up on Sterling's Nikki Library over at www.sterlingmetalworks.com He will walk you through every aspect of the carb, and what can be done to improve it. Or, you can look into the purchase of one of his Sterling Carbs which are very nice...

As I recall, I have to set my external Craftsman service tach to "4 cyl" to get it to read properly, so you should be fine instrument-wise. You're going to see two coil pulses per shaft rotation; which I think I remember being right for 4-cyls (never worked on one of those, myself).

You gonna call it an "MGX" when it's done?

definitely take lots of pics on the way ;)

Thanks all for the replies.

I guess I'm still interested on about the Vac advances leading vs trailing timing etc and if there is any truth to what I heard about leaving the trailing unhooked so it only used mechanical advance.

Yes I'll take some pics during the swap. Just finished an engine re + re in a Dennis Dart transit bus... anything is going to be easier than that.

I've read quite a bit over on sterling's site, I have the site bookmarked and I'll be reading more when I get some time.

I called a local parts supplier today to see about 20mm frost plugs but they go like 18, 24, 26 or something like that.... no 20 but I have some other sources.

Any thoughts on a good doner car for an electric fan? But it has to be a pusher due to space problems. I'm not fond of the "run it backwards" idea. What degree fan stat should a guy run if he's running a 160 thermostat?

Thanks

idk about pusher fans, but the taurus and mr2 fans are quite popular here. i'm running a taurus fan and it was a very easy install.

shutter valve is not for backfires. there are 2 anti afterburn valves to help with backfires.

the shutter valve is to keep the main cat temps from spiking on decel, RTFM! http://foxed.ca/rx7manual/manuals/RE...amoto-1981.pdf

the factory setup for the vacuum advance is they have a solenoid in the vacuum line so they can cut off the vacuum signal to the advances. and the advances are "off" at zero throttle.

so cruising down the street you get full vacuum advance, but full throttle it goes to zero, and at zero throttle it also is zero, cause the solenoid closes

So since the rats nest removal would get rid of the cutting the advance at idol.... is that a problem or...?

So with the rats nest gone what is the best thing to do with the vac advances?

only problem you might encounter is bucking on decel.

without the advance you'll be down on mileage and response

either way its very simple to hook up, so try it