Alright... I am new to rotary motors but have always been a fan of rx7 first and third gens. I have tons of experience with motors but they've all had pistons lol. So... I bought a 1985 rx7 that has a 12a with bad compression. Apex seals... got it... I'm not that stupid when it comes to rotaries... POINTS: I want reliability. I don't care about drivability. I love the sound of a bridgeport rotary. I want to remain carbureted. I'd like to rebuild the motor with as many new components as I can get. QUESTIONS: 13b motors seem to have more new parts offered but I'm not sure which are carbureted and which aren't. How's the reliability of a birdgeport 12a or 13b? Which one do you guys think I should go with to build? 12a seems cheaper... 13b seems to have more part availability... I'm not sure what to do.

 

12A housings will be hard to come by in good condition. One or both of your housings in your
12A may be trashed. You need to tear it down to determine before making a decision.

Other than that its a preference. Of course 13B gets you more power.
The 13Bs can be built for carbs by using 12A plates to make an old school 4 port 13B. Lots of
posts on how this can be done.

Are you sure the 12A is bad? Sometimes bad compression is the result of not doing it right
or sitting for a long period of time. You should try to revive it first to see if its really dead.

 

I've done the treatments and whatnot already. It wont start unless I pour about 3-4oz of ATF or oil into the carb. Then the oil helps seal it and gives it enough compression to crank. After it starts it has no power and can't idle on its own. If you move it you have to ease out on the clutch at about 2500rpm or it will stall. From what I've seen in my searching for parts, I figured it would be difficult to find 12a housings. That's why I thought it may be best to go with a 13b. I didn't know if there was anyone who could build up, mill, then resurface my current housings... I have no knowledge of what kind of machine work can be done to rotaries other than porting.

 

You may get lucky and find the housings are still good. You can look in the exhaust ports to get
an idea but a tear down will answer the question. It could just be seals, springs and bearings
plus whatever porting you want and you are good to go. I would go streetport and not bridgey,
but thats a choice you can make.

 

I just like the sound of a bridge better. I'm not doing it for power and I don't need a daily. I have 6 cars. I just want a fun attention getter thats sounds like a beast.

 

first and foremost, welcome to the forum. check the link to familiarize urself with the forum and how it operates.

RX7Club.com - Announcements in Forum : Introduce yourself

now to answer some questions. any 13b can be carbed, just get an intake that fits that specific block. pull the lower plug and listed to the air rushing out of the hole as ur cranking it on the starter. a steady 'woosh' of air should come from the plug hole, alternating front and rear. there will be 3 per rotor for each revolution. if it is apex, u loose compression on 2 rotor faces. putting a 13b in a 12a car, depending on which generation engine, u'll need to source a front mount, or mod the one u have, as the 13b is, if i remember right, bout 3/4" longer from the trans forward. not sure bout the fd engine, but fc engine will accept the 12a front cover.

 

The only reason i want to go with a carb is so I don't have to worry about all the wires and ECU. I want everything to be mechanical. I want to be able to run my motor on and engine stand with no computers. Is that possible with the 13b?

 

Yep its possible but you will need water and oil cooling if you are seriously running for any
length of time on the bench. Especially easy with a carb. Just need a dizzy and ignition with
coils and plug wires. If you run it with open headers you will have blood coming out your ears,
seriously put some kind of exhaust on it.

 

I gotcha. I don't actually plan on running it on a stand, I was just emphasizing the simplicity of the motor I want. I have too many cars to be battling electrical bull**** all day. I just want a reliable motor with a fuel pump and carb. So my best bet is a 86-95 nonturbo 13b? Is there a particular one that would be easier to convert to carb? I'd say theyre all about the same...

 

I cant say for sure, I'll let someone (Jeff20B?) with more experienced answer that one.

 

I appreciate all your help.

 

You want simplicity... You need an air cooled boxer engine from VW. 100% self-contained engine.


BUT since your car was designed for liquid cooling. I'd suggest a rotary engine like a 13B.

You can mate any carb you want to any 13B. It merely takes adapter plate. I put a Nikki (your factory stock 12A carb) on a 13B. All it took was a 1/4" plate of steel and a dremmel.

 

Aluminum plate from MZTX, add a 12A manifold to it like Qingdao did. Hogged out (and boost prepped maybe) Nikki. Seems to work great.

 

Is a Nikki what you would recommend? I was looking into the holly kits from racing beat. Money isn't an issue at all. I'm pretty sure racing beat makes a carb kit for a bridgeport 13b. So theres not a particular 13b "type" that I need to look for? Any of them will work the same? I just don't want to make a mistake ordering parts. That can get aggravating.

 

The RB holly kit for a 13b bridgeport is for a 74-85 13b...

 

But if I'm converting the 13b from efi to carb it should fit any i guess... I just don't know enough about these motors

 

The holley is turned 90 degrees from its orientation on a small block. So fuel slosh in the bowels during cornering is extremely aggravating. If you are going for an expensive carb get a pair of webbers or another side draft carb.



EDIT: if you are putting an engine from a 86-91 car into your bay then you will need a manifold to match that engine not your car.

 

Whatever you do, do not use the holley. I've yet to see one work well on a rotary.

Other than a modded nikki with intake a Weber IDA 48 or 51 downdraft is the easier,
next best solution. DCOE or Dellorto would be my third choice but it does fit the lower
intakes of 86-91 13Bs. Just cant get em new anymore (Dells I mean).

 

Which would provide more power on a 13b BP? The Weber 48 or 51?

 

Definitely the 51 for sure. Moar air/gas to feed the monster.

 

Would you recommend the Weber over the Nikki? I just think they look better. What's the benefits of running a down draft? Side draft? I'm guessing more low rpm torque with the side draft and less at top/less low rpm torque with the down draft more up top?

 

Nikki vs weber? You mean the slicked aluminum intake plate so I can machine it out to fit my setup?

 

Bump for my last two questions.

 

https://www.mazdatrix.com/r-intakefab.htm

 

That's what I thought you were talking about but I wanted to make sure I wasn't overlooking something. Their website is kind of shitty...