Andrew7dg

I have an 1982 RX-7 and a 1991 Miata and I switch between the two often.

It feels like the 12a is sluggish. It feels like the engine is choking out above 4,500 rpm and struggles to get to 6 when accelerating. I remember someone saying that Mazda installed a buzzer after 7 so they know when they have reached that point. I don't know how they would have!

Then I drive my Miata with a 1.6L. It is very happy after 3,000 RPM and is happy all the way to 6,500 and can reach 7,000 when accelerating without a problem. It just loves to sing and play at high RPM.

Doing some research and some side by side comparison

Rx7 Miata
100hp 115hp
105tq 100tq

Rear end
3.9 4.3

Weight
2,345 2,070

I am trying to figure out if there is something wrong with the 12a engine (carb choking airflow or timing off) or if that is the difference between the two. My goal is to have the RX-7 just as fun to drive as the Miata and right now it is not to that point. Looking at the specs it shouldn't be that far off. What are things I should watch out for that causes sluggish engine?

Or is this the nature of the car and I should look for a lower rear end gear? I thought rotorys like RPM though...

t_g_farrell

iTrader: (1)

My 12A easily revs to 8k if I let it. You have some sort of restriction going on. If you car is all
stock then you probably have a host of maintenance issues you need to deal with to get the
zip back in your car.

1. Verify the fuel is flowing at the right psi and rate. Change the fuel filter.
2. Make sure the carb is tuned correctly. FSM gives a good starting point. Go to Foxed.ca for
online manuals.
3. A tune up including cap, rotor and wires. Make sure the timing is correct too.
4. Verify your charging system and battery are up to snuff.

Start doing these things and see where it takes you. We can help you out when you have more
specific details on issues or functions.

lastphaseofthis

iTrader: (8)

seems like those weights are real far off... im guessing the rx7 is more like 2400 and the miata around 2700.

your car needs work.

j9fd3s

iTrader: (3)

the miata 1.6 is a sweet little engine, it does like to rev.

for the rotary my list looks a bit like t_g_'s list. i look at it like i'm building a showroom stock race car, so i look everything over for any problems/condition/etc and then i want to adjust anything adjustable to where it runs best.

i'll just start with the ignition, because its easy... you want to check and set the timing, the timing mark is wide so i set the timing to the advanced side of the mark. trailing should be looked at too. next you should check the wires, or if you're bucks up just buy new. they should be under about 14k Ohms resistance, any more and it'll misfire. cap and rotor should just be new. plugs should be new as well. if you beat the snot out of the thing, the GSL-SE plugs are the same, but in a colder heat range, so these are an option.

air filter should be new, paper is preferred.

fuel filter should be new. if its a stock car, i spend a while tuning the idle mixture, it effects mileage rather a lot.

the next step is to look at the exhaust. obviously you could just buy the RB street port system and call it good here, although depending on what you want there are options. hp = airflow. the stock cat is a pellet type, and these just suck, big benefits too swapping to a modern cat. next restriction is the pipe size, Mazda chose a 45mm pipe or something like that, and going to 60mm (2.25") pipe is better. next is the stock muffler, if you took one apart you'd wonder how air gets through it at all. then the stock manifold starts to not look like a header....

LongDuck

Congrats on your 15yr pin, j9fd3s! March is your anniversary month, and 15 shows real commitment.

Andrew7dg

My 1991 Miata at 2700? No, it is much lighter, probably more like 2100. The car is much smaller, less glass, and probably less metal.

1 and 3 are a lean or rich problem that would give weird running problems like lean surge (popping) or black smoke (rich)? I will double check it all though. Cap, rotors, wires, spark plugs have all been changed over.

The car was given to me with the carb taken apart in a box. I went through the very helpful manual giving me instructions on every gasket, metal check ball. I kept all the stock jects based on Jeff recommendation instead of the ones that come in the kit. I also stripped a couple components off of the carb that weren't needed. Car did not have rats nest when I recieved it.

I am wondering if the secondaries are opening up all the way? What is the best way of telling besides getting a friend to ride on the hood while I drive? Or should I just go all the way and modify the carb to have mechanical secondaries?

Charging system, does anyone trust the gauge on the dash?

I bought an RB header and a cat delete bullet muffler (RX-7 recommended). Everything from cat back (or now bullet muffler) is stock. I know the stock muffler is probably restrictive but I really don't want my ears bleeding (already started it up on the RB header and it was quite an unpleasant experience)

Qingdao

iTrader: (3)

Something is definitely wrong with your rx7. One, its full of lead cause I've weigh'd mine on the scales at 2250 with me in it. Dunno how much a first gen Miata weighs but my wife's '99 miata DOES NOT weigh 2k.

Although, I've never driven a 12a. I have to say that the miata will move but it is about as FAR from a rocket ship as you can get.

With electronic ignition and proper timing you shouldn't have a rev limit. Just keep spinning until the rotor bearings can't keep up anymore. This happens after prolonged exposure to 9k+ revving; I know from personal experience.

It's been said before, but you've got something clogging up the works in your RX7... be it fuel flow, exhaust flow, air restrictions or ignition. That's the difference between the Miata and the first gen RX7. The Miata is fuel injected and has a crank angle sensor for ignition; all that makes for a near maintenance free driving experience.

WANKfactor

iTrader: (1)

Yep, as gutless as they are, a bog-stock 12a should be all business (relatively speaking) after 6k through to redline. Something's wrong with it. And I found those weights you quoted strange too.

t_g_farrell

iTrader: (1)

The curb weight for a 1991 miata is ~2200 pounds, so his weights are not that far off if he has
removed or modified it slightly.

Used 1991 Mazda MX-5 Miata Features & Specs | Edmunds

1991 10Best Cars - 10Best Cars - Page 7 - Car and Driver

If you have vacuum secondaries that are not opening that could explain some of the issue. Although now
that you have told us you had to assemble the carb, I would recommend pulling the top off and making sure
you put the right jets in the right spots as well. They are easy to switch and we have all done it at one time
or another.

j9fd3s

iTrader: (3)

sort of. it works, but there tend to be voltage drops in the circuit. i'd put a voltmeter on the battery when its running. these cars, being older charge at about 13.6-13.9v, a newer car would be a little higher. due to the way the ignition is set up, it is sensitive to voltage, so i guess its quick to make sure that the coils are getting the same voltage as the battery.

a 99 Miata is just under 2400lbs the Rx7, stock is about 2330lbs

Joekaistoe

Your problem sounds a lot like one I had when I first got my car. My secondaries were (sort of) converted to mechanical secondaries but they weren't wired to the primaries, so they wouldn't open more than a crack. That bread tie was the cheapest, most effective power mod I've ever done.

My vote would be your secondaries aren't opening. You could put a dab of white grease where the vacuum actuator rod pivots; if the secondaries open you should be able to see the grease was pushed around.

Cameron38

iTrader: (2)

I gotta try that test on my SE. Run's out of breath above 6k.