Need some ideas on 12A engine

P

poohmanhome

New member
I have just rebuilt the 12A motor in my Rx-7. I have rebuilt 100's of "normal" engines but this is the first rotary I've done. I followed the manual and it went together well. I now have it in the car and I just cannot get it started. Its also been a pain to start but now I can't get it to do anything. The engine turns over and builds oil pressure. It has plenty of gas (the plugs come out soaked) and it has good spark. I am 99% sure that I have the initial ignition timing correct (lined up the marks on the distributer and inserted into engine with leading timing mark at line). The engine just won't fire. I get a little "huffing" back through the carb which I'm a little concerned about.

Is there anything one can do when rebuilding an engine to make this happen? Are there any external checks I can do to verify the internals? Any ideas, approaches? Anything is greatly appreciated.

Thanks.
 
Check the compression and get back with us. Do a search on this site and you'll get the info you need on doing the test and what numbers are good/bad. Did you use new seals, springs? What was the condition on the rotor housings?
 
If the plugs are "soaked", then you may have flooded the engine. The fuel washes the oil out and you lose compression.

Compression, fuel, spark. We know that you're getting one of these. :)

You may want to head over to www.rx7club.com and post in their first-gen section. There are also a couple other first gen RX-7 sites and lists out there if you google.

Good luck,

-bill
 
Sounds like the timing might be off. Did you pull the main pulley off with the 10mm bolts? The 12a's are all the same and can be put back 90 degrees off. The 2nd gen 13B's are off set and can only go back one way. Line up the small 10mm bolts with the timing pointer and remove the front spark plugs. Take a small wire or tie wrap and insert into the leading and trailing plug holes. When it is on dead top center the wire should go in the same amount trailing and leading. Check that and see what you find.
 
Thanks to everyone so far. I found a number of posts on checking compression with a regular compression guage. I will do that tonight. One thing I found is that there doesn't seem to be total agreement on which plug hole to use for the compression test, or even which plug hole is "leading" and which is "trailing". I've seen it both ways in various posts. Can someone definitively say which is which?

I did use all new apex seals and springs and new springs on the side seals. The rotors and rotor housings were in good shape (it took parts from several engines to get enough good parts for this engine). The front pulley is a racing pulley and has only 1 belt groove and has the timing marks in the right place. I cross-checked this with my other junk pulleys. Basically, the timing mark is aligned with the pointer when the key on the shaft is at 9:00.

Thanks again. I will report back once I check compression.
 
Definatly check compression, low compression rotaries are very hard to start.

Put the motor to 20 degrees BTDC (about 1/2" clockwise of TDC ) and take the distributor cap/rotor off. Look at where the magnetic triggers are in relation to the pickups, they should be very close or right on top of them. if they are not adjust the distributor until they are.

Have you tried putting a timing light on while cranking the engine to make sure you have spark?

Peak compression numbers should be the same in either plug hole, but you will probably get more accurate readings if you use the Leading (bottom) plug.

good motors will see around 100psi cold.

did you replace the corner seal plug or are you using solid corner seals? steel or carbon apex seals?

You can pour a small amount of 2 stroke oil in the intake to help raise compression to start it as well.
 
Yes, new engine compression is a bear. Starting can be tricky.

The crude but effective spark checking method is still out of the car and grounded with a jumper. That way you can tell if it's strong or weak.

Also, it lets teh crap blow out, LOL.

As said above leading on the bottom, Trailing on Top. (T=T...easy to remember)

#1 up fron, #2 is in back. (Just like a race) They run reversed, but not well!

2 cycle oil works, or even ATF, for getting compression on a new start up. But the smoke will kill a swamps worth of mosquitos with the ATF! A shot of ether can get things rolling too.
Finally, make sure the battery is in good shape.
 
poohman, if you were to give a name & your location there is most likely someone close who could support you effort to get the motor going. Looks like most things are covered in previous posts. BUT, if there is something in a post that is not understandable please speak up.
 
If you havn't already, try a set of fresh, new, standard heat range (BUR9EQP ) spark plugs. Don't even try to start it with really cold race plugs.

Is your carb a known entity? Has it run on a car in it's current condition before?
Accelerator pump adjustment can affect starting (and must be adjusted with the motor running--catch 22).
With the fuel pump on, shine a light down the carb and make sure fuel is not dripping. open the throttles and make sure that the addelerator pump is working.

The intake 'huffing' makes me wonder if you don't have 1 and 2 wires reversed on the distributer.

Good luck and let us know what you find!

Tak
#29 ITA Rx-7
SFR SCCA
 
As for which plug to pull for compression testing, the Mazda manual calls for the test to be done from the trailing hole. We've done back to back tests with either hole and have found zero difference between the two. So it doesn't matter....
 
Well, the motor passed the audio compression check. Removed a plug at a time and turned the engine over, got a nice "chug" from each hole on each rotation. Then tried a compression guage and got nothing. I then tried a fuel pressure guage, and each hole showed about 6 psi. Then squirted oil into each plug hole which didn't accomplish anything.

So, it seems like the compression on all holes is the same but very low. Turning the engine over with the plugs removed fills the garage with a fuel/air/oil mist very quickly, so the chambers are pretty soaked. I have cleaned the plugs a number of times with my glass beader.

Also, the concensus seems to be that the top plug is the "trailing" plug. However, the distributor cap has T1, T2, L1, and L2 on it and the L wires go to the top plugs and the T wires go to the lower plugs. This is the way the engine originally came and it did run. I'm really confused about that. The manual I'm using (Haynes) is even worse, it has a picture that totally confuses the wiring.

As I said above, the engine did originally run when I got it. At the track during the third session the carb went goofy and gas was pouring down the venturis and it would barely run. After a while of doing that the engine locked up tight. When I took it apart one of the apex seals had broken and gouged both the rotor and rotor housing (front).

When I rebuilt the motor I used another front rotor and housing, replaced all the apex seals, springs, corner seals, etc. with the 1st generation Rotor Kit sold by Atkins. The carb I rebuilt. WHen I first put the engine in the car the carb had the same problem. I took the carb top off, fiddled with the one float that was apparently sticking, and it now works OK. The fuel levels are in the middle of the windows.

There is another oddity that I don't understand. The line for the float vent goes to a black cylinder with a "petcock" on the bottom. Its a JAX product. I found it on the web but it didn't say what it was for or how to deploy the petcock. Any ideas on that?

I appreciate everyone's input. This is really starting to shake my confidence in my mechanical abilities (and making a Spec Miata look better).

Thanks.
 
I don't know 1st gen's.
Top is definitely Trailing, bottom is Leading, front is #1.
Scuttlebut has it and my experience confirms that with 2nd gen plugs, they can (and often do) foul to an unrecoverable state. New plugs may be required.
 
I might be reading this wrong, but did you do your compression test by hand??

It is done by cranking the motor. If you are doing rotor 1, put an old plug in the top, screw your comp tester in the bottom, and remove one of the rear plugs, and all wires. rank and watch the guage. It will "pump". If the "pumping isn't even, report back and I'll tell you how to decipher it. Repeat steps for the back rotor.

New plugs never hurt. Autolites work well, save $...

T is definatley trailing. T1, top front, L2, lower rear, and so on.

Did you test the sprark externally?
 
I used a screw-in compression guage and cranked the engine with the throttle open. The thing I might have done wrong is that I had all four plugs out of the engine when I cranked it. Will the compression bleed across the leading and trailing plug holes?

I did test the spark externally and it is strong. I'll have to figure out what the heck the deal is with the wires being backwards from what is labeled on the distributor cap. That's the way it came and it did run. Hmmmm.

I will try the compression test again with one plug at a time out and report back. I'll also get new spark plugs.

Thanks!
 
OK, stupid question about the compression bleeding. After I posted the message I realized that.

Anyhow, doing the test the right way I get about 80 psi in the front and 45 psi in the rear. Given that I have a normal compression guage I don't the individual chambers, but given the other tests I done and the regular "chugging" sound I would say they are pretty close.

So, next I need to figure out the plug wire issue. Does the leading plug fire before the trailing plug?

Any other ideas?

Thanks.
 
Did you take the distributor cap and rotor off and look at the pickups like I suggested in my previous post?

Trailing is definatly the top plug, put the wires on properly. Paul Yaw recommends trailing set to 16 degrees BTDC and leading set to 24 degrees BTDC (assuming you have locked timing). Mazda recommends that both leading and trailing fire at the same time, 20 degrees BTDC. It is very possible that whoever set your motor up before had both plugs firing at the same time (or close) so it wouldn't matter if the trailing wire was attached to the leading hole.
 
It has been a while since I've done a rotary motor but here are my suggestions, also emphasising some of what was already said.

Trailing is top and is the last one to fire
Leading is lower and is the first to fire
If they are crossed it will still run, just not well.
There is a separate ignitor and coil for each, make sure they are wired accordingly.

Turn off the fuel pump, remove the plugs and crank until you no longer get fuel out of the chambers. Once they flood they are a bear to start.

Put in a fresh set of plugs, re-energize the fuel pump, GOOD charge on the battery and crank for a little while. If it doesn't fire within 10-15 seconds, dont keep trying. You will only flood it again. In order to make good compression (especially initially) it takes a pretty high cranking speed. The rpm helps force the apex seals against the rotor housings. So here is what you do (at your own risk), put a tiny amount of ATF down the carb, turn off the fuel pump and have someone pull start you in 2nd gear. Pop the clutch and you will get enough RPM to ensure adequate compression. Once it starts reinergize the fuel pump and let it warm adequately.

Good luck and keep us posted!
 
OK, stupid question about the compression bleeding. After I posted the message I realized that.

Anyhow, doing the test the right way I get about 80 psi in the front and 45 psi in the rear. Given that I have a normal compression guage I don't the individual chambers, but given the other tests I done and the regular "chugging" sound I would say they are pretty close.

So, next I need to figure out the plug wire issue. Does the leading plug fire before the trailing plug?

Any other ideas?

Thanks.
[/b]
Click to expand...

Hmmmm. I expected a low reading, but the rear is bothersome. If you re-do the comp test, watch the rear. Use a comp guage that doesn't "Peak and hold", and you can watch the pressure "pulse". Good call on the compression...the chamber that the Trailing and the leading connect to are common...leaving both plugs out will yeild no compression. But leave the non tested rotor plugs out to aid cranking speed.

Here's the basic rule. You'll see three pulses per rotation.
-If they are high, high, high, it's all good.
-If they are High, low, high, then you likely have a side seal issue. If they are High low, low, then it's probably an apex seal.
-If they are low, low, low......you have a general sealing problem. Could be stuck apex seals, inadequate cranking speed, or scored housings. The rear is often the worst for wear after use.

That said, I have run and only been slightly off the pace with a blown seal on one rotor. Even less Tq than normal, but if you keep the revs up, it's run. And even on one rotor, it started.

Finally, nobody has asked yet...is the gas fresh???

And if in doubt about the mixture, Ether always works, if only to eliminate the compression and ignition questions. If it starts on Ether, you know it has compression and ignition.

(This is a great example of how leaving things like a rebuilt motor install until the last minute (not that you have) can be trouble right be fore a race...;) Ask me how I know....I am the King of the Last Minute.)

Good luck!
 
45PSI is pretty low on the rear. If you have a good spark get some new stock plugs to get it started. Try about a tablespoon of ATF in the rear and try to start it. It is starts it will smoke real good so do it outside. Sometimes you can free things up once it starts and get to normal temp.
 
You must log in or register to reply here.
Back
Top