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Old 10-26-07, 12:26 PM   #1
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12a Rotary Compression Check???

Can anyone advise me or point me to info. on exactly how to easily check the compression on a Rotary? I know its got to be different than a piston engine.
I just bought an 85 GSL thats sat for a couple years and now the only way it will start is by pulling it down the road with a chain.
I have narrowed it down to possible causes being:

1. Carb/fuel filter stopped up
2. Bad Positive Battery cable-Stock one is made from Alum. wire.
3. Starter weak
4. Low Compression

I am trying to salvage the 12a before scrapping it out to do a V6 Chevy swap, but I dont have much knowledge with how to check the compression on these Rotary engines, or what the actual Normal readings should be on the guage.
I guess my main questions would be what is the desired PSI on the guage when spinning over the engine, and which points should I check from on each rotor-Lead or Trail?
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Old 10-26-07, 12:49 PM   #2
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Here you go......>Compression Test
I just asked that not too long ago
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Old 10-26-07, 04:58 PM   #3
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Yup, thats the stuff. Thanks man.
I also did some digging today and found that the FSM says 85 psi on the compression for a stock 12a, and the differential between rotors is to go no greater than 21 psi. Of course this all should be at 250RPM cranking speed.
So you pull all 4 plugs to do the compression checks? Not clear on that, but thats what I think it says to do.
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Here you go......>Compression Test
I just asked that not too long ago

Last edited by BigDaddyJC; 10-26-07 at 05:04 PM.
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Old 10-26-07, 05:35 PM   #4
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Step # 2 says "Remove both lower plugs and wires"
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Old 10-27-07, 05:30 AM   #5
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If its been sitting for a long time, run some Seafoam through the tank to get sticky things loosed up again. I typically add a full can to 1/8 tank of gas and go for a drive. Results are nearly instantaneous...
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Old 10-30-07, 03:34 PM   #6
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Thanks, I'll give that a shot. I finally got my compression checked last Sat. and my back rotor only has 65 psi compression, and each apex seal pulse only hits 15 psi. I am thinking the back rotor has all 3 apex seals stuck since all 3 pulses are low.
Local mechanic told me to let the engine soak a few days with brake fluid poured down the carb while turning the engine over by hand to get all the front/rear rotors good and saturated. I am trying this through the week this week and plan to drain the oil out next Sat. and pull all the plugs to blow out the excess brake fluid before I take it for another drive. It will start when pulled off, but the low compression on the rear rotor is not enough to kick it off when cold. Plus the dang factory carb seems to be funked up also, since I cannot get the car to idle worth a dang below 1500 rpms. I know for a fact the accelerator pump is not spraying any gas when you pump the throttle.

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If its been sitting for a long time, run some Seafoam through the tank to get sticky things loosed up again. I typically add a full can to 1/8 tank of gas and go for a drive. Results are nearly instantaneous...
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Old 10-30-07, 03:47 PM   #7
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I wouldn't use brake fluid. That may unstick things, but it is bound to ruin seals. Check out what some of the other people have used to unstick things (ATF, MMO, Seafoam, etc.). Somethings are going to be harsher on your engine than others.
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Old 10-30-07, 10:34 PM   #8
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Brake fluid is extremely caustic. MMO or Seafoam are the best choices. ATF is also too caustic.
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Old 10-30-07, 11:41 PM   #9
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While there is an on going argurement as to whether atf damages the oil control o-rings, I don't believe it does and have used it on a number of engines with good results for cleaning out the carbon and freeing up seals, both apex, corner and side seals. If the engine is running decent, I often follow up with a qt of plain water, slowly drawn into the engine through a vacuum line to further steam clean the internals.

My personal opinion is that when there is excessive smoking after the atf, it's due to removing carbon that has sealed bad seals that were on thier way out and would allow oil to bypass the worn seals any way. Don't worry about your compression readings yet. Get the carbon removed and the seals freed up first. Even if the compression doesn't come up to minumum specs, the internals will nice and clean for the tear down and rebuild.

Regarding the other issue mentioned in your 1st post, low cranking speed is most likely due to a weak battery and/or bad cables/connections. Start with replacing the cables, positive in particular and cleaning all of the connections. Also check the extra grounds, i.e., the one from the firewall to the engine. I usually make a new one.

If the engine won't idle below 1500 rpm, you most likely have a vacuum leak. Find it and fix it, I you plan on doing the rats nest removal, now is the time as it will simplify finding a leak. If the AP is not squirting, and it's not leaking externally from the diaphram, then the carb is gummed up internally. This would be a good time for a carb rebuild to coincide with the rats nest removal.

Once this is all done, you should be able to start the engine without pulling it. If not, make sure the fuel system is working properly, fresh fuel filter, pump is good, adequate fuel volume and psi, along with a basic tuneup. Plugs, wires, cap and rotor. After that, then check the compression. Remove the lower plug from both housing, pull the engine fuse to to disable the fuel pump and remove the rotor from the the dizzy to disable spark.

While cranking for the comp test, observe the tach and note the rpms. This will only work if the trailing side of the ignition system is hooked up and complete, including the dizzy rotor. Fuel pump must then be disabled and the car bowls empty for this to work without the engine starting. Other lower plug should then be installed in case there is residual fuel that makes it into the combustion chamber. Fuel flames coming out of the spark plugs hole is a great wat to have engine bay fires. Don't ask how I know, lol. After that careless mistake I had to replace the complete engine harness, plug wires, brake reservoir and patch a number of other burnt wires on an 88 GTU. Not fun.

Per the FSM, compression test should be done on a warm engine with a cranking speed no less than 250 rpm. Make sure the battery is good and fully charged.

Report back your findings. The factory carb manual can be found in the FSM link in my sig line. Good luck.
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Old 10-31-07, 12:13 AM   #10
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If you have an opportunity to tow the vehicle again. Put whatever choice of chemical that you are going to use in the spark plug holes. Then tow the vehicle in 3rd gear.

I did this with several cars that had stuck apex seals in them. By the time I got back to the house, installed new plugs and fired her up. The seals were no longer stuck. Performed an compression test and the compression had returned.

Not saying that this will work on damaged seals or worn out seals but if they are indeed carboned up.
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Old 10-31-07, 12:17 AM   #11
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IMO it would be nuts to use a caustic like ATF or brake cleaner in your engine, scouring the combustion chamber walls and running down to eat the Big Bearings. YMMV.

For testing the compression you want a "Recording Compression Tester" that produces a time-base graph of compression. The "Motometer" is the Cadillac of recording testers and is usually about $700, but there's a guy on ebay selling his for $250, item #300163978868. It's the new version with quick-connect fittings and starter trigger switch. It's a fine piece of German craftsmanship.

But you have to make sure you have the screw in nipple for the sparkplug threads. If you have to buy it you'll spend about $100 with "samstagsales.com" which I think is the only supplier. I don't think that a conical rubber gizmo will be steady enough, if you can find one.

But if you're a cheap bastard like me and every other RX-7 owner I know, there's another solution. I have the OLD model of the Motometer which takes a screw-in nipple instead of the quick-connect and it doesn't have the electrical starter trigger (so you gotta get a $10 remote starter button at the autostore and press it with your free hand). Also, I don't have the adapter nipple to thread into the spark-plug hole and thread into the Motometer flexible hose. What I was planning to do is hunt up appropriate threaded pieces from hardware stores and autostores and splice them together by silver-solder or braze, as required. Maybe there's even a readymade nipple somewhere, but I wouldn't count on it. I wrote to "samstagsales" a couple years ago looking for it. I think they recommended their tradein program to get one of those $700 new style babes.

But I have too many projects, and none of my 4 rotaries seems to have poor compression, so if you want a neat-o project send me an eMail and make me an offer that makes it worth my while to ship it out, say $70-80. Maybe, eventually I'll put it up on eBay.

The part number on the side of the fine mahogany wooden box with dovetail joints is 623.002.1004. It contains the readout unit, a flexihose, an adapter guide written in German, and one pad of recording cards. It's kinda dusty from 40 years of testing Mercedes Diesels.

Disclaimer: I never actually used this thing, so I don't know exactly how to get the adapter parts, but I'm guessing they are some kind of standard metric threads. I don't know anything about the recording inkpen except it appears to be complete and intact and there appear to be some scribbles on the record card. Oh, I don't know if it's calibrated for high-pressure Diesel or low pressure petrol motors.

If there's interest I can snap a pix and post it, if necessary.

Oh, one of the other guys in the club got one a year or two ago and earned the money back in a weekend running around testing his buddies RX-7s. You can lookup the past lore on this subject by searching all over rx7club for "Motometer".
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Old 10-31-07, 08:31 AM   #12
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Biffle, please explain, justify, your comment of the caustic properties of atf and brake fluid on the engine internals. The automatic transmissions and power steering units, both of which use atf, are built from a wide variety of metals and rubber parts. None of which are different than the rotary engine internals. The same can be said for the hydraulic brake and clutch systems, though I've never tried brake fluid to remove carbon.

Atf has been used for decades to clean the carbon out of piston engines. I've been doing it myself for nearly 40 years with no ill effects. I do prefer using Seafoam first but will use atf on badly carboned/locked engines.
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Old 10-31-07, 08:52 AM   #13
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Lets see if I can add some more details on my troubleshooting so far.

Installed new fuel filter and replaced positive battery cable completely with a new one with COPPER wire instead of that cheap alum. crap they had on the car.
Grounds looked OK so I did not mess with them yet.
Carb is seeping fuel out around the accelerator pump to the outside of the carb, so I know its probably shot.
Water pump has also started dripping out the bottom now too.

Too late on using ATF. I ran about half a quart through the engine last Saturday after I found the rear rotor only pulsing 15 psi on each apex seal. Sort of thought that showed that all 3 of the seals were stuck. Motor smoked pretty bad for a while, then cleared up eventually.
Too late on using Brake Fluid also. I ran about a pint of that through the engine via the carb Monday night. Never have started the car up, just turned the engine by hand a few times while pouring in the fluid. Turned it a few times again last night when I got home. Going to turn it by hand again tonight when I get home also.
Once the weekend gets here, I plan to drain and change the oil again, start the car back up, dump the suggested amount of SeaFoam in the gas tank, pull it off again and run it around some more.

The car apparently has sat for about 2 years and just now been resurrected by some teenagers, whom I got it from in a trade.

What is this " Rats Nest Removal" I keep seeing mentioned? I would definitely like to do this before I tackle the carb rebuild if its pertaining to the removal of all the crap on top of this engine.
I remember back from a long time ago when my son fooled with these cars that most all the solenoids and vaccum stuff on these engines can be done away with and not affect the engines performance. I would like to strip mine down to only the necessities. None of the 3 cats on my car have guts in them now, and my location does not have inspections, so I want to get all the smog crap off my engine ASAP. I also plan to run a performance header on it once I make sure the engine can be salvaged.
Any help on links to doing this would be greatly appreciated. I know that the FSM will not show these things, but I also know that someone has done it before.

I appreciate everyones help and advice on this so far. I know very little about these rotary engines, and all info. supplied so far has been helpful.

I am getting sort of discouraged on the 12a engine though. I have my doubts it can be brought back at this point. I would really like to keep it stock though since its in great condition and even the A/C still works.
If the 12a cannot be salvaged though, I am afraid this cars life as a Rotary powered machine will have come to an end. I have a fresh built SBC 400 with Turbo 350 Tranny that is just about ready to drop in once I get the short water pump bolted on tonight and buy a small cap HEI dizzy for it. I was going to install it into an 86 RX7, but I really like the FB body better, plus its got a good title so I can drive it on the road. Like I said before, Alabama has no inspection so motor swaps here are fine.
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Old 10-31-07, 09:29 AM   #14
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Link for the rats nest removal is in the FAQs, which you should have read through by now, lol. Between the FAQS, archives, FSM and carb manual, you will most of the basics down. If the AP is leaking, then it's time for a carb rebuild.
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Old 11-04-07, 08:55 PM   #15
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Caustic or not, the Brake Fluid worked. Drove the car today for about 30 miles and it did fine.
Starting problems seem to now be due to no accelerator pump on the carb. You can pump it a zillion times and no spray. Car tries to start with choke, but not enough gas getting to it I think.
Going to get a carb rebuild kit next and try a cleanout and new accelerator pump.
Also got to install a new water pump now also, since mine has the dribbles.
Looks like I get to keep mine a Rotary for now......
I plan to still run some SeaFoam through it also once I get the gas level down to quarter tank again.
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Old 11-04-07, 08:55 PM
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