RX7Club.com - Mazda RX7 Forum - (OTHER) Decarbonizing A Rotary (FEATURED ARTICLE)

Hey Guys,

For a while now I've been working with a few others on this article. I aimed to dispel some of the "Just ATF It!" going around the forum, and replace it with some knowledge compiled from various posts and sources.

This is a work in progress, but here is the first "final" draft of my article - Decarbonizing A Rotary. I will be posting it and a "digest" version on my website, when it's finally ready to go online.

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Decarbonizing A Rotary

Hey everyone. There’s been a lot of talk on the forums about ATF, MMO, Amsoil Powerfoam, and Water being used to free our engines from Carbon. From Carbon Lock to Carbon Buildup, there’s been a need to try and sort everything out and put all the information we need into one place, so that’s what I’ve tried to do with this article. Anyone who has done a search on any of the words I mentioned above was bombarded with threads upon threads of information. Some of it good, some of it bad, some of it irrelevant, but all of it a royal pain to wade through to get what you want. So here’s my crack at putting it all together. If anyone has any add-ons, corrections, comments or general discussion about this article, you can e-mail me at vipernicus42@hotmail.com or PM me here at the forum.

Before I start I must emphasize:
Neither rx7club.com, any of it’s members, nor myself take ANY responsibility for your trying anything in this article. This information has been compiled from our own experiences and the posts of others. I can’t guarantee that what I did will work the same for you. I just hope it does!

So without further ado, let’s begin:

The Problems:
There are two main problems associated with Carbon:
1) Carbon Buildup and
2) Carbon Lock

While they both involve the same dreaded Element, they are caused by different things, give different symptoms and have slightly different approaches to resolve them.

Carbon Buildup:
Carbon Buildup, as the name implies, is a buildup of carbon and carbon-like deposits in the combustion chamber. These deposits can be caused by many things from the oil that we use to the type of driving we do. They get in the way of the proper functioning of the Apex Seals leading to loss of compression and inefficient burning of the air/fuel mix. Symptoms of Carbon buildup can be loss of power, engine missing, and loss of compression.

Our cars (at least the stock ones) use an oil-injection system to inject oil into the combustion chamber to lubricate the Apex Seals. So it makes sense that the kind of oil we inject could have an impact on what’s left behind after the oil has burned off. There’s long been a debate on the forum about what kind of oil to use, and it is not my place to re-start the holy war, but in my opinion (and the opinion of many of the posts and websites I have read) it’s best to use a good quality non-synthetic oil which has few additives of any type. The more additives something has, the more crap there is to be left behind after burning. If your car is having problems related to power, you should definitely check your Oil Metering Pump to make sure your Apex Seals are getting the lubrication they need. Lowe's Automotive makes a rebuild kit for the OMP for $20 and it’s quite an easy rebuild. See the thread "here" for more info on the Oil Injection System and rebuilding the OMP.

Our driving style also affects the buildup of carbon. It’s been known for a long time that on boingers at least, driving like a granny leads to buildup. The same is true for the rotary engine. Since our engines are so good at revving so high, it’s almost a shame to think that someone could be perpetually driving around and shifting at 3k. Now I’m not saying that we should drive our cars hard ALL the time, but from time to time putting the revs up doesn’t hurt.

The quality of the gas we use and the richness/leanness of our Air/Fuel mix can also have an effect on carbon. Since the whole process of making a car move involves Air/Fuel and Spark, a problem in any of these can lead to the Air/Fuel not burning completely away, leaving behind deposits of material. Keeping an eye on your air/fuel mixture, making sure your spark plugs, plug wires, distributor cap and rotor are in good condition and using only quality fuel can go a long way to preventing carbon buildup. It’s beyond the scope of this article to talk about how to adjust your air/fuel mix or how to tell if the fuel you’re using is “quality fuel” but general maintenance on these systems of your car can be found in the Heynes or Factory shop manuals, and there are plenty of threads on the www.rx7club.com forum about these topics.

So now we know what the problem is and what causes it, how do we fix it? Well the main key about Carbon Buildup is that it has built up, but it hasn’t locked the engine yet. Since the symptoms of carbon buildup are so similar to other problems in the Air/Fuel/Spark systems, these should be thoroughly checked first before trying any of the following. See Pele's "End All For No Start Threads" in the first generation section’s archive at www.rx7club.com for more information on cars that won’t start, or try reading some of Sterling’s articles on carburetors for cars that aren’t running properly.

The fact that the engine is still running, or at least turning, gives us one advantage over a carbon-locked engine. Since the engine is running there are a few things we should try before we resort to the de-carbonizing procedures for locked engines:

First, engine-cleaning gas additives are available which are designed to clean your engine through the gas. Any local NAPA, Canadian Tire, PepBoys or other local parts store should carry a range of these additives, many for less than $10. First off, make you’re your fuel filter is clean. You should change it at least once a year anyway, and for $5 and 5 minutes, you might as well. Another thing you can do in addition to the fuel additives is to take your car out for a good hard ride. Spend a half hour or an hour taking your car up to higher RPMs and back down again. Take it on the highway, get to speed, then shift down and floor it to almost redline. Do this a few times to help “blast” some of that carbon out the tailpipe. It may sound a little barbaric, using brute force to blast carbon out, but in many cases, these two simple things together will work. They work especially if the car has been exposed to mostly city driving, or extended periods of low-rpm highway driving. Vary the RPMs, take it up high a couple times and basically drive the crap out of it for an hour. Then go get an oil change, it can’t hurt!

If your carbon problem still persists, you can try “steaming” it out. This method seems a little safer than the harsher methods outlined below in the “Carbon Lock” section, but it’s still more direct than just driving your car hard. The basic theory is this:
You take an old windex spray bottle (any old spray bottle will do) and clean it out thoroughly. Fill it with water. Get an assistant to sit in with the car parked, and rev the engine to about 3k. With the top off the air cleaner assembly start spraying the heck out of that bottle. The engine will bog and you’ll be introducing water into the Air/Fuel mix. This water turns to steam inside the combustion chamber and the steam helps lift the carbon and send it out the tailpipe. Many people have reported good results with this method, but it’s still a little vague how much water you need or how long to do it for. Until I get more info and update the article, I say just use an entire bottle or two of water. You hand will hurt from all the squeezing, but hopefully your car will thank you.

Tried this “steaming” method? E-mail me with your info, so I can update the article.

At this point in the article, WankleGuy would have my head if I didn’t at least mention Amsoil Powerfoam. This stuff is available again at any NAPA or CanadianTire, and is specifically designed for engine cleaning. The basic idea is that you spray this foamy penetrating/cleaning oil into your carburetor barrels while the car is running and it cleans the inside of your engine.

There are a few things to be aware of with this Amsoil approach. One is that it was originally designed for cleaning piston engines. Nobody really knows what it does in our engines, though some people on the forum have had great success with it. Since it’s hard to know if it requires specific heat or compression requirements to fully burn off, there’s always the possibility that the powerfoam won’t completely burn off. Chances are that it will leave much less build-up than what you currently have though. Another thing to beware of is the fact that it does indeed have to burn. If you are not running a straight-piped exhaust, the by-product of burning powerfoam has to pass through your catalytic converters. These converters were made to deal with air/fuel burns only and so anything else might contribute to clogging the cats.

This foam is the midway point between the stuff I’ve mentioned above and the slightly harsher procedures outlined below. I don’t have enough first-hand (or even second-hand) experience with PowerFoam to fully endorse it here, but if the above haven’t worked I definitely recommend it before you try anything in the Carbon Lock section. Personally I cringe at the idea of putting anything like this into a running engine, but many people have had success with it and it’s definitely better than running an engine with mass carbon buildup.

Tried this “Amsoil” method? E-mail me with your info so I can update the article.

The last resort if none of this has worked is to read down this article to the carbon lock solutions and give them a try. If they’re done properly, these methods are about as safe as any. However, I repeat :

Neither the form, any of its members, nor myself take ANY responsibility for your trying anything in this article. This information has been compiled from our own experiences and the posts of others. I can’t guarantee that what I did will work the same for you. I just hope it does!

Carbon Lock:
Carbon Lock most commonly occurs in cars which have been sitting for long periods of time. As far as I can tell, this “Carbon Lock” is actually a combination of a few things. For starters, any lubricant which might have allowed the apex seals to travel along the inside walls of the combustion chamber would have long since evaporated, crusted up or drained down into the oil pan. Any carbon buildup which was present before the car was left to sit would solidify, and anything which could dry up and lock the apex seals in place would do so. It’s sort of like what happens when you leave an old plate in the sink too long. No matter how oily that spaghetti sauce was when you put it there, a few days later it’s crust. This process can take a great many weeks or months (usually months) to happen depending on many factors.

The symptoms are pretty simple – your engine won’t turn. At all. Period. Okay, well maybe in some cases it can turn with great difficulty, but it definitely isn’t a good idea to turn it. The first thing you have to realize when you’re diagnosing a sitting engine is that force hurts. Unlike the Carbon Buildup above, you can’t blast it out. If the engine does turn, push/pull starting it or cranking the starter are the absolute WORST things you can do. Forcing a carbon locked engine can easily pop an apex seal and turn a $10 2-day problem into a $2500 two-week rebuild problem.

Start by diagnosing how seized the engine is. Take all four spark plugs out and try to turn the main pulley (the biggest one) by hand. Under normal circumstances this pulley would turn relatively easily by hand if the engine is in good condition. If you can’t turn it at all by hand, then the engine is probably carbon locked or has another, more serious problem. You may be thinking “what do you mean more serious problem?”. Well, if you’re in a junkyard, that problem can be almost anything. I have read posts by people who have dropped bolts down the intake to purposefully seize the engine in order to take the flywheel off, for instance. Engines with blown apexes which have been run after the seal blew can have massive amounts of internal damage and not be able to turn. The carbon un-locking procedures in this article can’t help those engines, but they can help any engine that would otherwise be healthy, if it wasn’t for the carbon (and carbon-like deposits).

So what do you do? Well the main thing is that you need to get some form of lubricant and penetrating oil into the chamber to start eating away at the carbon and lubricating the walls of the combustion chamber so your Apex Seals can move again. There are three main fluids which you can use to do this, which I will discuss a little later on. First there are a few things which must be said with respect to how this approach is applied.

The rotary engine spins in a sense, most people know that. What that means though, is that anything you put into the engine has the potential of being scooped up and shunted out the exhaust manifold – not a good thing if you have catalytic converters you’re trying to preserve. What we want to do is get a lot of penetrating/lubricating oil in there to eat up the carbon, but also get as much of it safely back out of the engine as possible. If we can get almost all of the oil out through someplace safe like the bottom spark plug holes, then the carbon/oil sludge won’t clog or mess up anything.

So here’s arguably the best (safest) way to un-lock a Carbon-Locked engine:

1. Take out all four spark plugs and try to turn the engine’s main pulley by hand. Under normal conditions this would be a relatively easy thing to do, but you should be working with a carbon locked engine. If it doesn’t turn, good. If it barely turns, good, don’t force it. If it turns easily, find another article – your engine isn’t Carbon Locked. (If you’ve come here from the above section, make sure you’ve checked all other problems before proceeding).

2. Take your trusty windex bottle (any spray bottle will do) and clean it thoroughly. Fill it ¼ to ½ full of your chosen penetrating/lubricating oil. Please read about the oils below, don’t use just any penetrating oil because we just don’t know what it will do.

3. Spray the oil liberally into all four spark plug holes, don’t be shy we’re going to try and remove as much as we can later. About 3 good squirts ought to do it.

4. Let the oil sit for a couple of hours. The best success is had with the first sitting overnight, but at least a few hours.

5. Try to turn the main pulley again, but here’s the extremely important part: you want to be turning the pulley so that you’re pushing the top of the pulley towards the passenger side (left side, assume it’s a US/Canadian model car) of the vehicle. What this will do is turn the rotors so that the apex seals scoop up the oil/carbon sludge from the bottom of the combustion chamber and push it out the bottom spark plug holes.
When you try to turn the pulley this time, you might get only an inch or two of movement, don’t despair, that’s normal. This inch or two will move the apexes just enough that the next time you spray, you’ll be sending oil into different parts of the chamber. So turn the pulley as far as you can to the left (passenger’s) side and spray another round. If your pulley turns more than a few inches, turn it about half a turn.

6. Repeat the spray-wait-and-turn until the pulley moves freely around. You should wait at least an hour or two between tries, if not more. When it finally moves freely all the way around, give it at least three full rotations to the left. The eccentric shaft pulley turns three times to one full turn of a rotor, so you want to make sure that the rotor can turn fully around, and that you’ve purged as much of the oil/carbon sludge from the engine as possible. About six to nine turns should do it. You might want to wear light gloves for this, the stock fan sure can pinch sometimes if you aren’t careful.

7. After making sure that the pulley turns freely and that you’ve purged as much sludge from the combustion chamber as possible, put your spark plugs in and wire it up to get ready to give it a try. The left-over sludge will probably foul your spark plugs so have an extra set handy. Before you do try to turn it though, you want to check a few things if your car has been sitting:

a. Change your fuel filter and check fuel flow to the carburetor. If you have an 81-83 model, fuel will flow with the ignition in the “on” positions. Unfortunately, all trim levels of 84-85 cars require the engine to be turning to flow fuel, so this isn’t possible with those cars.
If the car has been sitting a while, purge your gas tank of it’s fuel and put new stuff in. Old fuel turns to a thick varnish-like substance which doesn’t burn well in the engine (though it does burn well on trees.. but don’t do that – it’s dangerous and only for people who don’t know better. And of course, you have an Rx7, so you know better)

b. Though you can’t properly check it until the engine is running, take a look at your Oil Metering Pump. If your engine was turning but had a lot of resistance to begin with, it’s possible this system needs some attention. Our cars inject minute amounts of oil in with the Air/Fuel mix to help lubricate the seals, and if this OMP fails, it can quickly lead to a snapped Apex Seal. The rebuild kit is only $20 (worth every penny), and it takes very little time to do the simple rebuild. Check BrownMound's OMP Rebuild and The Mazspeed OMP Rebuild Article for more info.

c. Change your oil and filter. This is imperative. If the engine’s been sitting for a while, the oil is probably thicker than molasses. Thinner oil will lubricate the rotors on the eccentric shaft so that they will turn more easily. 10w30 non-synthetic is good, especially if it doesn’t have lots of additives. Besides, if you’re OMP is working you will need that clean oil to lubricate those apex seals!

d. The air filter is a good thing to change at this point. Remember Air+Fuel+Spark = Power. Clean and unrestricted air is easier to burn.

e. Have an extra set of spark plugs on hand as I mentioned above, and also replace your wires, distributor cap and rotor. This will ensure a good, powerful spark.

f. Coolant, though less critical for the time being, is very important once the engine is running. Rotary engine do not take well to the heat.

8. Start it up!

So here’s where we talk about the oil/lubricant. The three most common to use for this task are Automatic Transmission Fluid (ATF), Marvel Mystery Oil (MMO) and Amsoil Powerfoam. Most of these are very common, though MMO can be a tad tricky to find. I think www.summitracing.com has it, but check with the guys at your local parts stores.

ATF – This has been around for a long time. About as long as Automatic Transmissions I’d guess! The reason it’s used for decarbonizing is because it can easily be found in any mechanic’s garage in quantity, it’s cheap, and back in the 1960s someone discovered that it could be used to decarbonize piston engines. The down-side to ATF is that it’s main design purpose was not killing carbon, it was designed to lubricate the gears inside an Automatic Transmission. It contains many detergents and other agents that act on carbon because the components of an automatic transmission work under high stress with very low tolerances for things like carbon. This oil is very harsh on rubber (such as the rubber oil seals which seal our rotors to the side housings) making it swell up and lose some of it’s elasticity. It sure does work on carbon though, but when it burns it produces a smoke cloud that can surely worry the neighbors for a good few minutes. This smoke contributes to the clogging of catalytic converters. The good news is that many people (including myself) have had success with ATF in killing carbon, and if you use the method above very little fluid is left inside the engine to burn off. I got a small puff of smoke when I did mine, not nearly enough to kill my catalytic converters, or worry the neighbors. The important thing is to purge it. So if ATF is all you can get your hands on, go for it, just be careful and know what you’re getting into.

Note: Neither ATF nor MMO should ever, EVER be poured into a running engine or down through the barrels of the carburetor. If anyone posts and tells you otherwise they obviously haven’t considered what they’re doing to their car. Yes some people may have done this, yes it may have worked, but you’re much safer off not doing it. Just ask Rx7Carl or Sterling if putting foreign oils into their precious carburetors is the best idea in the world. Just say NO to the ATF/MMO pushers who say “just pour it down the carb barrels, it worked for my friend’s friend and his car!”. ATF/MMO is not the solution for everything.

MMO – Marvel Mystery Oil. Not as hard on rubber, supposedly just as hard on carbon. MMO is quite frankly a mystery. It doesn’t seem to have been designed with a single purpose in mind like ATF, and according to the guys on the forum it works. It sure seems to provide the lubrication and penetrating qualities one would want to unlock an engine, but since it isn’t as harsh of a penetrating oil it may not be as harsh on the carbon. Some will be left, but that should easily be handled by the “Carbon Buildup” section’s remedies.

Amsoil Powerfoam – I’ve mentioned it before, and now here it is again. Amsoil Powerfoam is made to clean engines. I don’t know if Amsoil needs heat or compression to work it’s magic, but it deserves to mentioned here. The fact that it’s designed to clean engines says something about it’s power. It’s not harsh on rubber, and if it can be purged out as easily as MMO and ATF, and can soak deep behind those Apex seals and springs, it’s probably the best of the three to use. According to Atkins Rotary, this is what Mazda uses to free seized engines. I’m not sure how accurate that is, but it is definitely a vote for powerfoam if I ever heard one.

The above procedure is the safest way to apply ATF/MMO/Amsoil to your engine, and should work in the majority of carbon lock cases. In extremely bad cases, spraying through the spark plug holes may not offer enough exposure for more than one or two apex seals. In this case, removing the engine can provide you with the ability to spray through the intake, the exhaust, and the spark plug holes. Again, you should make sure to purge absolutely as much of the oil out as you can before putting the engine back on the car.

Having the engine off the car also offers the unique opportunity to inspect the interior of the engine without disassembling it. Through the exhaust ports with a flashlight, one can see the apex seals and parts of the faces of the rotor. A good inspection should quickly reveal whether or not all the apex seals are intact. I should warn that it is harder to turn the main pulley with the engine out, so you may not be able to put as much force on it. However, you can turn the engine over, and coat the insides with the penetrating oil to properly free it. This is obviously the last resort before a rebuild. At this point if nothing of the above has worked, you already have the engine out so you can either pick up a rebuild kit, take the engine in to a rotary shop, or get another engine.

Well, that pretty much wraps up my article on Decarbonizing a Rotary engine. As you can see it’s still a work in progress. We need many more first-hand accounts and a bit more research to give more definite answers to some of the nagging questions you may still have left over. We welcome all kinds of feedback, be it first-hand accounts, alternate methods, positive criticism, or questions you still have unanswered. Please e-mail or PM me online at vipernicus42@hotmail.com or Vipernicus42 on the forum. If I can’t be reached for a long time (which is always a possibility) Inittab (Bob) would be more than willing to take your comments. PM him.

As the French in Canada say: “À la prochaine chicane!”
Jon