Carburetor Facts By Tom Turner
I have been working on cars and motorcycles for the last 26 years, and have been working on 1st generation RX-7s for the last 17 years. On the RX-7, I have only a little experience with the stock Nikki and the Holley 4 barrels, but several years with the Sole/Mikuni 44 side draft, and several years with the Weber 48IDA two barrels. I have also worked on SU , Holley, Rochester, Carter, Mikuni, Keihin, and Bendix carbs on piston engines. The one thing I know for sure is that the more I learn about carbs, the more I realize that I don’t know about carbs and the more I realize how hard it is to tune a carb from scratch on a new application. My observations and recommendations here are based on my personal experiences and reinforced by what I have read from other people’s experiences. This is not a complete know-it-all article, as it would take many, many books to cover all about carbs, but I am going to write this article the best that I can. I strongly recommend reading other tech articles about carbs, plus books about the specific brand of carb you happen to own. If you want a modified Nikki 4 barrel for your street or ITA racer, I recommend reading Paul Yaws tech articles at the Yaw Power website about the work he has done for others and he can do for you.
If you are thinking about changing from the stock Nikki carb, the single best piece of advice that I can give about carbs is this: unless you have lots of experience and time and patience, buy a complete carb / air filter / intake manifold kit from a shop that specializes in rotaries. They will have all the bugs worked out of the system, they know will supply the right size venturies and jets you need, in short they have already worked out all of the problems. If it all possible, drive someone elses RX-7 who already has installed the carb and manifold that you are thinking about buying. That way you can get a feel for how it is going to behave. If you are going to run it on the street, spend some time driving it at part throttle easy cruising speeds, because that is the way that the car is going to be operated most of the time anyways, and that is where most of the bogs / flat spots / poor carburetion will show up. You may think you are going to save big bucks by buying a used carb from some other non rotary application and adapt it to the rotary, but by the time you get all the bugs out of it (if you ever do get it running right,) you will have spent more time and money than if you had just went out and bought the right thing to begin with. With the time you will have spent trying to work all the bugs out of a untried carb system, you could have spent that time working at your day job and earned the extra money it would take to go ahead and buy the right system from an experienced rotary carb shop.
What is the Right Carb for My Application?
If you are in the process of modifying your rotary to get more power out of it, first put a header and a free flowing exhaust system on it. After you do the exhaust, then you need to improve the flow through your intake system. Before you decide what you are going to do with your intake system, you need to ask yourself these questions, and come up with honest answers for them. How much money can you afford to spend? Are you still going to drive your car on the street, and if so, is this modified car going to still be your daily driver or are you only going to drive this car very occasionally? How important are gas mileage and smoothness going to be for your modified car? Are you going to be drag racing or road racing this car? What do the rules for your racing organization allow you to do?
The following recommendations are my opinion, based on what I have done myself, read about, and heard others say:
Carbs for 12A Stock or Street Ported, ITA Racing and 13B 4 port Stock or Street Ported Applications. Modify your Existing Nikki 4 barrel: I believe that you will get the best power increase for your money and smooth operation with the Paul Yaw modified Nikki 4 barrel. He will take your old carb and modify it for your application (stock or street ported engine.) He works opens up the venturies to make the carb flow more air, he puts in different jets, and he does a few other changes to make it run smoother and flow more air. It takes him a few weeks to do this, so if you don’t want your car down while you wait on him, then buy a used cheap junkyard carb and send that to him to modify. Another reason this system is much cheaper is because you can use your existing Mazda intake manifold. For a very decent price, Paul Yaw can also rework your stock Mazda manifold and make it flow better, for a little bit more power too. There are other people who modify the Nikki 4 barrel, but before you give them your hard earned money, you better get some good references from other people who have tried their product and believe in the quality of their work. For more Yaw info see Paul Yaw’s web page for more information:
Solex Mikuni 44 PHH side draft 2 barrel: These carbs are no longer available, but there are still some used ones that come up for sale occasionally. The performance will be comparable to the Weber 45 DCOE side draft 2 barrel. I ran one for several years on first a stock ported 12A, then on a street ported 12A. Weber 45 DCOE side draft 2 barrel: These carbs are a good compromise between power and smoothness or driveability on the street. Gas mileage may be slightly less than using the modified Nikki 4 barrel. As with all big 2 barrel carbs, you cannot open the throttle quickly at low engine speeds. The minimum engine speed at which a big 2 barrel will not bog varies depending on the size of the venturies, but it usually you cannot floor them below 3500 rpm without the engine stumbling. This is usually a rude shock to someone new to a 2 barrel, but that is just the laws of physics. You can overcome the bog by learning how quickly you can open the throttle and when you can give it full throttle. These carbs require a special intake manifold made just for this carb and your engine. I strongly recommend that you buy a complete kit with carb, air filter, intake manifold, and the linkage required. If you buy a carb off of some other application, it will take you time and money to get all the other necessary pieces rounded up, and a lot of time to get it all running smooth.
In January of 2000, these complete kits cost about $700 for a 12A and about $750 for a 13B from Fast Freddie’s (714) 540-3801. Mazdatrix also sells these kits for about $50 more, and Mazdatrix can be reached at (562) 426-7960 Weber 48IDA down draft 2 barrel: From my own personal experience, this carb makes more high rpm power than anything else on a street ported 12A, but it suffers from the usual big 2 barrel problem of flat spots and bogs during low rpm street operation. Most people use 37 mm venturies with 12A engines and 38 mm venturies with 13B engines. Owners of highly modified 13B street engines may want to try the 51 IDA version. The Weber 51 IDA is a 48 IDA that has the throttle body bored out to 51mm, and special 51 mm throttle butterfly plates installed. The 48 IDA is no longer available new, but there are still several ones for sale used. Used prices range from $300 to $600 for the carb, and Mazdatrix still sells new intake manifolds for $311 as of January 2000. Used intake manifolds typically range from $100 to $200. Holley 4 barrel carbs: Racing Beat developed several of these years ago, and Mazdatrix also sells them now. In general, a 4 barrel will offer a little better gas mileage and better low and mid-range power than the big 2 barrels do. For a stock ported 12A or 13B, Mazdatrix sells a 465 cfm Holley complete kit for $735 as of January 2000. The vacuum operated secondary throttles that come on the smaller street Holleys are very good at boosting low rpm and mid rpm power by preventing over carburetion. The last time I drove one of the Holley kits was in the mid 80’s when they first came out, and I thought that it drove very smooth on the street. Mazdatrix also sells kits for street ported 12A and bridge ported 12A engines for about $844 as of January 2000. Jim Downing recommends using a 550 cfm Holley for 13B engines on the street. In America, Holleys have been around the hot rod crowd for years, so some people feel more comfortable working with them than they do the Italian Webers; this is probably why they are popular with American rotor heads. One bad thing about Holleys is they are not as tunable as the Webers or Solex/Mikuni or Dellorto carbs. On the Holleys, you can not change the venturi size, the emulsion tubes, or the air jets. These things are fixed, cast in place. Another thing to beware of is buying a used Holley. The kits that Racing Beat and Mazdatrix sell have several modifications done to their carbs to make them run good on rotaries. In general, if you buy one that came from some where else, you will have a hard time getting it to run right. One last bad point about Holleys is the float bowls. If you go around curves hard (doesn’t everybody with an RX-7?) the standard Holley float bowl will make the engine die out. The standard float bowls are okay for drag racing, but for street and road racing you need to get the Holley center pivot float bowls, which will cost you another $150.
Carbs for Bridge Ported 12A and 13B Race Engines
Holley 4 barrel carbs: Since the bridge port engine has 4 intake ports, it is natural to use a carb with 4 barrels and an intake manifold with 4 runners. You need a larger carb for bridge ported engines. Some experts recommend you need a 600 cfm Holley for bridge ported 12A engines, and a 650 cfm Holley for bridge ported 13B engines. As I said above, Racing Beat and Mazdatrix does several modifications to a Holley that they put in their kits. In general, if you buy one that came from some where else, you will have a hard time getting it to run right. I’ll say it again: do not buy a used Holley that came from anywhere else but off an RX-7. If they people selling it to you do not have the intake manifold that mates it to the rotary, then it didn’t come off of a rotary and it is going to cause you a lot of headaches! In general, a well tuned Holley 4 barrel will make more mid-range power, and more useful horsepower across a wider rpm band than a Weber IDA. However, one bad point about Holleys is the float bowls. If you go around curves hard (doesn’t everybody with an RX-7?) the standard Holley float bowl will make the engine die out. The standard float bowls are okay for drag racing, but for street and road racing you need to get the Holley center pivot float bowls, which will cost you another $150. . Another bad thing about Holleys is they are not as tunable as the Weber. On the Holleys, you can not change the venturi size, the emulsion tubes, or the air jets. These things are fixed, cast in place. As of this writing, I don’t think that Mazdatrix still sells new Holley kits for the 13B bridge port engine, but there are used carbs and manifolds for sale out there.
Weber 48IDA and 51IDA
This carb suffers from the usual big 2 barrel problem of flat spots and bogs during low rpm operation, but most people with bridge ported engines don’t care about low rpm drivability, they are just looking for all out high rpm peak horsepower. Some experts claim that they can get a little more peak horsepower out of a Bridgeported engine with a Weber than the same engine with a Holley. Most people with bridge ported 12A engines put 42 mm venturies in their Weber 48 IDA.. Owners of bridge ported 13B race engines usually use the 51 IDA version, with 45 mm venturies installed. The Weber 51 IDA is a 48 IDA that has the throttle body bored out to 51mm, and special 51 mm throttle butterfly plates installed. The 48 IDA (and the modified 51 IDA) is no longer available new, but there are still several ones for sale used. Used prices range from $300 to $600 for the carb, and Mazdatrix still sells new intake manifolds for $311 as of January 2000. Used intake manifolds typically range from $100 to $200. Carbs for Peripheral Ported 12A and 13B Race Engines. Since these engines only have two intake ports, it is just natural to use a big carb with 2 barrels like the Weber IDA. For a 12A peripheral port racer, use the 48IDA with 43 mm venturies. For a 13B peripheral port racer, use the 51 IDA with 46 mm venturies. Note that the peripheral port engines need a slightly larger venturi than the bridge ported engines, that is because the peripheral port engine can flow more so it needs a bigger venturi throat to let it develop it’s full horsepower potential.
What About Fuel Injection?
Hey! This article is about carbs not fuel injection! But seriously, the stock GSL-SE 13B fuel injection system cannot flow as much air as a Weber or Holley can. There are aftermarket systems that can flow a lot of air, but I imagine that by the time you pay a professional to run the car on a dyno and map the fuel injection for you and get it running perfect, you will have spent $2000 for the whole system. Most SCCA road racing classes will not let you run fuel injection on the Mazda Rotary. Last years state of the art Can-Am 4 rotor road racer run by Jim Downing used fuel injection, and God only knows how much that system cost. The most powerful 13B peripheral port road racer I have seen, in 1998 when there was still a GT3 class in Professional Sportscar Racing, made 380 horsepower with an $8000 flat slide racing fuel injection system (by the way, that engine had a design life of 6 hours.) $8000 just for the fuel injection system! That is why, at my lowly income, that I use carburetors.
What Jets Should I Use in my Carb?
Sorry, but I can’t answer that question for Mazda Nikki 4 barrels and Holley 4 barrels. For those of you who do have these carbs and have them running GOOD, then please e-mail me with your jet sizes and we will add them to our data base. Remember, these are only recommendations. Every engine is a little different, and if your engine porting, or air cleaner, or exhaust system or the altitude you live at is different, then these will not be exactly right. However, they are a good starting point. These jets will get your engine running. To get maximum smoothness and max. power, you will need to continue to fine tune it from here. Note: if you don’t use EXACTLY the same size venturies and emulsion tubes as listed here, then these jets sizes are useless. Not sure what size your venturi is? Check for a number on it, or pull it out and measure it. For Solex/Mikuni 44PHH two barrel side draft carb, used on stock or street ported 12A with header and free flowing exhaust:
Venturi 39 mm
Main fuel jet #200
Air Jet #240
Idle Jet #62.5
Accel. Pump #90
Emulsion Tube OA
The following Weber recommendation come from Racing Beat in Anaheim CA:
Weber 48IDA on street ported 12A with headers and free flowing mufflers
Main fuel jet #170
Air Jet #150
Emulsion Tube #F-11
Weber 48IDA on street ported 13B with headers and free flowing mufflers
Main fuel jet #190
Air Jet #160
Emulsion Tube #F-11
Weber 48IDA on bridge ported 12A with headers and open exhaust
Main fuel jet #240
Air Jet #170
Emulsion Tube #F-11
Use larger #300 float needle valve
Weber 48IDA on peripheral ported 12A with headers and open exhaust
Main fuel jet #230
Air Jet #125
Emulsion Tube #F-8
Use larger #300 float needle valve
Weber 51IDA on bridge ported 13B with headers and open exhaust
Main fuel jet #235
Air Jet #165
Emulsion Tube #F-11
Use larger #300 float needle valve
Weber 51IDA on peripheral ported 13B with headers and open exhaust
Main fuel jet #240
Air Jet #110
Use larger #300 float needle valve
How Do I Fine Tune My Carb?
WARNING: If you have no ear or feel for tuning an engine, then you better leave this entire subject alone and let someone else do it for you.
Before you can tune your carb, you need to know the basic principles of how they work. These basic principles of carb operation apply to all carbs. A few terms: The terms twin choke and two barrel mean the same thing; likewise the terms four choke and four barrel mean the same thing. The term hesitation or stumble or bog or flat spot is usually used interchangeably here; it refers to a combination of engine rpm and throttle setting where the engine suddenly runs rough because the carburetion is not perfect. The engine may recover from this roughness after a second of time, or it may not recover at all until the engine rpm / throttle setting condition changes. The combination of engine rpm and throttle position is very important when trying to tune or trouble shoot a carb. If you don’t pay attention to what throttle opening AND what rpm your engine is running rough at, you won’t know what circuit in the carb is causing your problem.
Basic Carb Operating Principles
Everybody knows that their rotary engine breathes air in, adds gas to the air in the carb, and burns the air/fuel mixture in the engine to make power. Carburetion is the process of adding the exact right amount of gas to the air flowing into the engine, at all possible engine rpm and throttle combinations. Trying to get that air/fuel mixture in the perfect proportion all the time is a lot harder than it seems.
Having more fuel than the engine needs for the amount of air it is breathing is called running too rich. Not having enough fuel for the amount of air the engine is breathing is called running too lean.
The fuel gets introduced to the air in the venturi. The venturi is also called a choke by some people The venturi is the part of the body of the carb that necks down to a smaller diameter than the throttle butterfly plate, and is located upstream of the throttle. As the air flowing through the carb passes through the smaller diameter of the venturi, it has to speed up slightly. As the air speeds up, the air pressure drops, and it sucks fuel out of the float bowl and into the air stream. When running at throttle to full throttle, the engine is getting the fuel it needs through the venturi in this way; this system is called the main run circuit. Most carbs have a booster venturi suspended right in the middle of the venturi. Since the air is flowing faster in the center of the carb than along the carb walls, the booster venturi gets an even stronger pressure drop, and it is where the fuel is usually introduced into the air flow. The booster venturi is usually a small cast piece that is suspended by one or two legs or arms that attach it to the main carb body. The fuel travels from the main carb body to the center of the booster ventrui thru a small passage in one of the legs or arms.
Since the venturi only sucks gas out of the carb float bowl when air is flowing through it rapidly, a different method of providing the engine with fuel is needed at idle. That system is called the idle circuit; it will be explained later on. To handle the condition in between idle and the main run circuit, most carbs have special passages called transistion circuits. Most street driving is done using the idle circuit, the transition circuit, and just barely getting into the main run circuit. Most carb flat spots / bogs occur in during these operating times, when the fuel flow in the various circuits cannot instantaneously keep up with the changing conditions. Like I said above the combination of engine rpm and throttle position is very important when trying to tune or trouble shoot a carb. You must pay attention to both how much the throttle is open and how many rpm the engine is turning when trying to figure out how carb is working and to trouble shoot any carb problem.
Since the venturi is smaller in diameter than the main carb body, it is slightly constricting the flow of air through the carb, and thus it slightly restricts the flow of air thru the engine. We all know that to maximize the power our engine puts out, we need to maximize the amount of air that flows through it. Therefore, to help the engine flow more air, racing carbs have bigger venturies than a stock carb. However, it is very easy to put too big a venturi on and have poor carburetion. If the venturi is too big, then the air does not have to speed up enough as it flows through the venturi. If the air does not speed up fast enough, then it does not get sufficient pressure drop, and then it does not want to suck the fuel out of the float bowl proplerly at low air flow (part throttle / low rpm) conditions. The most common mistake someone can make with a street car is to put a carb on with venturies that are too big. If your venturies are too big, you will have a bad bogs / flat spots at part throttle / low rpm conditions. That is why it is best for most people to buy a carb from a reputable rotary shop, because they will know from their own trial and error experiences as to what is the best size carb with the best size venturi for your application. If you have a carb that offers different size venturis like the Weber and the Weber copies (Solex/Mikuni and Dellorto,) then it is best to run with the size venturi that is recommended by someone that has been through all the trouble of tuning that carb. If you change venturi size, then you must start tuning all over in finding the correct size main jets and air jets to go with your new venturi size. Production carbs (like the stock RX-7 Nikki and the Holley) have the venturi cast into the body of the carb, and that venturi cannot be exchanged with a different sized venturi. Aftermarket Holley copies like BG and others offer carbs that look just like the Holley except they have replaceable venturies and other changes to increase tunability.
Float Bowl Level
The float bowl level is critical in getting the carb to run right. As stated above, during throttle and above operating conditions, the venturi sucks the fuel out of the float bowl. If the fuel level in the float bowl is too high, fuel will run out of the float bowl, through the venturi and down in the engine all the time, flooding out the engine. This is exactly what happens when your needle and seat go bad, making the float bowl level rise above the booster venturi and flood out the engine. Besides flooding out an engine, an improper float bowl level will show up when the carb is making the transition from the idle circuit to the transition circuit to the main run circuit. Since the venturi is sucking fuel up out of the float bowl, if the float level is too low, then the venturi must suck harder since it has to raise the fuel higher up before it starts flowing out of the venturi. If the venturi has to suck harder, then it will not reach that higher suction until you reach higher air velocity through the venturi, which means that the main run circuit won’t start flowing fuel until higher rpm / throttle more open conditions. A too low float bowl fuel level is sometimes the cause of a stumble or bog or hesitation or flat spot. Too high of a float bowl level may make the main run circuit start adding fuel too soon, before the engine is flowing enough air to need that much fuel. This will cause too rich of a mixture during the transition periods. If your carb was running right and you suddenly develop a carb problem like these, then check the float bowl level. Always run the float bowl level that is recommended by the manufacturer of the carb, or the level recommended by the person who put together the complete carb/intake manifold kit that you are running.
Main Run Circuit
As stated above, the main run circuit is where the engine gets most of it’s fuel and air from throttle to wide open throttle, and covers most engine rpms from about 2500 rpm all the way to engine redline. The devices in the main run circuit that let you meter or control the fuel flow are called the main fuel jet or main jet, the emulsion tube, and the air correction jet or the air jet. The bottom of the emulsion tube is down in the fuel in the float bowl, and the fuel enters the emulsion tube through the main jet. The top of the emulsion tube is located in air, and the air entering the tube must pass through the air jet. Air and fuel enter and mix together in the emulsion tube, and then this air/fuel premix then flows thru a passage into the booster venturi, and then into the main air stream, and then on into the engine. Production carbs like the Nikki and the Holley usually do not have replaceable emulsion tubes; you are stuck with what they cast in place. Webers and Weber copies have several different emulsion tubes to choose from that are easy to replace but I strongly recommend that you use the emulsion tube that the experts recommend for your particular applicatrion. As stated above, the main run circuit is where the engine gets most of it’s fuel and air from throttle to wide open throttle, and from most engine rpms from about 2500 rpm all the way to engine redline. Both the main jet and the air jet control air/fuel mixture during these operating conditions, but the main jet is the most dominant in controlling mixture during mid-throttle / mid rpm conditions, and the air jet is the most dominant in controlling air/fuel mixture during full throttle / high rpm conditions. In general, if you have a carb jetted right for mid rpm mid throttle conditions, it is hard to get it to run too rich at full throttle high rpm, during that last 1000 rpm right before redline.
A larger main jet has a bigger number on it and will make the engine run richer.
A smaller main jet has a smaller number on it and will make the engine run leaner.
A larger air jet has a bigger number on it and will make the engine run leaner.
A smaller air jet has a smaller number on it and will make the engine run richer.
When the engine is idling, the throttle plate is almost closed, and almost no air is flowing through the venturi. Because not enough air is flowing through the venturi for it provide fuel for the engine, the idle circuit was created. Basically, all idle circuits have a small air orifice, a small fuel orifice or jet, a mixture screw, and also there is the throttle stop for the throttle butterfly valve. The fuel enters the air stream at a small hole just below the throttle butterfly valve. Most production carbs do not have adjustable, replaceable idle air jets and idle fuel jets; they only have the idle mixture screw. Webers and Weber copies have replaceable idle (fuel) jets to increase their tenability. In general, run the idle mixture jet that the experts recommend for your particular application.
Adjusting the idle of an engine is a simple job that anybody can screw up. You adjust the mixture screw(s) until the engine idles the best. Your final setting of the screw must be done with the engine really hot after a hard run. If you try and set the idle mixture with the engine mildly warm, the engine will be too rich once it gets up to full hot operation, and it will tend to foul spark plugs. The universal starting point to set most carbs at is to set the idle mixture screw 1 and turns out. This means turn the screw gently all the way in, and then back it out 1 and turns. If you are ham fisted and turn the screw in too hard against it’s seat, you will ruin the screw and the seat and you might not ever get your car to idle right again. If your engine wants the idle mixture screws more than 2 turns out, then you might need to install a bigger idle fuel jet. If your engine wants the idle mixture screws less than turns out, then you might need to install smaller idle fuel jets.
If you have a vacuum leak from any source, it will not let the engine idle properly, and you will not be able to adjust the idle mixture properly. If you have a problem idling, first make sure that you do not have a vacuum leak. Check engine to manifold gaskets, carb to intake manifold gaskets, and every hose and every fitting that is attached to the intake manifold.
The transition circuit is also sometimes called the progression circuit. The transition circuits covers the operating condition when you are just barely cracking the throttle open, when the throttle is opening just slightly more than idle, before there is enough air flow through the carb for the venturi to suck fuel through the main run circuit. I have found that this circuit can make a profound difference on whether one high performance carb is more streetable than another. The transition circuit usually gets its fuel from the same passages that feed the idle circuit. The fuel then enters the air stream in a hole, or a series of holes, or a slot that is located right where the tip of the throttle butterfly touches the carb. As you open the throttle, the throttle will uncover the transition circuit holes, and feed fuel to the engine during this condition of the throttle being just barely open. Since the transition circuit gets it’s fuel from the idle circuit, sometimes you can adjust the transition mixture by changing the size of the idle jet. A good test of the transition circuit is to open the throttle very slowly by turning the throttle stop (idle speed) screw. If the transition circuit is working properly, the engine should smoothly increase in rpm with no hesitation or roughness. If the float level is not correct, then there is no way you can get a smooth transition from idle to the main run circuit. If the float level is too high, the main circuit will begin delivering fuel too soon, and the mixture will be too rich during the transition period. If the float level is too low, the transition circuit will stop delivering fuel before the main circuit starts up, and the lean mixture will cause the engine to run very rough.
I prefer the type of transition circuit that lets the throttle uncover a series of holes to deliver the fuel. When I was running a 12A street port engine, my Weber 48IDA only came with one transition hole. This carb had an awful flat spot on the transition from idle to the main run circuit. In 1994 I sent the carb to Advanced Engine Management, phone (310) 327-9336, in Gardena, California. For $25, they fixed my problem by drilling two extra transition holes in each barrel of the carb. Their fix did not get rid of all the transition stumble, but they got rid of about 75% of it, and made the car much more pleasant to drive on the street. Never try to drill out or modify the existing transition port holes yourself.
Other Factors That Can Make Your Car Run Bad
If your car was running okay with your carb, and all of a sudden it started acting up, odds are it is not the carbs fault. Are you sure that you have no vacuum leaks? Is the ignition in perfect condition? Are the sparkplugs new, and of the correct heat range for your application? Is the carb getting a nice, steady decent supply of fuel pressure?
If you put a NEW carb on, and it is not running right, are you sure you read and followed all the steps in the instructions? Are you sure you did not create any vacuum leaks? Are you sure you are feeding the carb with EXACTLY the fuel pressure as specified in the instructions? Are you sure you did not put this new performance carb on a marginal, wore out engine, and the extra strain of you now trying to rev the hell out of it did not damage the tired out old engine?
To Really Fine Tune It, Get an Air/Fuel Meter
It is very, very hard to tell whether your mixture is too rich or too lean without an air/fuel ratio meter, especially on a street car. The Mazda rotary can run under a wide variation of mixtures, from very lean to very rich, but it only makes maximum horsepower when it is running slightly rich. You cannot always tell if your car is too rich or too lean by reading the sparkplugs, especially on a street car. If you have a problem with a mixture not being correct, in order to fix it you need to figure out which of the circuits in the car need to be corrected (idle, transition, main run circuit, etc.) Let’s say your street car runs rough at 3000 rpm in 5th gear, when you are steady cruising 60mph on a level highway. At a steady 60 mph, your engine is only producing about 20 hp, so you are not making enough heat to be able to tell mixture by your exhaust gas temperature (unless you already have a data base of egt vs. engine power.) If you don’t have an air/fuel ratio meter, and you want to check your mixture by reading the spark plugs you are going to have to warm up your car, put in a new set of plugs, drive it only at steady 60 mph for many many miles, the push the clutch in, pull over on the side of the road and remove and read your sparkplugs. The reason I say this is because if you do any idling, or any throttle open hard acceleration during this test, then the different mixtures from those conditions will mask the conditions that you are really trying to test for (your steady 60 mph misfire.) If you have an air/fuel ratio meter, you can instantly read the air/fuel ratio, at any throttle opening, at any rpm. Exhaust Gas Temperature gauges respond slowly, and in my opinion are only good for steady engine operation on a dyno, or on a race track with a VERY long straightaway. Also, like I said above, unless you already have a data base of normal, correct egt vs. engine power, you have no idea of what that EGT number really means. An air/fuel meter tells you straight out: you are running just right, a little rich, way rich, pegged the scale off the rich side, etc. You don’t have to be a genius to read the air/fuel gauge. If you do have a mixture problem, and if you want to fix your problem, you do have to pay attention to how far open you have the throttle open and at what rpm the engine is running when you see the mixture problem on your air/fuel ratio meter.
Haltech sells a nice air/fuel meter with 30 LEDs, but they want $150 for theirs. You can buy an Intelletronix air/fuel ratio meter for as cheap as $40 from Summit. The Intellitronix gauge only has 10 LEDs for a read out, and that level of resolution seemed good enough for me. The gauge is the standard 2-1/16″ diameter round, and fits in standard gauge holes. Intelletronix also sells the oxygen sensor to drive the gauge, but they want about $40 for theirs. I found ou that 99% of oxygen sensors have the same output, so I went and bought the most popular (mid 80’s GM,) cheapest oxygen sensor from my local auto parts store for about $25.
The hardest part of the job is installing the oxygen sensor in your exhaust pipe. You need to drill a 5/8 dia hole in the side of your exhaust pipe, then weld a threaded bushing on to hold the oxygen sensor. You can buy a bushing for the sensor can be obtained from some muffler or auto parts stores. If you are a do-it yourself type you can make your own fitting. Most all oxygen sensors have M18x1.5 threads. I took a low strength 5/8-11 nut, drilled it out to 21/32 diameter, then welded it on my exhaust after the collector. Then I tapped it M18x1.5 When screwing in the sensor, be sure the threads have some anti-seize on them.
Wiring is simple. Connect the wire coming out of the oxygen sensor to the gray wire on the meter. Connect the wire from the meter to positive 12 volt power. Connect the black meter wire to ground. Mount the gauge in an easy to read spot, and you are ready to go!
If you are tuning for maximum hi-rpm horsepower, you need to run your car in a high gear (3rd or 4th or 5th ) so that you can get a good stable reading. If you try and do it in 1st & 2nd gear on a slalom course or on the street, engine rpm and intake air velocity are changing way too fast for you to read it and get a good data for peak hi-rpm horsepower.
What Fuel Pressure Does My New Carb Need?
If you buy a new carb kit, the instructions should tell you what the fuel pressure requirements are. Racing Beat offers the following recommendations:
Stock Mazda Nikki carb, 1976-1983 . 4.5 psi
Stock Mazda Nikki carb, 1984-1985… 3.5 psi
Weber, Dellorto, Solex/Mikuni. 4.5 psi
Holley.. 6.0 psi
To ensure that your fuel pressure is correct, you must check it with a pressure gauge connected to a tee right at the carb. In almost all performance applications, you will need to install a regulator with your performance pump, so that you can set the pressure just right.
My Car Seems to Run Lean After Several Seconds of Full Throttle Hi RPM Operation
A common complaint of people who put big carbs on is that the power seems strong at high rpm in the lower gears, but by the time they get up to 5th gear the power falls off. This can be caused by too small of a fuel pump, unable to keep up with the fuel demands of the bigger carburetor. You need a bigger than stock fuel pump if you expect to make more than stock horsepower. To ensure that you have enough fuel to feed that big pump you put on, you need a fuel pressure gauge to measure fuel pressure at the carburetor. Put a tee in the fuel line very close to the carburetor. For temporary street use, you can tape the fuel pressure gauge to the windshield, so that you can read it while you are driving. If you have a race car, you need to install a permanent fuel pressure gauge in your instrument panel. While you are running your car full throttle at hi rpm in the upper gears, your fuel pump should be able to maintain the fuel pressure at the carb to within 1 psi of the carb’s normal fuel pressure. If your pressure is low, and you have a high performance pump in place, check your fuel filter. Always replace your fuel filter once a year in street applications.
I can See Gas Dripping From the Venturies at Idle or When the Fuel Pump is On
Possible causes are 1) a piece of trash is making the float needle valve stick open, 2) your fuel pressure is too high, 3) the float level is set too high
My Car Runs Lean at Mid Throttle to Full Throttle, Mid RPM to Hi RPM Conditions
Install a larger main fuel jet to make the mixture richer.
My Car Runs Rich at Mid Throttle to Full Throttle, Mid RPM to Hi RPM Conditions
Install a smaller main fuel jet to make the mixture leaner.
My Car Runs Lean at Full Throttle, Hi RPM Conditions
Install a smaller air jet to make the mixture richer. This is a common problem.
My Car Runs Rich at Full Throttle, Hi RPM Conditions
Install a larger air jet to make the mixture leaner. This problem is not very common.
My Car was Idling Fine, but now it Runs Very Rough at Idle
This is a common complaint. If your car was running okay, and all of a sudden it started acting up, odds are it is not the carbs fault. Are you sure that you have no vacuum leaks? A Vacuum leak will make any car idle awful. Check the carb to intake gaskets, and the intake manifold to engine gaskets. Check every single fitting and hose connected to the intake manifold. Check the vacuum hose going to the power brake booster. Is the ignition in perfect condition? Are the sparkplugs new, and of the correct heat range for your application? If the spark plugs look dirty, put in a new set of plugs before you do anything else!
Possible carb problems can be a piece of trash has plugged up something in the idle or transition circuits. Blow out the idle jets with compressed air or a can of carb cleaner, and blow out the passages that the idle jets were screwed into. Look down the throat of the carb while you are blowing thru the idle passages, and make sure you can see fuel/air blowing out of the ports in each throat by the throttle butterfly valves.
My Car Idles Fine, but When I Open the Throttle It Wants to Die, and Then It Recovers and Runs Strong
1) With the engine shut off, check that your accelerator pump is giving a nice, strong, long squirt of gas down the venturis when you open the throttle by hand. If you don’t see a nice, long squirt down the venturies, then fix your accelerator pump.
2) Make sure your float bowl level is not too low
3) Make sure the transition circuits are not plugged up by trash. Blow out the idle jets with compressed air or a can of carb cleaner, and blow out the passages that the idle jets were screwed into. Look down the throat of the carb while you are blowing thru the idle passages, and make sure you can see fuel/air blowing out of the ports in each throat by the throttle butterfly valves.
4) Your carburetor is too big! You went overboard when you put that monster carb on your street driven engine. You need to put a carb with smaller venturies on your car.
5) You put on a carb that was not made for a rotary, and it does not have sufficient accelerator pump volume and/or the wrong transition circuit for a rotary. Good luck on trying to get it tuned right.