Once the carburetor is installed on the engine, the throttle stop must be set to insure proper operation of the carburetor. The throttle stop on the carburetor is on the throttle shaft, opposite the throttle cable. This is preset with the butterflies just past vertical at full throttle. If the gas pedal is allowed to travel past the full throttle setting of the carburetor, the shaft will bend.

There is also a throttle stop located underneath the gas pedal. This should be adjusted so that the primary butterflies are vertical with the gas pedal all the way to the floor. This will leave a small amount of travel before the throttle stop on the carburetor is reached.

The throttle stop on the gas pedal is flexible, and excessive pedal pressure will allow the throttle to open too far. If your class rules allow, you should fabricate an adjustable stop under the pedal itself.

We do this by cutting a short length of 2" square tubing, and welding a cap on one end. Weld a nut on the capped end, and then thread a bolt, and jam nut into the end. This piece should be welded to the floor of the car so that the underside of the pedal makes contact with the bolt head. This gives a positive stop with no flex, and can be adjusted by loosening the jam nut and threading the bolt in our out.

If this seems like overkill, get down on one knee outside of the car so that you can see the assembly, and actuate the pedal by hand so that you can see just how much it flexes. Once you do this, you will wonder why Mazda even bothered, as the assembly is very flexible.

The first time you stomp the pedal to the floor and feel a positive stop, you will be glad you made the effort. Additionally, you can be sure that the primary butterflies are exactly vertical (Which results in the highest airflow.) no matter how hard you press on the pedal.

Note: We are currently working on a bolt in piece which we will make available as soon as we do the first production run.

All unused hose fittings on the carburetor should be covered with vacuum caps. The black rubber caps do not hold up well and will eventually crack causing vacuum leaks. The colored vinyl caps available from most auto parts stores will last much longer, and are recommended.

Before attempting to start the car, double check all fasteners and hose clamps to insure that there are no leaks, or anything that could cause the throttle to hang wide open.

The needle and seat can occasionally cause problems by hanging up during shipping. To avoid flooding the motor on the first start up, turn the adjustment screw on the fuel pressure regulator almost all the way out. This will give about .5 psi. Turn the pump on, and watch the float bowls through the windows to make sure they don’t overflow.

If one or both of the bowls are not filling, tap the top of the carburetor with a screwdriver handle to pop the needle loose. If one or both of the bowls overfill, remove the top of the carburetor, and check for foreign matter on the needle, or in the bowl.

The needle and seat are vacuum tested before shipping. If you find that one or both needles are leaking you can be sure that there are particles of something floating around in the fuel system. All it takes is a very small piece of rust or dirt to keep the needle from sealing, and these pieces can often get broken loose from old fuel lines during installation.

We recommend that you use new fuel lines from the firewall to the regulator/carburetor when you install your carburetor.

Once the bowls fill, you can start the engine, and raise the fuel pressure to the correct level.

The fuel pressure gauge supplied with the carburetor is a very accurate measuring device, and should be treated with care. It should not be permanently mounted under the hood, as vibration will affect its accuracy. The gauge should be used to set/check the fuel pressure, and then placed somewhere safe.

To check/set your fuel pressure, attach a short length of ¼" fuel line to the gauge. The plumbing section of nearly any hardware store should have a ¼" NPT female, to ¼" hose barb fitting available to do this. If you can only source a ¼" NPT male to ¼" hose barb fitting, you can use a ¼" NPT coupler to complete the connection.

The fuel pressure can then be monitored by removing the return line from the carburetor, and sliding the length of ¼" hose on to the return line.

Before setting the float level, adjust the fuel pressure to the recommended setting (Last Page of Manual.) with the engine idling, and the pressure gauge temporarily attached to the return line.

The fuel level should be approximately 1/16" below the middle of the sight glass. Looking in the window, the fuel against the glass will be flat in the middle, and curl up on either end to meet the edge of the window assembly. This "curl" is called the meniscus, and should be right in the middle of the sight glass, with the flat portion of the fuel approximately 1/16" below center.

The float level can be adjusted by removing the top of the carburetor, removing the float, and bending the tab that actuates the needle. Do not bend the pivot arms. It is important that you do not nick the surface of the tab. We suggest using a piece of tape to cover the jaws of a pair of needle nose pliers.

The floats are dry set before shipping, but some minor adjustment is normally needed. This process can sometimes be a real pain, going back and forth until the correct level is attained. This requires some patience, but it is critical to the operation of your carburetor. Once it is properly set, it should not need to be adjusted again.

Note: This process can be made easier by holding the lid upside down, and measuring from the lid to the float to give yourself a reference before and after bending the float tab.

Once the float level is set, the float drop should be adjusted. This is detailed in the accompanying illustration. The needle protrusion should be set between .110", and .125" If the needle does not protrude far enough, you will suffer fuel starvation at high rpm.

Your carburetor is now ready for tuning. The first step is to set the idle speed and mixture which should be done with the engine fully warmed up. The idle speed is adjusted with the screw to the far right (As viewed from the drivers side.) of the throttle plate. Clockwise increases the idle speed, counter clockwise decreases the idle speed.

The idle mixture is adjusted with the screw in the middle of the throttle plate. Clockwise leans the mixture, counter clockwise richens the mixture. For street cars, the mixture should be adjusted for best idle quality. For race cars, the mixture should be lean enough that the idle is just slightly rough. This lean idle mixture is required for best throttle response when you get back on the throttle after rolling into the corner.

Your carburetor was shipped with a selection of jets to allow for fine tuning. You should have jets one size richer, and one size leaner for both the primaries, and the secondaries. Extra jets are available in increments of .001" for $2.50 each.

The fuel jets are located in the float bowl, and the lid of the carburetor must be removed to get to them. Once the lid is removed, you can access the jets with a stubby screwdriver by removing the float bowl plugs.

Always double check the float bowl plugs after making a jet change. If you leave one loose, it will work loose on the track, and fuel will spill on to the exhaust manifold, and you will have a fire!

The primary jets are used to tune the part throttle, and low rpm mixture. Drive the car normally and note the response and power at part throttle/low rpm operation. Change to the next larger jet, and drive the car again noting the power and response. If it has improved, leave that jet in place, and go on to tuning the secondaries. If the power and response are worse, try one jet size smaller than what was supplied in the carburetor, and again note the power and response. At this point, simply install the jet that gave the best results, and go on to tuning the secondaries.

Note: If your engine is ported, you should request a larger jet selection, as no 2 ported motors are alike, and a ported motor will normally require more time spent tuning. We simply cannot guess what will make your motor happy as we can with stock ports.

Tuning the secondaries is essentially the same, but in this case drive the car hard, and pay attention to full throttle power above 5,000 rpm. As with the primaries, try one step richer, and one step leaner, and run the jets that give the best results.

If your motor is unported, in most cases the correct primary jet will be installed, and can be left in place unless weather conditions require changes greater than what can be accomplished with the selection of secondary jets.

Tuning the secondaries is simply a matter of running a few laps to get a feel for the power, and then changing to a richer jet, noting the difference in power. If the power is worse, try going one step leaner and note the results. Simply run the jet that gives the best results.

If your motor is unported, you can successfully jet the carburetor at the track. Track time being as expensive as it is (Not to mention tires.) renting a chassis dyno is a sure way to make sure your jetting is optimum, and not waste any track time.

If your motor is ported, dyno time is mandatory. You can easily waste an entire race weekend arriving at proper jetting without the help of a chassis dyno. Because no two ported motors are the same, we cannot make an informed decision as to optimum jetting like we can with stock ports.

We want to be sure that you are happy with your purchase, and that can only happen if your carburetor is jetted properly.

Once your carburetor is optimally jetted, make note of the weather conditions. Many customers make use expensive of weather stations which show humidity, barometric pressure, and temperature. Other have good results with a simple air density gauge, and some just pay attention to how the air feels.

Whatever your approach, if you take careful notes, you will soon have a small data base, and you will be able to make appropriate jet changes before you go on track for a session.

The best approach is to tune the engine on the dyno with an exhaust gas temperature gauge in the car. Most stock 12A’s will show exhaust gas temps from 1,650 to 1,700 at 8,000 rpm with optimum jetting. This is a rough guideline. There is no correct egt, because probe type, and placement will vary from car to car.

If you tune for optimum power on the dyno, and then note the EGT, you can use that for a reference on track. Whatever the number is (When tuned for optimum power on the dyno) 1,550, 1,600, or even 1,750, this should be your target while running on the track, and you can use this as a guide for optimum jetting.

Keep in mind that weather conditions affect jetting requirements. High humidity, or high temperatures will require smaller fuel jets to account for the reduction in oxygen content by volume.

Again, take notes so that you can refer to them should you see the same weather conditions again.

Over time, you will get a feel for this, and intuitively know what jetting to run under all conditions, and you will be a step ahead of your competition.

The accelerator pump is adjustable, and spending a bit of time with this will be required for best throttle response. The accelerator pump is on the right side of the main body as viewed from the drivers side of the car.

The pump body is approximately 1" square, and is held in place by 4 small button head screws.

There is a small steel lever attached to a spring loaded rod which pivots from the throttle linkage. There is a 7mm. Nylock nut at the end of the rod which forces the lever against the spring. Turning this nut clockwise will increase the total amount of fuel injected into the engine during rapid throttle opening. Turning it counter clockwise will have the opposite effect.

The throttle response can be set by revving the motor from just off idle (approx. 2500 rpm) and noting the response. Make adjustments in both directions until you get the best response.

Rotary engines are fairly sensitive to ignition timing, and the difference between optimum, and 4 or 5 degrees off can easily be 5 to 10 horsepower. If the centrifugal advance has been disabled, the timing can be set at idle. If the centrifugal advance is still active, the engine must be revved to approximately 5,000 rpm to set total advance. If your engine still has vacuum advance, the vacuum lines should be disconnected before setting the timing.

Our dyno testing shows that a stock port 12A will make best power with the leading set to 24 degrees before top dead center, and the trailing set to 16 degrees before top dead center.

The stock timing marks on the pulley are at top dead center, and 20 degrees after top dead center, and are meant for setting the timing at idle.

The total advance numbers listed above can be arrived at by the use of a "dial back" timing light, but this is not recommended. Very few dial back lights are accurate (Only the MAC, and Snap On have so far proven to be accurate.) We have seen a variance of as much as 7 degrees between different brands of dial back lights.

Additionally, most lights are designed for a 4 stroke engine which fires each cylinder every other revolution. As a result, if you wanted to set your timing at 24 degrees, you would have to dial 48 degrees into the light. The more timing you dial into the light, the less accurate it is likely to be.

A better solution is to add additional timing marks to your pulley. This is detailed on the website.

If you are using your carburetor for a street application, the factory fast idle mechanism will be maintained. The fast idle speed can be adjusted with the cable adjustment nut at the carburetor cable bracket.

Stock Ports 2.0 psi.
Ported 12A 2.5 psi.

Note: Fuel pressure should always be set with the engine idling.

If you have any questions, please contact us. We want you to be fast.

Paul Yaw
Yaw Power Products

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