On the stock Rx-7, fuel is pumped from the tank to the carburetor by means of small piston pump that rates at roughly 5.5 psi. The capability of the stock fuel pump to actually produce that pressure varies with the age of the vehicle, as the pumps do wear out. Generally, even a worn stock fuel pump can generate the necessary volume of fuel into an open bucket, but as soon as the slightest resistance is encountered, the volume diminishes as the pressure increases. Therefore, the fuel pump output "bucket test" cannot accurately distinguish between a stock pump capable of pushing 5.5 psi and one that can only push 4.75 psi when flow is restricted. If a flow restrictor in the form of a smaller diameter hose is applied to the end, then the test may be more accurate. However, since there is no standard by which to guage, the test would have to be done first without the restriction, and then with the restrictor in place... -an awful lot of work to test the pressure of the pump when you can simply hook a guage up to the carburetor.

The Nikki carburetor is meant to have a constant feed of fuel about 1 - 2 psi over what the actual useable internal fuel pressure should be. The Nikki incorporates an internal fuel regulator, as simple in design as they come. It's nothing more than a bypass hole drilled in the internal plumbing through which all of the fuel going back to the tank via the return line must pass. I have found three diameters of return bypass orifice used throughout the Mazda Nikkis, according to dates manufactured. The larger the hole is, the lower the pressure of the fuel inside the carb, and visa versa. Indeed, the most trouble free manor in which to guaranty that the pressure inside the carburetor falls with a relatively narrow range according to specifications is to utilize a fuel pump with only a slightly higher pressure rating, and then bleed off the excess pressure using this simple bypass design for the fuel return line. While for a mass manufacturing application this "cookie-cut" design helps to eliminate the need for more precise fuel system metering, for performance applications where fuel demand goes beyond the specifications, the system as is simply cannot provide adequate flow or consistent pressure.

The problems with using the stock fuel delivery system for a performance carbureted application are three-fold; First, the stock fuel pump, even when new, simply cannot move the volume necessary to maintain the needed pressure for wide open throttle (WOT) fuel demand. The second problem is that even if it could produce enough volume and pressure, -or if it is replaced with an aftermarket pump that produces only slightly more volume than the maximum fuel needed at WOT, the internal regulator is not designed for the higher pressure. The orifice would have to be recalibrated to tolerate the higher pressure and volume flowing through the system. The third problem, most particularly important for track racers, is that even if the internal fuel bypass orifice were carefully recalibrated to tolerate and regulate the new higher volume & pressure, the variable range of pressure inside the carburetor would be much wider than the stock system, and with a modified Nikki carburetor, it would be enough to severely interfere with tuning on the track. The owner of a street performance application can tolerate a little bit of "slop" in the internal carburetor fuel pressure without much notice, but the track racer jet tuning in an effort to shave a few seconds of off lap times will be chasing his tail.

The stock fuel delivery system needn't be completely revamped in order to work with a performance application. All that is needed is an upgraded fuel pump, a fuel pressure regulator (FPR) and a reliable fuel gauge. Since the internal regulator of the Nikki carburetor is calibrated to maintain between 2.75 and 4.25 psi with a pump output of 5.5 psi, the internal bypass will allow for no pressure build-up while the performance Nikki is fed between 1.75 and 2.5 psi. This is the "theraputic range" of fuel pressure that should be maintained inside the performance Nikki. The pump must be up to the task of supplying the fuel necessary to maintain this pressure at WOT fuel demands, and the regulator is set between the pump and the carburetor fuel inlet. At this point, there are two ways to finish the plumbing; The return line can be left intact, running from the carburetor to the tank, or the carburetor can be "dead-headed". If the return line is kept hooked to the carburetor, then the pressure gauge can be ported off the third port of the FPR. This is very convenient, and the simplest (and therefore most common) set-up. However, the guage will then only read what the pressure is going to the carburetor, not what the actual pressure within the carburetor is. Chances are that if the FPR reads 3.5 psi, the internal carburetor fuel pressure is lower at WOT due to the return line being such a ready escape for fuel.

The option to dead-head the carburetor can be done two ways, each with the FPR running in-line from the tank to the carburetor inlet. The carburetor return line should be capped with the fuel pressure guage, and the third port on the FPR (depending on the model) can either be blocked off completely, or it can feed back into the return line. Capping the return line of the carburetor with the fuel pressure guage is a convenient way of getting a very accurate reading of the pressure going to the carburetor. I personally have had little luck completely dead-heading the fuel delivery system, and I choose to use the tank return line. If a pump such as the Carter 7lb is used, the return line to the tank may not be necessary. I say "may" because finding a fluctuation of greater than .75 psi throughout the power band while testing can be a concern for some and not for others. Any higher capacity pump used in the fuel delivery system should definitely make use of the tank return.

Fuel pressure in the performance Nikki carburetor should be kept as low as possible while still meeting the demands of maximum engine RPM & load. I'll state that again; Fuel pressure in the performance Nikki carburetor should be kept as low as possible while still meeting the demands of maximum engine RPM & load. Anything beyond that pressure only serves to overwhelm the emulsion system inside the carburetor and make it less effective than it's full potential. In the stock Nikki, a marginally performing emulsion system may present an indiscernible loss in performance, but the fuel consumption may be noticeably higher. This is due to the fact that the fuel is not being emulsified before the carburetor atomizes it, and the mist is not optimal. Consequently, the engine runs rich when it needs power, but doesn't always run leaner as it should for low load and cruising. There may be power in the mid to high range, but the low end will suffer. Correcting the fuel pressure will require a full jet tune up. This is why in the beginning stages of performance tuning larger jets should be used in order to find the correct fuel pressure. The pressure should be reduced incrementally between runs with no jet changes until the optimal pressure is found, which is the point at which if the fuel pressure is reduced by .25 psi or less, high end performance will suffer. Once the pressure is set, the emulsion system will be working optimally, and the carburetor is then ready for jet tuning. The pressure inside the carburetor will probably fall somewhere between 1.75 and 2.75 psi, but only if adequate fuel volume is supplied to maintain that pressure under the heaviest fuel demands.

Fuel gauges and regulators should never be skimped on when it comes time for purchase. Poorly calibrated gauges are not uncommon, and some can be off by 5% or more. But an un-calibrated gauge would still tell you a measurement for comparison from one adjustment to the next.The biggest problem is that gauges are also very inconsistent. Fuel itself is very inconsistent, so it stands to reason that measuring fuel pressure needs a bit of attention. Be sure that the temperature of the fuel and the gauge are the same. or as close as possible, to the last time it was read. Pretty difficult, I know, but necessary for serious tuning.