No Start:

Check ignition including plugs, plug wire routing, igniter wiring, etc. Check for spark & clean/change plugs. Check timing. If you never saw the engine run and do not know the history, the distributor may have been pulled and reinstalled incorrectly, supplying healthy spark, but at the wrong timing.

High Idle:

There is one or more vacuum leaks. Systematically try to track these down. Port plugging should be done with appropriate caps and not with make-shift caps using old bits of hose plugged with bolts. These do let small amounts of air in, and if many are used, can add up to a substantial leak. Do not re-route hoses from one port to another in an effort to cap both, as one port may be subject to a stronger vacuum signal, which will continue to draw in air.

All too often someone decides to remove the rat's nest (the network of steel hoses that control valves related to the auto-choke and emissions control), and instead of following a tutorial (or at least doing it systematically), they simply tear it all out and cap all the ports later. Invariably, they miss one, but because they did not follow the tubes & hoses, they have no idea where it is.

An entire picture essay of the rat's nest removal is available on mazspeed.com. It was written by two of the most knowledgeable forum & rotary community members known, and serves as a great troubleshooting guide for vacuum leak issues as well because these leaks are most likely associated with the rat's nest; removed or not.

There are, however, a few things associated directly with a carburetor rebuild that can cause vacuum leaks.

A comprehensive list of vacuum leak sources will be written to this page.

Power Loss:

The fuel filter is most often the culprit for high end power loss. A clogged fuel filter on a stock set-up 12a seems to have the thumbprint of consistently presenting itself as a power loss at between 5000 & 5500 RPM.

But that's not the only possible culprit. Other causes include linkage problems, fuel delivery issues, improper fuel pressure, and ignition problems.

There's a piece of linkage on the stock Nikki carburetor commonly referred to as the "teeter-totter". It seems that if this linkage slips off, it causes a power loss. Also, any linkage binding that may be causing the secondaries not to open will obviously cause a power loss up top.

The vacuum secondary throttle shaft is prompted by a lever attached to a large diaphragm in what's commonly referred to as the "vacuum secondary box". The circuit for this diaphragm box begins at an orifice at the apex inside the front rotor primary venturi. It follows a hole drilled all the way down through the main body, and at the bottom on the underside of the main body is a small channel cast around the front rotor secondary venturi. At the other end of the channel, another hole is drilled up through the main body, and then sideways out the backside where it finally terminates as a port where the vacuum box mounts to the main body. If the carburetor has gotten severely flooded and then sat for a long period of time (more than a year), then the channel at the bottom of the main body could be clogged with fuel varnish, keeping the vacuum signal the starts in the left primary venturi from ever reaching the secondary vacuum box.

If the vacuum box doesn't have a gasket installed in between it and the main body, a tiny amount of the prompting signal can be bled off, though usually not enough to make much difference. If the link that goes from the vacuum box lever to the secondary throttle shaft is not installed correctly, it will bind the shaft, not allowing it to fully open. The link should be installed with the bend facing upwards, towards the top of the carburetor. If the gasket between the main body and the throttle body is flipped around, then the channel won't be sealed, and the signal that's supposed to open the secondaries will be bled off. This does not present an idle vacuum leak.

Fuel pressure is also a possibility. Obviously you should check for adequate pressure, but too much pressure will over-rich the mixture, and you will lose power, especially up top. If the fuel level in the carburetor is too high (which can be caused by too much fuel pressure, among other things) the emulsion system will not function correctly, and the mixture will be over-rich. In order for the emulsion system to function as efficiently as possible, fuel pressure should be kept at the lowest pressure needed for the application, and the carburetor fuel level should be dead center of the sight glasses. This will make a big difference.

The other 'nasty" culprit for high end power loss is a misbehaving ignition system. Problems include old plugs, dirty plugs, the wrong plug gap, bad igniters, bad points,bad coils,  the wrong timing, spark loss due to arcing, and on & on. Ideally for carbureted performance, the 12a should be timed at 24* BTDC leading & 16* BTDC trailing, according to the Master, Paul Yaw. I will try to get more info on ignition soon, as it is not my forte.

Carburetor Float Level Diagnosis

Check the fuel bowl sight glasses on the sides of the carburetor. These are the best indication tools you have for what's going on inside the carburetor. The fuel level should always be dead center. The fuel level can be correct, too low, too high, uneven, even, and consistent or inconsistent. All of these behaviors are extremely important for diagnosing your carburetor problem, and it's normally the first carburetor-specific question helpers will ask.

Both Levels are Correct: Usually associated with a no-start situation or high idle.

Both Levels are Low: This is a fuel delivery issue, usually associated with a fuel flow impendence, plumbing or a fuel pump electrical problem.

Make sure the pump runs when you first try to start the engine. If it does not, check the fuse and the relay. If it's neither of these, it's either the ignition switch itself, the wiring to the pump, or the pump is bad.

If the pump does run, remove the fuel supply line to check fuel output. If there is adequate fuel coming out, then there is a blockage inside the carburetor fuel rail, or somehow your floats are pushed up, closing off the needle valves. If there is fuel flow, but it's inadequate, then you have either a blockage or crimp in the fuel supply line, a blockage in the tank, a plugged fuel filter, or a blockage in the tank ventilation line. If there is no fuel at all, you may have any combination of the previous scenario, or you may have the pump installed backwards. -You could be out of fuel, too. (It happens!)

Both levels are High: This has by far the longest list of possibilities and warrants a proper approach utilizing the process of elimination. There are well over a dozen variables that must be checked, in the proper order. Usually this does end up being a carburetor-specific issue, but not always. In either case, it's the symptom where most complications due to unnecessary adjustments come from.

Starting with the simplest, the float bowl vent solenoid (FBVS) must be hooked up for the carburetor to vent. If the carburetor cannot vent, fuel will often siphon at high demand, which continues even when demand falls lower. A disconnected FBVS will usually cause an very erratic fuel level, but usually both bowls will be effected the same way.

The FBVS is the electrical cylinder mounted to the front of the airhorn. On older models it is mounted by three screws and has a green ground wire attached to one of them. These can be easily removed along with a brass bung inside the air horn, and then a penny can be inserted into the vent hole. It will stop at a ledge inside, and then needs to be epoxied into place. However, on later models, the solenoid is screwed into a much different brass bung that's pressed into place. Unfortunately, the vent solenoid plunger is trapped inside the bung and it's extremely difficult to remove them. Trying and failing will result in a very nasty mess.

If the FBVS is retained, it must be hooked up. They all have a single black & white wire which needs to be hooked up to an ignition-switched 12V+ source. Fuel tank venting can also be attributed to erratic float bowl fuel levels, though not usually a high level.

The stock fuel pump should be able to handle "dead-heading", the elimination of the fuel return line, but without a fuel pressure regulator the pressure inside the carburetor will not be consistent. It can overcome the needle / seat assembly (actually a direct relation to the float buoyancy) sporadically and cause minor flooding during driving. Usually during idle a dead-headed stock Nikki with no regulator (running the stock pump) will not flood, but it can. This is pertinent because it is related to fuel return line impendence / blockage. A blocked return line would effectively be the same as dead-heading the carburetor. The return line hose has in it what's called a "restrictor" by the Work Shop manual. It's a small cylinder in between the two short lengths of hose, and it's actually a check valve, not a restrictor. This must be installed correctly in order for the return line to function.

The pressure should never go past 3.75 psi. Over pressurizing will surely flood the carburetor, and if the pressure is just at the crest of over pressurizing, the carburetor will alternate between behaving  fine and causing inconsistencies such as high end power loss, outright flooding, and a hunting idle that changes and / or dies. If you are running an after market pump, it should not be larger than 7 psi. A regulator will be a must, and even with the carburetor dead-headed, the rest of the fuel delivery system will have more consistent pressure if the regulator is plumbed to spit the fuel back into the tank. This is the best way to set up the fuel system for a Sterling Nikki. The regulator has consistent pressure going to it and doesn't get taxed so much. I have found (and read) that fuel pressure regulators tend to be a bit fragile. Blasting 15 psi at a 4 psi regulator is eventually going to yield an inconsistent pressure on the other side of it. It's just too much.

Another culprit can be clogged air jets on the top of the carburetor, though it's unusual for a carb to even run at all if air jets on both sides of the carburetor are plugged. Plugged air bleeds basically have the same impact as the FBVS not being hooked up, only on a smaller level; the system fails to vent, and can begin to siphon furl as a result.

Float adjustment should be the last thing you consider unless you've already gone and adjusted them. If you've just recently rebuilt the carburetor, then it's very probable that either the floats are binding on the needle / seat assemblies, or the assemblies themselves are binding. In either case, simply moving them up and down for a while over the range of motion can break in the parts. I also explain what can be done to the needles themselves to help with the break-in on my Nikki Rebuilding page. If you rebuilt the carburetor, be sure you didn't accidentally install the crush washers over top one or both of the originals, or leave them out altogether. If you look at distance the needle itself can travel, you'll realize how much of a difference this error can make. Float drop need not be excessive, and is not particularly crucial in a stock Nikki / stock 12a application. What is crucial is that they are both consistent.

Only One Level is Low, & the other is Correct: Most likely an internal problem with the carburetor. This is usually due to a misadjusted float, but can be due to a blockage in the fuel rail , especially if the level if it's inconsistent in the offending bowl. Both floats should be exactly the same when setting them. If they are, then there is some kind of needle binding issue. This can happen during a rebuild when the offending needle seat has been replaced. The original aluminum crush washers don't always fall out and I have found carburetors with two washers under one seat and one under the other seat, indicating that the builder was unaware the original washer never came out. This effectively lowers the float drop, limiting how far the needle valve can open, even though the float levels are the same.

Another possibility is a (the) weight(s) and / or check ball(s) are missing or not functioning in the accelerator pump. As a result, fuel may be siphoning into the carburetor through the accelerator pump circuit. The source of fuel for the AP is the right hand fuel bowl.

Only One Level is High, & the other is Correct: Most likely an internal problem with the carburetor. More than likely the cause is binding of the needle / seat assembly in the offending bowl. Usually a good smack over the fuel bowl with a plastic mallet (not a rubber one) or the plastic handle of a screwdriver will shock it back into place. It may not seat properly and you will have to go through that again several times before it finally "breaks in". Needle & seat binding trouble in a fresh rebuild is usually a result of friction between the new parts due to the microscopic matte finish of the needles and the microscopic machine finish of the insides of the seats. I demonstrate how to help facilitate the normal break-in of these parts during a rebuild in the Nikki Rebuilding section of my site.

It could be, of course, that the float is completely out of whack, and needs adjusting, but most likely not if it's rest & drop measurements are dead even with the other float. It's possible, just as the scenario above, that the needle did not get a crush washer installed underneath it during a recent rebuild. If that were the case, and the float was adjusted to match the other, then the fuel level would be consistently higher.

If there is an inconsistency associated with the high fuel level of only one bowl, then there can be a clogged emulsion tube or air bleed. These can cause siphoning at high demand, but the siphoning doesn't stop once it starts, so you might have reasonable fuel levels in both bowls, drive the car, and then find one bowl full. Carefully (gently because they are soft brass) remove the suspected jets and clean with carb cleaner. Do NOT use metal drill bits or steel wire to ream them out because they are precise sizes.

"Follow the Fuel" Carburetor Diagnosis

We can often locate the source of our carburetor woes simply by following the fuel...

Obviously it starts off in the tank. The fuel can't leave the tank at any effective consistent rate without a vent of some sort. Faulty tank ventilation can cause a lean condition in the mid RPM range to present itself rather suddenly, even though you started the trip with power all through the band. This is because at first, there may be an actual over-pressurization occurring within the fuel tank, so the carburetor is getting adequate fuel (the over pressurization being too little to overwhelm the carburetor's needle valves). But when the fuel leaving the tank starts to create a vacuum, the fuel supply is cut down to a trickle, and the carburetor may starve for fuel.

Even within the carburetor, there needs to ventilation for the fuel to flow into the main circuit. The result of faulty carburetor ventilation is an erratic fuel level in the fuel bowls, resulting in everything from a failing idle to outright flooding.

The tank vents through a line that runs parallel with the fuel delivery and return lines to the engine bay, and terminates into one of 3 different charcoal canister designs. The charcoal canister absorbs and traps the fuel vapor that comes out of the line while the car is not running, or the engine is at idle. Even when the car is just sitting, the fuel tank and system can become warm, and the gasoline vapors would cause the tank to expand if it were not vented. The charcoal canister is ported to the carburetor air horn or air filter unit, and as the engine warms up, the fuel trapped within the canister vaporizes again, and gets vacuumed up into the engine.

If the canister becomes saturated with water, it will fail to work and can effectively plug the fuel tank vent.

The fuel flows from the tank through a plastic safety rollover fuel-cut unit. These rarely cause any trouble, and they are best left in place. A check ball inside shuts off fuel flow if the car rolls over on it's side or top.

Next is the fuel filter. A clogged fuel filter will present itself as a loss of power at about 5500 RPMs. This magical number seems to be when power demand opens the vacuum secondaries, and the demand for fuel increases. nearly every case I've ever read about where changing the fuel filter remedied a power loss problem on a stock Nikki carburetor, the power loss always occurred at between 5000 and 5500 RPM. If you experience a high end power loss that is not remedied by changing the fuel filter, and it's later determined that you do not have a fuel pressure / delivery problem, it is most likely attributed to one of several linkage and non-fuel related carburetor issues explained in the paragraphs under 'Power Loss' above.

Next is the fuel pump. The stock fuel pump is a piston design, and when new, it put out about 5 psi. They do wear out, and they can put out the same volume when worn, but not the same pressure. So if there's no resistance, the old stock pump will deliver adequate fuel for a stock set up. But if you're running a performance carburetor or modified Nikki that demands more fuel on tap, the stock pump will disappoint you.

Rarely does the stock pump outright quit. But you can tell if either it, or the relay are not working by listening when you first turn the key to the accessory position. If the pump doesn't run, then you have an electrical issue.

The line running from the tank to the carb should be adequate for most performance carburetors, provided they have a strong fuel pump. Upgrading the hose to a larger diameter won't increase fuel delivery to your carburetor, but what it will do is help keep the pressure consistent. Of course, it's expected that your fuel pressure regulator will take care of that job, but every little bit helps.

As the fuel feeds into the fuel rail on the carburetor, it flows right back to the fuel tank via the fuel return line. Inside the fuel rail is a tiny orifice which provides impedance to the flow of fuel exiting the fuel rail into the return line. This is the Nikki carburetor's internal fuel pressure regulator. There are three different size orifices used between 1974 and 1985 Mazda Nikkis, each corresponding to a different fuel pressure specified for that model. The smaller the hole, the higher the fuel pressure is within the carburetor.

While the fuel rail is for the most part trouble free, there are screens inside the banjo fittings over top each fuel bowl. It takes quite a bit of rust and/or varnish to clog these screen to the point where fuel delivery to the carburetor is impeded, but it does happen. Of course, if they are that bad, then chances are the rest of the components on the carburetor are already begging for a rebuild.

Once the fuel is screened in the fuel rail, it can enter the needle seat assembly. Here, too, is a screen over top of each seat, though of much finer mesh. The fuel fills up the bowl and the float rises and closes the valve. As fuel is used, the float drops down, allowing the needle to open and the bowl to fill to the same level before the supply is cut off. This happens constantly, and it's really only at idle that the floats are closed for any appreciable amount of time.

I mentioned earlier that the float bowls need ventilation just as the fuel tank does. But leaving the carburetor floats vented to atmosphere presents both an emissions and a safety hazard, so a special solenoid is utilized on the Nikki carburetor that effectively seals the carburetor float bowls when the engine is off. This solenoid is called the Float Bowl ventilation Solenoid (FBVS).

The FBVS is the electrical cylinder mounted to the front of the airhorn. On older models it is mounted by three screws and has a green ground wire attached to one of them. These can be easily removed along with a brass bung inside the air horn, and then a penny can be inserted into the vent hole. It will stop at a ledge inside, and then needs to be epoxied into place. However, on later models, the solenoid is screwed into a much different brass bung that's pressed into place. Unfortunately, the vent solenoid plunger is trapped inside the bung and it's extremely difficult to remove them. Trying and failing will result in a very nasty mess.

If the FBVS is retained, it must be hooked up. They all have a single black & white wire which needs to be hooked up to an ignition-switched 12V+ source.

Fuel tank venting can also be attributed to erratic float bowl fuel levels, though not usually a high level.

The return line from the carburetor back to the tank maintains a steady flowing "stream" of fuel for the carburetor to draw from. It can be closed off, or "dead-headed", but there's no performance gain by doing this with a stock Nikki in a stock 12a application, and can actually cause more problems. In the return line hose that runs from the carburetor to the metal tank return line is what's called a "restrictor" by the Work Shop manual. It's a small cylinder in between the two short lengths of hose, and it's actually a check valve, not a restrictor. This must be installed correctly in order for the return line to function.

If the fuel delivery system passes the "Follow the Fuel" testing, then it's time to look at the carburetor itself. The manifold is considered to be part of the carburetor with regards to troubleshooting high idle problems as well as high end leaning issues. The manifold contains all sorts of components that should be properly sealed if and when they are removed. Simply removing extraneous components and jamming bolts into the hoses never works out very well. The threads on screws cause vacuum leaks, thye hoses are not pliable anymore and may be leaking at the junction to the carburetor, and ports that need to be capped get overlooked.

The carburetor can be misadjusted in many ways to cause vacuum leaking, but unless it's been freshly rebuilt, I strongly suggest that no adjustments other than the idle be made until all other variables have been eliminated.

Vacuum leak causes can also come from bent or binding linkage, cables not having any slack in them, and a handful of things associated with the throttle shafts. At that point, it is advised that you seek the advice of those on my forum, and it will be best if you have photos at hand, ready to post.

Vacuum Leak Source Diagnosis

All ports on the ported spacer in between the carburetor and the intake manifold are potential vacuum leak sources, as well as all ports on the intake manifold and the throttle body. The intake manifold is complicated on later model Rx-7s due to an anti-backfiring system that utilizes manifold vacuum to prompt the closing of a valve inside that blocks of the rear primary runner. These things are simply a real pain in the ass, and can be difficult and frustrating to remove and correctly seal. Problems arise if the vacuum pot that controls the valve shaft is removed but the valve is left in place. It must then be wired in the open position to ensure it does not close. The valve, even in the open position, obviously presents a large air flow impediment, and it is suggested it be removed in it's entirety.

The two worst ways to cap vacuum ports is by putting screws or bolts into the ends of the hoses, or to reroute the hoses to other ports that need to be capped. Though the threads on screws and bolts often allow for only a minuscule vacuum leak, the practice of this method over several other ports can add up to a substantial leak overall. As for rerouting hoses to other ports, the port the hose has been rerouted to may have a substantially higher vacuum signal than the other port. Consequently, the other port succumbs and provides a vacuum leak source for the one of higher signal. Further complicating these issues are that the port with the stronger vacuum signal is more likely to develop the strength of that signal as engine RPM or load increases, so the resultant symptoms can have you chasing your tail trying to pinpoint the problem.

A common idle problem culprit often overlooked is the Richer Circuit. The Nikki carburetor Richer Circuit, usually found on Rx7s with automatic transmissions, is a bit complex, as one can tell simply by examining the casting on the exterior of the main body. The circuit is designed to enrich the fuel mixture when the transmission is removed from the neutral position. Just as you need to give fuel to the engine when shifting out of neutral in a manual transmission, the automatic transmission contains a converter that also requires a more power to keep running. The richer circuit supplies this enrichment automatically with the use of an electric solenoid connected to a switch on the transmission. When the solenoid opens, the circuit is exposed to vacuum and a metered amount of fuel is provided to the rear rotor via a port in the throttle body.

Keep in mind that the richer solenoid and the top portion of the circuit are all located on the secondary end (back side) of the carburetor. The circuit then follows through a port in the throttle body that is drilled straight down, between the secondary bores, and the horizontally and diagonally, to the rear rotor primary bore. At the top of this circuit, on the main body, it terminates in five places; the richer solenoid bung, a capped port directly over the richer solenoid, inside the back lip of the main body, in which there is a large brass weight; a parallel port milled right beside it, in which a long air bleed jet is screwed; and also a port that leads from there towards the rear rotor fuel bowl and branches off into two separate ports, one terminating at the top of the fuel bowl above the level of the fuel, and one terminating low in the fuel bowl, submerged in fuel.

This complex circuit is probably the most usual cause for never-ending high idle frustration. Having packed the richer solenoid bung full of epoxy, as well as the two ports on the top of the secondary end outer lip of the main body, most people are certain they have effectively sealed this circuit. Many un unclear whether it can even present a vacuum leak issue. But the problem is that three of the six ends of this maze of a circuit are not blocked. The internal port that allows the presence of the vacuum signal, located inside the rear rotor primary bore, and the two ports in the fuel bowl. The port in the fuel bowl that is well above the level of fuel will be the actual source of the vacuum leak. Since the fuel bowls are not sealed off, and the fuel bowl vent is plenty large enough to accommodate such a leak, it is unlikely that the atmospheric pressure acting on the fuel itself within the carburetor is subject to change even at high engine RPM or load. However, the vacuum signal within the richer circuit can get high enough to begin to draw fuel into the circuit from the port that terminates at the bottom of the fuel bowl and is submerged in fuel. With the rest of the circuit sealed, an outright siphoning can occur. Obviously, this is another of those "difficult to pin-point" troubles that can cause everything from the leaning of the rear rotor to stumbling from flooding at high RPMs which presents itself as power loss and is often misdiagnosed as running lean.

There is no reason to disassemble the richer solenoid circuit unless the carburetor has been sitting for some time and needs a cleaning and a rebuild. If the richer solenoid is not installed correctly, some of the aforementioned problems can occur, less the high end flooding. The richer solenoid should be assembled with the spring and piston and reinstalled first, followed by the other richer circuit components.

The surest way to block off the richer circuit is to disassemble the main body from the throttle body and plug the large round port located between the secondary bores, on the outside edge. The top portions of the circuit will still need to be blocked to keep fuel contained within the carburetor.

Two circuits that will not present themselves as idle vacuum leaks are the subzero starting assist (SSA) circuit and the vacuum secondary circuit. The vacuum secondary throttle shaft is prompted by a lever attached to a large diaphragm in what's commonly referred to as the "vacuum secondary box". The circuit for this diaphragm box begins at an orifice at the apex inside the front rotor primary venturi. It follows a hole drilled all the way down through the main body, and at the bottom on the underside of the main body is a small channel cast around the front rotor secondary venturi. At the other end of the channel, another hole is drilled up through the main body, and then sideways out the backside where it finally terminates as a port where the vacuum box mounts to the main body. Although if the gasket in between the throttle body and the main body is reversed, the circuit will leak causing the secondary throttle shaft not to open, since it goes nowhere near the idle circuit, it will not present itself as an idle vacuum leak.

The SSA circuit is a very simple one. It begins at the rather large brass nipple located directly in the center of the front of the main body, in between and beneath the OMP input lines. This was an idea proposed to help with carburetor icing, a problem more likely to inhibit engine starting on Mars, or at least Siberia. ( It didn't work all that well! ) Not all Mazda spec Nikkis have the SSA nipple, and they have a brass block-off bolt in place instead. The circuit is drilled straight in for about an inch, and then goes straight up. It terminates in the back end of the accelerator pump nozzle. It poses no idle vacuum leak threat, and the nipple even contains a check ball, so it won't let anything in unless it's pumped in anyway.

Flooding Issue Diagnosis

There are enough causes of flooding in the Nikki carburetor that it's best to first diagnose the type of flooding you have before you start looking for the cause. Basically, for our purposes, there are only a handful of variables we need concern ourselves with while categorizing flooding; one bowl vs. both, inconsistent vs. consistent, and if it changes with engine RPM / load. Flooding can be anything from an unusually high fuel level that's only slightly higher than the half way point in the sight glass to fuel outright spilling from the carburetor. Even if it's slight, if the level changed and you didn't adjust the floats, then something's wrong.

Very generally speaking, the following rules seem to apply, so they can help guide you in the right direction;

One bowl floods:

If the flooding is only in one bowl, during idle, it is likely a float, needle and seat assembly issue. The float may simply not be seating properly for some reason.  Often this is a temporary situation that can be remedied by a few whacks over the offending needle valve with a hard plastic handle of a screwdriver.

If the flooding is only happening in one bowl, but only after a drive where heavy engine load or higher RPMs were experienced, then it may be a clogged air jet or emulsion tube. This will cause siphoning. Check all of the jets in the top of the carburetor associated with that main circuit (that same side) for debris.

Both bowls flood:

If both bowls flood at idle, then it's most likely one of three problems; The fuel bowl ventilation solenoid (FBVS) is not hooked up, the needles are not seating, which is a usual occurrence after a fresh rebuild with new parts, or your fuel is over pressurized, which can be caused by incorrect fuel plumbing, an oversized pump, a failing or misadjusted regulator, or incorrect dead-heading of the carburetor (again, basically incorrect plumbing).

Misbehaving new needle & seat assemblies often need that shock from a good smack or two to get themselves set correctly. This may be required several times, but there's a tip on how to help facilitate this break-in on my rebuilding page.

The pressure needed for a stock Nikki in a stock 12a application need not ever exceed about 3.75 psi, though I believe some Nikki carburetors' internal regulators were speced at 4.25 psi. The fact of the matter regarding fuel pressure is that running the least pressure you need, determined by the lowest pressure that will still deliver enough fuel for your set-up at WOT (wide open throttle), will allow your emulsions system to work as efficiently as it can, which will yield you the most power and best fuel economy. Many Sterling Nikki owners have their pressure set at as low as 2.25 psi for auto cross applications, and 2.75 for track applications.

If the fuel plumbing is not done correctly, the regulator cannot do it's job properly. Venting the excess pressure via the return line is the best way to ensure a consistent pressure to the carburetor under all demands. Dead-heading the carburetor, that is, running it without a return line, is usually not the best for the fuel pump (though most are designed for that capability). It's far easier to incorrectly plumb the fuel delivery system if you are dead-heading the carburetor.

Inconsistent flooding:

Mostly this can be attributed to a ventilation problem. There are three ventilation systems to examine; the fuel tank, the internal carburetor fuel delivery system, and the main circuits' ventilation.

The fuel tank has three lines running from it to the engine bay. One is the fuel supply line, one is the return line, and one is simply a vent that terminates at the charcoal canister designed to capture and condense escaping fuel tank vapors when the engine is off. If the charcoal canister system is not functioning correctly, or there is a blockage in the tank vent line, the tank will obviously fail to vent. Usually this will cause a low-delivery situation to the carburetor, presenting itself as a low fuel level in the carb during higher fuel demand, but in rare cases at idle there can also be sporadic surges of fuel coming from the tank, depending on the problem.

The most likely culprit is the FBVS. They rarely malfunction; in fact, hardly ever. But if is not hooked up to a ignition switched12V+ source, this will keep the carburetor's internal fuel delivery system sealed. Fuel level behavior is erratic, but usually the same for both bowls, the only complication being perhaps a needle / seat assembly that is also misbehaving either from a fresh rebuild or as a result of the flooding itself. The FBVS is the electrical cylinder mounted to the front of the airhorn. On older models it is mounted by three screws and has a green ground wire attached to one of them. These can be easily removed along with a brass bung inside the air horn, and then a penny can be inserted into the vent hole. It will stop at a ledge inside, and then needs to be epoxied into place. However, on later models, the solenoid is screwed into a much different brass bung that's pressed into place. Unfortunately, the vent solenoid plunger is trapped inside the bung and it's extremely difficult to remove them. Trying and failing will result in a very nasty mess.

The next most likely cause of flooding inconsistently is one or more clogged air bleeds associated with the offending bowls. If the primary emulsion jets are clogged (and to a lesser degree, the secondary emulsion jets), then the carburetor can flood when the engine is revved. There may also be dripping of fuel from the fuel pipe associated with the offending circuit. Dirty jets can even cause a low or no idle problem. Jets should be cleaned carefully and handled delicately. They are made of soft brass, and a small scratch inside the orifice is large relative to the jet size itself. The emulsion tubes are also rather thin, and soldered together.

Flooding Oddities:

If the carburetor seems to hemorrhage fuel for no apparent reason, it's usually a fuel delivery system issue (over pressurization). If the carburetor drips fuel consistently out from the accelerator pump (AP) nozzle, then one or more of the components of the AP is damaged, missing, or installed incorrectly, and the draft through the carburetor is siphoning fuel through the AP. The fuel source for the AP is the fuel bowl to which the AP housing is attached.

Things that are NOT idle issue related...

Two things immediately come to mind here; The sub-zero assist nozzle, and the idle compensation circuit.

The sub-zero assist nozzle does create a vacuum leak within the carburetor that effects idle. It's the brass nozzle located in the center of the front of the main body of the carb, just below the oil feed line. The other end of that circuit terminates at the back of the AP nozzle, well above anything that could effect the idle.

The idle compensation circuit is tied in with the emulsion venting circuit, and should not be completely blocked. On very early nikkis, the emulsion system is vented utilizing the idle step bleed and transition circuitry, but as the model years progress, so does the complexity of both the idle altitude compensation circuit and the emulsion vent. If the idle compensation valve is removed, simply for the sake of keeping unfiltered air out of the carburetor, the external ports should be blocked, and any associated tubes (either one or two, depending on the year of the carb) should be capped. The jet under the rear lip of the air horn, however, should remain open to vent, as in some cases, it may serve as the only emulsion system ventilation. Without promer ventilation to the emulsion system inside the carb, a variety of problems will plague the idle, including inconsistencies due to fuel pressure and temperature changes, and shutting off by itself.

Special Notes...

The "Teetor-Tottor"

There has been a lot of misinformation surrounding this bit of linkage, simply due to the ignorance of what it's function really is. It's most commonly blamed for lack of power at WOT, and in some cases, a lack of throttle response. In fact, it has nothing to do with either, the former usually being caused by a fuel delivery problem or a blocked secondary vacuum circuit, and the latter almost always caused by a faulty accelerator pump. The most common falicy is that if the shaft nut falls out of rocker on the "teetor - totter" it causes the secondary throttle linkage to lock, presumably as some sort of safety feature.

The actual function of this extraneous piece of linkage is simply to provide the ability to fine tune the tension of the throttle return spring. The reason this is necessary is that the vacuum or spring  forces acting on various components that make direct contact with the primary throttle shaft need a means of ensuring consistency from carburetor to carburetor. The throttle return springs may be from different sources, or wear out over time, but the vacuum measurement form the system that pulls the AC idle compensation valve, for example, are all about the same, even over time.

Backfiring

The rotary exhaust runs inherently rich, and with the fairly broad  RPM band, particularly with a manual transmission, backfiring is to be expected. There are a few extraneous components, both on the stock carburetor as well as the intake manifold, designed to minimize or eliminate backfiring, particularly upon deceleration. When the entire emissions control system is removed, however, some of the key components designed to minimize backfiring are are also eliminated. Ultimately on the modified Nikki carburetor, the only component left that can help minimize backfiring is the deceleration dashpot.

The deceleration dashpot is a fairly precisely manufactured component, and my guess is that they are all pretty consistent. It contains a rubber bellows filled with oil and an internal baffle with a small orifice(s). The oil is forced through the orifice as the piston on the dashpot is closed, slowing the closing of the throttle shaft. The oil is thicker at cold temperature, so adjusting the deceleration dashpot should be done with the engine at running temperature.

I hope this has been helpful, and if you have any questions, comments or suggestions, please do not hesitate to let me know on my forum. I will continue to update this and other sections of my site with new information that you all share with me.

Copyright © 2009 Dennis Williams, Sterling Metal Works. All rights reserved.

gorealfast@sterlingmetalworks.com