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Because this section is such an important resource, I've chosen to publish it even though it's still in it's rough draft. I'll chip away at cleaning it up. Last update: 3/08/09
So you've decided to finally rebuild it, huh?
Good for you. It can be very intimidating at
first, but it's really not that
difficult. Even though I break down the rebuild in stages, it's best to read this section in it's entirety, as I list some little tricks to get you out of trouble. You also need to understand that even if you've done everything correctly, the carburetor may still take some coaxing, soft gentle caring words of encouragement and a few smacks to get working. But when all is finished and you're back behind the wheel, you'll swear your car has at least 15 more horsepower - 10 from the rebuild, and 5 because you're the one who did it! The satisfaction is great.
Before you start...
Plan this out. Set up
a workspace that's clean, well lit, and out of
the reach of tiny, curious fingers. I have a
white Formica workbench, which is great
because it makes all the parts easy to see.
I've tried news paper on a table top in the
past, and the newsprint makes it very easy for
check balls and weights to get lost. I suggest
white meat packing paper if you can get it, or
a light colored sheet if you can sacrifice
one. If you're using a sheet or cover on the
table/bench, don't let it hang down where it
could accidentally get snagged and yank all
your organized carb parts into a big pile.
(It's that law of averages, folks. It would
probably never happen unless you had open-carb
surgery going on!) Beware of a home made wooden work bench which is often a series of planks put together that has cracks that can gobble up check balls and tiny weights.
Use containers with lids for tiny parts. A
block of 2x4 serves well to set the throttle
body down on. This way the weight of the
carburetor won't be resting on the linkages.
Bending a throttle shaft is a real
no-no! Buy the kit first. Don't have your carburetor strewn about the work bench in pieces waiting for a rebuild kit. I used to suggest Hygrade kits, but I now recommend the GP Sorenson kits. These are available online from Autozone, and they have two available for the Nikki. Both will work, but the more expensive one contains replacement check balls for the accelerator pump, while the cheaper one doesn't. This is not detailed in the product description, and I was disappointed after believing I had saved money on a shipment of ten kits! The better GP Sorenson kits cost $27 and change, and can be shipped to your door in two days.
Have a notebook handy during disassembly and
take notes, no matter how stupid or crappy the
drawings are. Snapping a few shots with your
digital camera is good too, but I believe that
the effort of having drawn or written
something to paper does something to my brain
to cause me to better remember the
process/subject later on. Often, I would not
even have to look at my notes afterwards, but
had I not bothered to take them, I surely
would've forgotten. Study the carburetor really well first, making note of all the tools you'll need. Roughly, you'll need the following;
Optional tools:
Removing the Nikki-
Either all of the emissions components work, or the whole emission system does not work at all. Even if all the individual components do work, a small vacuum leak in the system can cause idle problems. If you need the system working, you should inspect all hoses for blockage and cracks. I will be adding a whole separate section to my site eventually just for inspecting & troubleshooting the emissions system. But for now I'm focusing mostly on the carburetor due to the fact that I estimate that about 80% of the people seeking advice here don't need to retain their stock emissions control system, and most of them have already removed it. A great emissions system removal tutorial is outlined here , on the Mazspeed website. You should go ahead and read that before removing the carburetor. Most of the hoses and wires going to the carb from the engine are pretty straight forward as to where they go; the exception being what goes to the ports on the base plate. Since there's no reason to remove the base plate (other than cleaning it), the hoses to it should be left connected. If you do need to remove the plate for cleaning, etc, do NOT try to remove the gasket. They are adhered to the plate. Also k eep in mind while cleaning & inspecting the hoses & wires that they have been exposed to over 20 years of temperature cycle and oil. Consequently, they crack very easily, particularly the insulation on the wires.
The fuel rail is bolted onto the carburetor very tightly. Before removing the carb from the engine, take a 14mm box end wrench and crack the two banjo bolts that hold the fuel pipes on. These will need to be removed later to inspect & clean the screens inside. Complete removal of the fuel rail is optional, and should depend on how dirty the rest of the inside of the carburetor is. You'll have to make an educated guess as to whether or not you should remove it completely if you want to take advantage of the carburetor being anchored well. It can be very difficult to remove the assembly from the carb on the work bench because the bolts are on so tightly. With the exception of the air horn screws, the rest of the carburetor bolts aren't so bad. The least accessible is one of the nuts that hold the carb to the manifold. If you happen to have an extra cheap wrench you can sacrifice for this, putting a 45* bend in the open end of a 14mm wrench with a torch will make a tool that can get in there easily. If you can't make that sacrifice, use patience so as not to round off the corners on the nut(s). Once the carburetor is removed, be attentive to how you set it down. Ideally, no weight should be placed on the linkage from the bottom, and certainly not any force (from an accidental drop of even a few inches) as primary throttle shaft has circlip grooves milled into it, making for some fragile bending points. If the primary shaft bends more than about 3 or 4 degrees, it will crack, and then there's no bending it back straight without breaking it. (Ask me how I know!) Don't forget to place a towel over the open manifold on the engine.
Cleaning the exterior-
After removing the carb, it needs to be
cleaned before disassembly otherwise grit and
gunk will find its way into the carb and cause
all kinds of problems later
on. Aerosol carburetor cleaner is great, but should be used sparingly just for really stubborn areas, otherwise you'll need so much of it that it will become an unnecessary expense. Besides, it will give you a headache, ultimately end up in at least one of your eyes, -at least once (even if you're wearing goggles, it seems!), and is just generally evil stuff that causes cancer. -By no means am I "Captain Health", but I'll have to wonder if my use of such products are not the cause of cancer should I be diagnosed with it 20 years from now. Obviously the use of these chemicals should be done in a well ventilated place, such as the garage with the door open, and in case you've forgotten about physics, chemistry, and the law of averages, NO SMOKING! If you would rather use something more environmentally friendly, you can use Simple Green to clean carb parts, but you have to rinse them off very well in hot water. Simple Green is a great degreaser, but it contains ammonia salts and will etch aluminum, which the air horn is made of: zinc. which is a primary ingredient in the pot-metal main body casting, and even the glass bowl sights! It will also oxidize the brass jets & pipes. So do NOT soak the carb or parts for any length of time. And don't use Simple Green straight, either. "Royal Purple" contains the same active ingredients as Simple Green. A few toothbrushes with hard bristles are good to have. Apply heat to the neck of a brush just before the bristles, and bend the bristle side out at 45*. This makes a great tool that will even scrub the bottom of the float bowls. (If you try to curve just the tip with the bristles, they'll fall out.) Definitely do not use a steel brush, and even a brass brush should be avoided unless completely necessary. Be aware of stray bristles finding their way into the carburetor.
Disassembly-
The disassembly can be done from two perspectives, depending on your needs. If you need the carburetor reassembled in it's entirety, then it can be broken down into four basic components, the idea being to disassemble the least amount necessary during the rebuild. You can then address each of the four sections individually and further break them down to replace what's needed. You may have chosen to remove all of your emissions components, or perhaps you want to toss the auto-choke. In either case, the approach to disassembly is more component-specific. Because the screws used on the Nikki are fairly soft but usually very tight, removing unwanted components is best done while the carburetor is all together because you'll have better leverage for bearing down on the screw driver. Once the extraneous components have been removed, the rest of the disassembly can still be broken down into the same four sub-assemblies.
The four sub-assemblies are the auto-choke and cable bracket assembly, the air horn, the main body, and the throttle body assembly.
Removing the auto-choke / cable bracket assembly:
The idea here is to remove the throttle cable bracket and auto-choke from the carburetor main body, but not from each other. It just cuts down on the number of parts to reassemble. The AC idle comp valve will stay on the bracket assembly (if equipped), but the deceleration dashpot and EGR valve will come off by themselves. The altitude idle comp valve hose will need to be disconnected from the valve and the throttle body (at the other end). The auto-choke vacuum line will also need to be disconnected from the throttle body. If more cleaning needs to be done to the exterior now that the bracket is removed, it should happen before the air horn is removed.
Removing the air horn / float assembly:
First remove the two banjo bolts that hold the fuel rail ends to the tops of each fuel bowl. Inside you'll find small brass screens. Unless the carburetor sat dormant out in the elements for a long while, these are not usually very dirty. If they are dirty or corroded, then that is an indication that whole fuel rail should be removed and cleaned. If the screens are clean, and the rail was not leaking, it's not necessary to remove the rail from the air horn. The screens often get distorted when they are removed. Just slide them around a pen or something similar and roll the sides out flat with your thumb. Remove the tiny cotter pin from the fast idle linkage that attached to the choke shaft assembly. There should be two tiny washers that come off, also. Remove the center bolt and the seven screws that hold the air horn on the carburetor. The gasket is most likely frozen to the parts, so a gentle tap with a screw driver handle or piece of wood may be necessary to break it free. Lift it up and out of the main body carefully so as not damage the floats. The floats are plastic, but the hinges and tabs are soft metal. Be careful while flipping the air horn assembly over that the float hinge pins don't fall out. It should never be set down on the floats.
Rebuilding the air horn assembly:
Just for an immediate sense of accomplishment, go ahead and rebuild the air horn. It's really the best time to do it because your work space is uncluttered, and still nice and clean. With the air horn upside down, remove the pins that hold the floats in. As you lift out the floats, and the needle assemblies will come out with them because the retaining clips will be hanging on the tabs. Set the floats, hinge pins, and needle assemblies aside, preferably in a container. Don't throw any parts that you'll be replacing away just yet. Remove the gasket. If you need to do any removal of stubborn gasket material or gunk from the air horn, keep in mind that it is cast aluminum and very soft. Any scraping with a razor, etc, should be done with great care.
The brass needle seats have four cross drilled holes in them, as well as the slotted edges for a removal tool. The actual tool used to remove these is not your standard "big boy" slotted screw driver. It doesn't really matter if you mangle these, as they are being replaced anyway. But, they do tend to be a bit stuck, much like those air horn screws you just fought with, and there are some tricks to ease them out. Mashing the tops with a screw driver does pose the risk of leaving tiny brass flecks around, which could ultimately wind up inside your freshly rebuilt carburetor. You can use the shank end of a drill bit to remove (and install) the needle seats by inserting through the cross-drilled holes and turning. You can also use a small metal rule on the slot on the top, if it's thick & strong enough. A good sideways smack with the screw driver handle will usually help loosen the seats. Be certain to remove the old aluminum washer under each of them, and install new ones when reinstalling. If this is over looked and you wind up with a doubled up washer, it will effectively lower the float drop for that needle, and you'll run into problems later on. When you remove the seats, they'll either be plastic conical or metal cylindrical screens fitted to the tops at the fuel inlets. These need to be carefully removed and refitted to the replacement seats. Installing the needle assembly is easier than removing the old ones, but make sure they're tight.
Install the new gasket. Pay attention to how it lines up. With the exception of very early Rx-7 Nikkis and all previous Nikkis, the air horn gasket needs to go on one way. Before installing the needles, here's a tip that can help you avoid needle the binding that causes the flooding that so often follows a fresh rebuild; The seats have very small, nearly microscopic concentric tooling marks on the inside. Again, on a microscopic level, this makes for a very rough surface that's not conducive to having another brass part slide easily across it's surface. While there's not much we can do about the inside of the needle seats, we can do something to the needles themselves to decrease the friction as they move in and out of the seats. The seats are not polished inside, but the surface is at least consisten t, and there is no way to polish the insides to that same consistency, even with my watchmakers lathe. Please do not try. While polishing the insides of the cylindrical needle seats sounds like a great idea, it's a fruitless, messy effort that won't yield results worth the time. They can even end up sticking worse. But what you can do that's very quick, easy & effective, is burnish the four corners of each needle. The needles are "tumble polished" after they are spit out of a manufacturing machine, and as a result they have a matt finish. This is where the friction comes from. Coat the outside of the needle in about a finger-print's worth of oil, and take something small and highly polished and lightly rub it along the shaft on each corner very lightly. You should start to see the corner get brighter. This is burnishing the surface, and it's the same as "breaking in" the part. Try not to rub hard as that can cause tiny gouges in the surface, defeating what your trying to accomplish. All in all, this process should need more than about 4 or 5 minutes, but it can save an hour or more of aggravation later on.
Assemble the needle, springs, pins, and retaining clips just the same way as the old ones, and drop them into the seats. Turn the clips so that they both face the same direction, and install the floats so that the tabs are in between the pin heads and the clips. Be sure that the floats are the correct ones for their respective bowls. With the exception of pre-'79 Nikkis, there is an indentation cast into the sides of each float for carbs with richer valves installed in the bowls. With the air horn upside down, the top front edges of the floats should be about 1/16 of an inch above parallel with the gasket on the air horn. If they raise too high in the float bowls when closing the needle valves, the fuel level will be high, and you won't be using the emulsion system to it's full potential. Fuel mileage will suffer noticeably from having a float bowl fuel level that's too high. If it's set too low, then high end mixture will be lean. When flipping it over ( careful that those float pins don't slide out! ) the floats should drop down equally. The distance they drop down is not nearly as critical as them dropping down equally, as well as pulling on the needle assemble retaining clips enough to open the needles. Do NOT start readjusting the floats unless you absolutely have to! Of all the vital components within the Nikki carburetor, I have to admit, they really skimped on the floats. They are not high quality parts, and they can take only so much tweaking. The float level is adjusted by bending the tabs that make contact with the needle valve pins. Float drop is adjusted by bending the other tabs that rest against the air horn mounting/pivot casting.
You can breathe easy as you've just completed the "surgery" part. The rest is very straight forward. Remove the aluminum crush washers that are more than likely stuck to the heads of the fuel rail banjo bolts. There should be four washers total; one on the top and bottom of each rail banjo. Replace these with the new ones, being careful to choose the correct ones. It's best to finger-tighten these now and wait until the carb is assembled and on the engine before using a wrench on them. If you've decided to remove the fuel rail assembly, there are replacement aluminum washers for each junction.
Separating the main body from the throttle body:
Remove the accelerator pump linkage. There is a pin through it that holds the lever in place, along with two very thin washers that I'm certain are only designed to be a gigantic pain in the ass. (You'll see!). On the pin is a tiny circlip retainer which you must not lose because the replacement clip in the rebuild kit seems to be the wrong size. The pin will only slide out once the lower left cover screw is removed. Turn the main body upside down and remove the cotter pin that holds the linkage to the secondary throttle vacuum box. Turn the main body back over, and remove the four bolts that hold it to the throttle body. A tap on the corner of the throttle body may be necessary to separate the two. Never hit the throttle shaft or use it for leverage.
Inspection, cleaning, and rebuilding the main body:
The vacuum box is held onto the main body by three screws, and has a small gasket at the join. There should be no reason to remove it, but if you do, just be sure to use the replacement gasket. There is nothing you can do to the inside of the vacuum box, and it is best left assembled. Inspection of the secondary vacuum box is easy enough without taking it apart. The kits don't come with a new diaphragm for these, but they hardly fail. A simple test is to remove it, plug the vacuum tube with your finger ( it sticks out about 3/16ths inch and can either be metal or plastic ), and try to operate the lever. Don't use too much force. You just want to tell if there's an appreciable leak. Truth is, even with a tiny leak, the vacuum box will still do its job. Without your finger plugging the hole, the lever should return to it's original position due to the spring inside.
Removing the accelerator pump components:
There is a spring and diaphragm inside the housing and there's a replacement diaphragm in the rebuild kit, but not a replacement spring. Remove the brass plug on the top edge of the main body, right over & behind the accelerator pump housing. Under the plug is a small, skinny brass weight, and under that is a very small check ball. Pick the carb up side down and catch the weight & ball with your other hand. Next, remove the accelerator pump nozzle, located between the two primary venturis. That assembly consists of a small brass banjo screw with an aluminum crush washer between it and the nozzle banjo, and a paper gasket under the nozzle, Inside the circuit is a larger diameter brass weight that is much shorter than the other one. Double check to be sure there are replacement check balls supplied with the kit. -There are no replacement weights supplied in the kit. Sometimes the check ball and even the weight get stuck from varnish. A toothpick should be used to wiggle the weight out while the main body is turned upside down. If the check ball is stuck, a very light tap on the top of it with a finishing nail will often free it up. A soak in carb cleaner may be necessary. If you still can't remove it, leave it for later. It may sound like a sensible idea to keep them from rolling away, but NEVER (!) use a magnet on check balls that you are going to be using or re-using. As very minute particles of rust go through the carburetor, they will start collecting on the check balls, keeping them from sealing well.
Inspect all of the jets to be sure that the
orifices are clean. I remove them and look at
them, and then try to blow through them if I
have any doubts. Tooth picks are good for
getting junk out of some of them, but many are
too small. A small section of guitar string
makes a great tool for this. Be careful not to
score the insides of the jets. Do not use
drill bits to clean them! They are very
specifically sized to deliver precise air to
your carburetor idle and transition circuits,
and you can seriously mess up the idle if you
open these up. With the exception of '74 and earlier Nikkis, the only jets in the top of the main body that can be accidentally switched are the very short No. 2 step jets located on both the front and back edges of the carb. Another pair of jets in the center can fit in the wrong places, but then the jets that belong in those places can't fit anywhere else, so you would discover the problem eventually. As for the No 2 step jets, the pair that goes on the secondary venturi side are nickel plated, and the ones that go on the primary side are plain brass. With the exception of pre-'75 Nikkis, the secondary No. 2 step jets are the only nickel plated jets on the top of the main body. The only other nickel plated jets are the secondary fuel jets. The long tubes screwed into the booster venturis are the emulsion tubes with air jets on the top. The four jets in the center of the top of the main body are dual purpose transition jets that function differently as the fuel level within the carb drops. It's very important that the bottoms of these jets, which have tiny orifices drilled into them, are clear of debris. These are definitely not jets you want to accidentally ream out by sticking anything too big in them. Jets should be soaked in carb cleaner. The ammonia based cleaners like Simple Green and Royal Purple will oxidize them and not clean them very well. I have had success with boiling them in Dawn dish detergent. Don't forget the fuel jets. These can be accessed either by a tiny right angle ratchet screw driver, or by removing the access plugs on the sides of the bowls. The access plugs have a locking clip that needs to be popped off. It can be cleaned and reinstalled later, but I usually discard them. There are copper crush washers under the plug bolts to seal the bowl. These can be reused, but there should be replacements in the kit. There are also small aluminum sealing washers on each fuel jet. These should be replaced or reused when the fuel jets are reinstalled. Tighten the fuel jets well, but carefully.
The Nikki carburetors on automatic transmission equipped Rx-7s have an "richer circuit" designed to enrich the mixture when the transmission is taken out of Park. This is located between the secondary venturis, on the edge of the main body, and consists of a richer solenoid, a plug screw, a long brass weight, and a long step jet. Some pre '79 Nikki carburetors were equipped with a vacuum operated power valve, located on the secondary end of the throttle body. Neither of these should be dirty inside, but if you do remove them for inspection, be aware of all the parts and springs. If the richer circuit is to be disassembled, always reinstall the solenoid first when reassembling.
Replace the seals in the float glasses. Behind the steel frames that retain the glasses are paper gaskets to cushion the glass from the metal. Under the glass piece is an "O"-ring set into a recess cast into the housing. These rubber rings and the paper gaskets are supplied with the kit. For some reason, the paper gaskets have never been cut correctly in any kit I've ever tried. I always have to trim each corner with scissors to get them to fit correctly.
The sub zero start assist nipple is located low between the OMP lines. It will not cause a vacuum leak if it isn't plugged. The circuit snakes up to the back side of the AP nozzle, so it can't ever interfere with the idle circuit, and there is a check ball inside the nipple, so it's not going to take anything in that isn't pumped in.
Once the carburetor main body is cleaned to your satisfaction, rebuild the accelerator pump using the replacement diaphragm, check balls, nozzle gasket, and aluminum washer for the brass banjo screw. Drop a new check ball (first) and the skinny weight into the plug hole above & behind the AP housing. You should be able to see them through a small hole that's drilled sideways through the AP casting. Reinstall the brass plug. These tend to leak if they are not nice and tight. Insert the AP diaphragm into the AP pump cover, and hold the spring in the housing as you put the cover on. Don't install the lower left screw because the lever hinge pin will not clear the screw head.. Secure the cover tightly with the other three screws, and thread the pin through the housing, the lever, and the two washers. Next drop in the second new check ball under where the AP nozzle banjo screw goes, followed by the short, fat brass weight. Use the new nozzle gasket, and the new aluminum crush washer for the brass banjo screw, and reassemble it. It's not necessary to get this super tight. If you have no replacements for the check balls, or lost one, and the ones you have are not very bright and shiny, then replacements can be found in the spray gun tops of most household cleaners. The check balls just happen to be the same size (usually). If you've lost one of the accelerator pump weights, there are a few creative alternatives; To replace the skinny weight, take one of the old pins from the original float needle assemblies and clip off the head. It's best to apply a small amount of pressure to clippers and slowly work your way around the pin, effectively girdling the whole thing before you finally snip off the top. You should be left with no burrs sticking out. Another check ball can work as a weight for under the nozzle, but a BB is better. Whatever you use, it has to be small and smooth.
When replacing the jets, be certain to tighten them, but use caution so as not to mar the tops. Some jets, particularly the No. 1 step jets in the center of the main body, nearest the primary venturis, have very thin and fragile heads that can get torn completely off with a screwdriver.
Nothing needs to be done to the throttle body except a good cleaning.
If you use compressed air to blow out the carb body, be mindful of fuel spaying into your eyes, and be careful not to overload the vacuum box. If you plan on using compressed air later when the carb is fully disassembled, you need to be aware of internal parts that you may blow out into oblivion. There include plastic sleeves inserted into each emulsion tube circuit. Sometimes they're in there really hard, and other times they'll come right out. It's not necessary that they be removed at all, but the carburetor won't run very well at high flow if it's missing one. Also, if a check ball, and even a weight in some cases, are stuck in the main housing, they can get blown out into Never-Never land, too.
Reassembly-
Apply a light film of oil on the top of the throttle body and install the new gasket, making certain that it is orientated correctly. You can check by looking at the bottom of the main body. If the gasket is install wrong, there will be a leak in the vacuum secondary prompting circuit, and your secondaries will not open. The curved channel on the bottom of the main body is part of this circuit. There are two indexing pins on the top surface of the throttle body. Reassemble the main body to the throttle body, and tighten the four bolts up. Install the vacuum box and connect the linkage, and install the accelerator pump lever. Once the pin is in, you can screw in the last of the four housing screws. If you've lost the tiny circlip that holds the accelerator pump lever pin in the housing cover, just use a piece of wire. It should be a bigger gauge than a bread-bag wire tie, but not by much. Wrap it around the pin just once where it fits in the groove, and twist the ends together until it's tight. Clip off the excess. The pin will probably never try to move sideways, and even if it did, the head of the lower left accelerator pump housing cover screw won't let it fall out anyway. I never bother to reinstall the two thin washers that go on either side of the accelerator pump lever arm inside the cover. They are there to keep the steel lever from wearing away at the aluminum housing.
Check all linkages and double check that all jets are in place and tight. Next is the air horn. Be certain that the float hinge pins are centered so they don't hang on the edge of the main body during assembly. Do not force anything. The air horn assembly should fit right together with the main body with no effort. If it does not, a slight shift from side to side will let it drop right in. Secure the air horn lightly with the center bolt, and then screw in the corner & edge screws. The center bolt can now be tightened. Wait until last to reconnect the fast idle linkage. Be sure to replace all of the aluminum crush washers in the fuel rail assembly where it's been taken apart. If it was completely removed, be sure to keep everything loose at first and tighten it all up a little at a time in order to avoid misalignment problems.
Last is the cable bracket & auto-choke assembly, along with the deceleration dashpot and EGR valve. Reinstall these just as they came off, and then reconnect the fast idle linkage, being certain that the cable link hangs over the top of the cable bracket.
Time to reinstall the carburetor!
A rebuilt carburetor should last another ten -
fifteen years in the hands of someone who
maintains their car well.
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Copyright © 2009 Dennis Williams, Sterling Metal Works. All rights reserved. gorealfast@sterlingmetalworks.com |
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