All PL510 models sold in North America came equipped with a Hitachi dual-throat carburetor, which is now 20-some odd years old. If you still have one, it no doubt needs to be overhauled or replaced by now. These carburetors worked well when new and can still be made to function well. However, after inquiring about the cost of a new one from Nissan, and then pricing a rebuilt unit, most people decide to buy a Weber 32/36 DGV. The Weber is more cost effective than a rebuilt Hitachi or a new one from Nissan, even if you do get a 20% club discount.

Anyone who has had both will also probably tell you that the performance with the Weber is better anyway. I would agree that this is true when comparing the Weber 32/36 DGV to the Hitachi DAF 328 which was fitted to 510’s through the 1972 model year. However, there are other Hitachis to choose from that offer worthwhile features, including good performance. In 1973, the 510’s rated horsepower moved up to 100 from 92 the previous year. Part of the increase came from the use of larger intake valves and a different cam, and part came from a new type of Hitachi carburetor, the DCH 340. It was first used on ‘73 1600's and 1800’s, and on all the L20B's that followed.

The Hitachis are more complex than the common replacement Weber downdraft carburetor, mainly because they had to meet emission standards of the time.

Another difference is that the designers at Nissan seemed to be especially concerned with how their stock carburetor would work in city traffic.

Because of the extra parts, it takes a bit more patience and understanding to rebuild a Hitachi properly than to rebuild a Weber 32/36 DGV, but it may have to be done in areas with emission inspections. If you are interested in a performance upgrade to your 510 that is equivalent to installing a 32/36 DGV, but would like to retain the appearance of the stock carburetor with legal emissions levels, a later-model Hitachi DCH 340 may be for you. All of these carburetors can be tuned to work on modified motors, but you have to understand how they work to modify them correctly. You also need some knowledge of the available jets and various small parts which are interchangeable between different model-year applications and different Datsun models. That material will be covered in a follow-up article on custom tuning.

This article is motivated by the proposition that if you understand how a carburetor works, and what all the parts are for and how they fit together as you take the carburetor apart, it will be easier to remember where they all go when you put it back together. We can’t afford the space to include detailed drawings of every part of these carburetors, so consider this article as a supplement to a shop manual and the instructions included with a rebuild kit. The information presented here may also help you choose a replacement carburetor to fit your needs.

Hitachi vs. Weber: Carburetor Basics and Design Comparison.

A carburetor is a venturi device through which air is drawn and mixed with fuel to create the fuel-air aerosol known as the mixture. The pressure drop which occurs when air accelerates rapidly through the narrow throat of the venturi draws fuel up from a chamber where the fuel is maintained at a constant height below the venturi. The air rushing through the venturi atomizes the fuel stream into an aerosol of tiny droplets. Both the Hitachi and the Weber downdraft carbs (vertical venturis) have two venturi throats of fixed size. The constant-height chamber is called the float chamber, and the height of the gasoline in the chamber is maintained by a float-operated valve, just like in your toilet tank. One of the nicest features of the Hitachi design is the glass window of the float bowl. You can always check to see if the float valve is working correctly and if fuel is actually reaching the carburetor by looking in the window on the front of the carburetor. The Weber and Hitachi carburetors share the same basic design consisting of 3 fuel “circuits” which work together to maintain the proper air-fuel ratio under all operating conditions. These circuits are the main circuit, primary and secondary, the idle circuit, primary and secondary, and the high speed enrichment circuit. They are explained in some detail below.

The main and booster venturis - the main fuel circuit

Most fixed-venturi carburetors have a small cone-shaped nozzle extending out over the middle of the main venturi, which contains the hole where the fuel comes out. This nozzle is called the booster venturi, and it helps develop the pressure drop that pulls fuel up from the float chamber at a lower air-flow rate than the main venturi would do by itself. The booster venturi is very important in obtaining good low-speed running and throttle response. It also spreads the partially-formed mixture out over a larger area, then atomizes it further at the transition to the second venturi for more thorough mixing with air.

The Hitachi used on L16 engines in North America is the DAF 328. It has an unusual primary venturi design called a Solex where, instead of a normal nozzle-style booster venturi, a central post with a bulge centered at the narrowest point of the main venturi helps create the small cross-section of the venturi, shown in figure 1. This "post" has holes at several points on a circle just above the narrowest section of the main venturi, from which the fuel is drawn out into the air stream. The secondary throat of the Hitachi has the more standard Zenith/Stromberg-style booster venturi, like both throats of the Weber. A single passage enters the booster venturi bore just above its narrowest point to introduce the fuel from the float bowl into the airstream. When you open the throttle plate on the primary throat, air is drawn in by whichever cylinder is on an intake stroke, a partial vacuum is produced in the venturis, and fuel is "sucked" up from the float chamber and atomized into the airstream to produce a flammable air-fuel mixture of just the right ratio of fuel to air. This action is diagrammed in figure 3A. The main trick to achieving that perfect ratio is to meter the fuel through a jet with a hole of a precise size: the primary main jet. This sounds simple enough, but is actually fairly tricky because the airflow rate is constantly changing, which means that the amount of vacuum in the venturi is not constant, and thus the fuel flow, even through a jet, isn't constant. Not only is it not constant, but the fuel flow will not vary in an exact relationship to the airspeed in the venturi.

The natural tendency is for the mixture to become richer as airflow through the venturi increases, creating a stronger vacuum in the venturi. This effect has to be balanced to maintain a perfect mixture at all engine speeds. This is accomplished by leaking a small amount of air into the fuel passage between the main jet and the booster venturi, through the combination of the primary main air bleed jet and an emulsion tube, shown in figure 3A. Imagine drinking water through a straw. Now prick a hole in the straw - you have to suck a lot harder to get the same amount of water. This is the function of the air bleed jet in the main fuel circuit. The emulsion tube is located in the main well under the air bleed to set the point in the development of the venturi vacuum where the bleed hole is opened up. That point can be varied by changing the position of the holes in the sides of the emulsion tube - higher holes mean that the air starts bleeding into the fuel stream sooner, lower holes mean air starts mixing in later.

This is a fairly subtle design point where Weber carburetors really excel. Weber has a large assortment of emulsion tube designs with holes of varying sizes at different positions, and with tubes of varying diameters. The amount of fuel which can sit in the emulsion tube chamber (called the main well) also makes a difference. If it holds a lot of fuel, it will take longer for that fuel to be drawn down to where a hole is uncovered. Most Weber emulsion tubes are fat in some sections, skinny in others, to carefully fine-tune the rate at which the mixture changes (or the precision with which it is maintained) with the airflow rate into the engine. Hitachi emulsion tubes are simple, straight tubes, with small holes all of the same size, placed at different positions on the tube. The reason these pieces are called "emulsion" tubes is that, besides controlling mixture, they produce lots of bubbles, creating an air-fuel emulsion which atomizes in the main venturi better than a stream would.

The idle and progression circuits, and the accelerator pump.

So far, we have covered the workings of the primary fuel delivery circuit. What is happening when the engine is idling and the throttle is almost completely closed? Under these conditions, a strong vacuum is developed in the manifold below the throttle plate, and too little air is flowing though the main venturi to create a pressure drop. The idle circuit doesn't need a venturi because of the high vacuum present. It just needs a jet and an air bleed to create the right air-fuel mixture, and a passage to direct that mixture into the throttle bore below the throttle plate.

As most of you who have worked on a car know, there is a small adjusting screw on the side of the carburetor base to set the idle mixture, it functions as a needle valve by allowing you to fine tune the amount of fuel flowing into the throttle bore. The idle mixture enters the throttle bore through a small hole placed just below the edge of the throttle plate in its idle position, and which the needle screw pokes into from behind, figure 4. Due to the position of this hole near the edge of the throttle plate in it's almost-closed idle position, the functioning of the idle circuit drops off rapidly as the throttle plate is opened and the hole is exposed to atmospheric pressure. This is why it is critical to have the throttle plate in the correct position at idle. If, for instance, you have a vacuum leak somewhere in the intake tract, you may have lowed the idle speed to compensate, and you could be partially covering the idle hole. Or if you compensated for a problem which makes the engine run slower, (maybe retarded timing?), by opening the idle position of the throttle plate, you would be getting less vacuum on the idle hole than you should. In either case, you would then have to compensate for the incorrect throttle position by making the idle richer or leaner by turning the idle mixture screw. Then you might create a hesitation as you open the throttle just past its idle position. As throttle shafts wear, the extra clearance causes a vacuum leak at the shaft, changing the position of the throttle plate. This is the main reason old carburetors generally don't allow a smooth idle. This is something you have to judge when deciding whether or not to rebuild an old Hitachi. Nissan doesn’t sell new throttle sections or shafts separately. It wouldn’t be difficult to rebush an old throttle section with a plastic like Teflon. Any machine shop with a lathe and mill could do it. It's just a matter of cost and whether such a small job is worth their effort.

So, the engine is idling and you step on the gas. As the throttle plate opens, the idle circuit starts losing the vacuum which made it work, but the engine speed hasn't increased yet, so the airflow that is building in the main venturi is still fairly weak. This would cause a big hesitation, (flat spot) if it weren't for two fixes: the progression hole (see figure 4) and the accelerator pump. The progression hole is another opening that leads from the idle mixture passage above the needle valve into the carburetor bore just above the edge of the throttle plate in its idle position. It continues to allow fuel into the airstream when the throttle is cracked open just past idle position. The accelerator pump squirts a small jet of fuel into the carburetor throat when the throttle plate is opened father to allow the engine enough fuel to increase its speed to the point where the main circuit can take over. On the Hitachi carburetors, the accelerator pump is a small piston pump with a couple of ball bearings and springs which act as check valves. The ball check valves don't seal perfectly, so the pump generally squirts a bit more when the piston is compressed rapidly than when it is compressed slowly.

The accelerator pump on the Weber is a rubber diaphragm instead of a piston, much like the original fuel pump. They seem to last longer than the piston type because they don't have the sliding seal. On the Hitachi carbs, you can adjust the pump volume by changing which holes are used in the accelerator pump linkage. There are three choices on the long end of the main arm for setting the ratio of stroke length to throttle rotation, and two holes on the piston rod to set the full stroke length. There are no such adjustments on the Weber.

There are also differences in the design of the throttle circuits between the Weber and Hitachi carbs. The Weber idle circuit has a single idle jet and air bleed. The Hitachis have an idle jet and air bleed, but the mixture then passes through an economizer jet, where more air is mixed in from a second air bleed. This arrangement should allow tighter control over the idle mixture as the engine speed varies, and helps the idle emissions stay within specs.

Full power valve

Besides the main and secondary fuel circuits and idle circuits, most fixed-venturi carburetors also have full-power valves. These are designed to open only when the manifold vacuum drops below a certain point, as happens when you open the throttle nearly all the way. The full-power valve is a little plug-like device screwed into the bottom of the float chamber. It has a hole through the top which is plugged from below by a small spring-loaded needle. When the needle is pushed down, gas can flow down through the hole into the body of the valve, then out into the main well through a calibrated orifice in the side of the body. It provides a richer mixture than the main jet alone, to give you decent acceleration when you put your foot into it. The full-power valve is operated by a piston or diaphragm connected to manifold vacuum by a hole. A rod extends down from the piston to the button on the valve. It has a spring wrapped around it which pushes the rod down onto the valve button to open the valve, shown in figure 3A. The spring tension sets the amount of vacuum which opens the full power valve.

When manifold vacuum is high, as it is at cruising speed and idle, the piston is pulled up away from the valve button. When vacuum weakens as you approach wide-open-throttle (WOT) the spring pushes the rod down and opens the valve. The valve typically opens at about 6" to 8" of mercury.

All Hitachi and Weber carburetors have these valves, all working on the same principle. The Hitachi’s have a piston in the top section of the carburetor (figures 2 and 5), and the Weber has a diaphragm in the cover section (figure 3A). Both styles have a spring on the rod to provide the opposing force to the vacuum and set the vacuum level at which the valve is opened. For the Weber, the full-power valve diaphragm assembly is sold as a rebuild part.

Secondary circuit

These downdraft carburetors both have a secondary throat with a larger venturi and throttle plate than the primary. The secondary throat is used to feed the engine the extra mixture required at high speeds and loads. It is preferable to have a second similar-sized venturi rather than one big venturi, because it would take much more airflow to produce a vacuum in the single big venturi than it does in the smaller primary one. This would create a much bigger problem getting a smooth transition from idle to the primary fuel circuit The secondary venturi is just a slightly larger copy of the primary, with a main jet, air bleed and emulsion tube, and an idle circuit. Unlike the primary throttle plate, the secondary plate is fully closed most of the time, so the idle circuit has only a progression hole just above the closed throttle plate.

One of the main differences between the Weber and the Hitachi is that the secondary throttle plate is opened by manifold vacuum on the Hitachis and by a mechanical linkage on the Weber. The Weber’s secondary throttle begins to open when the primary is 2/3 open, on the Hitachi the secondary opens when two conditions are met- the primary is open more than two-thirds and the manifold vacuum increases beyond a certain point. A simple way to put it is that the Weber secondary throttle opens when you think it should, whereas on the Hitachi it opens when the engine thinks it should. {The point at which the vacuum-operated secondary opens can be altered for quicker throttle response, to be discussed in the custom-tuning article.}

High speed enrichment device

Some fixed-venturi carbs also have a high-speed enrichment device. This is a nozzle located in the choke bore at the top of the main venturi. It is connected to a hole which extends down to the float bowl. If you really rev the motor to the point where you are getting a bit of vacuum up in the choke housing area, extra fuel can be drawn out of the float bowl through the high-speed enrichment nozzle. This is to prevent a high-speed lean out, which could cause damaging detonation. The DCH 340 carburetors from L20B motors, and the Weber 32/36 DGV, have high-speed enrichment nozzles. The Hitachi DAF 328 from the L16 and DCH 340 carburetors from the L18 do not.

Emission controls and other carburetor accessories

The only part of the Hitachi carbs that qualifies as an emission control is the Boost-Controlled Deceleration Device or BCDD. There is no equivalent device or function on the Weber 32/36 DGV. The BCDD is the oval-shaped cylinder on the side of the carburetor body facing away from the valve cover. It functions as an auxiliary idle circuit that is activated by the high manifold vacuum which occurs when you decelerate with your foot off the accelerator. Under these conditions, your motor is turning much faster than it is at idle, and the idle circuit can't feed it enough mixture to fill the cylinders enough for anything approaching complete combustion to occur. The result is lots of hydrocarbon and CO emissions. This isn't particularly good for the motor either, since you are sucking oil past the rings and valve guide stem seals, and then not burning it off. Residues could bake onto to to various surfaces.

If you look into throttle bore of the Hitachi carburetor, you will see one hole that is bigger than any of the vacuum ports. That is the giant "idle" passage that the BCDD feeds the mixture through. The point of this oversize idle circuit is to feed enough mixture into the engine to allow complete combustion during deceleration. The air that is mixed with the fuel to create this extra mixture comes through various drillings in the carburetor center section. One of them has a threaded rod with a tapered point and a locknut. The tapered point sticks into a jet, where it sets the air feed.

Do not undo the locknut and rod when you take things apart. Retuning a BCDD is an unnecessary hassle. I refer you to a Factory Service Manual or Haynes Manual for diagrams and tuning instructions for the BCDD. Gaskets for the BCDD section aren't included in most carburetor rebuild kits. I just reuse the old gaskets when I take apart a BCDD to solvent soak the metal parts.

The only way a problem with the BCDD might show up is on a dynamometer emissions test which included deceleration.

The rest of the emission-control devices aren’t really parts of the carburetor. They are located in the stock air cleaner, but they still modify the mixture. There are more of these devices on carbs from 1972 and later model years, including the L18 and L20B-powered vehicles which have the temperature-controlled air cleaners. They are very good at maintaining a proper idle mixture when it is hot under the hood, and at controlling the intake of air from the manifold heat stove. You won't have trouble with carburetor icing using these air cleaners.

Another accessory is the vacuum-controlled choke opening unit, called the automatic choke unloader. It uses a small diaphragm with opposing spring to pull a linkage which opens the choke plate temporarily when you accelerate. You can check the condition of this diaphragm by compressing the rod and placing your finger over the vacuum port opening. If the rod stays in, then the diaphragm is still good. If it instantly pops back out, you need a new one as you've got a vacuum leak that could affect your idle adjustments. It is OK for the rod to slowly re-extend, but it shouldn't pop right back out. The Hitachis also have idle cut-off solenoids,an option you can order for Webers as well. Finally, some L20B engines equipped with A/C had a solenoid on the side of the BCDD to give a little boost at idle when the A/C is running.

Comparison of Basic Carburetor Specifications

Note: Fuel and air jet sizes are stated in mm.
1. 1970 510. 2. 1973 610. 3. 1978 200SX (non-CA).

Performance comparisons

I have had all three of these carburetors on my 510 wagons at different times. My general impressions have been that the Hitachis run as well or better than the Weber in city driving. The city mileage is somewhat better. However, the Weber may atomize the fuel more thoroughly, as my 32/36 DGV has always given me slightly better highway mileage. As far as performance, the DCH 340 from an L18, when jetted properly for a 510, gives about the same power as the Weber. I would say that it accelerates better off the line, and has about the same top-end power. The larger-bore DCH 340 from an L20B makes noticeably more top-end power than the Weber. Any of these carbs will give you a significant performance improvement over the stock DAF 328. The Hitachi also runs perfectly smooth, with no apparent transision between circuits, whereas the Weber has a noticable boost effect, and sometimes a slight stumble, when the secondary opens. These observations make sense when you look at the venturi sizes listed in the specifications table above. A smaller primary venturi should give better low-end acceleration, and a larger combined cross-section should give more top-end power In addition to the performance improvements, the larger Hitachis have the same emissions functions as the original carburetor so they are an environmentally-friendly upgrade. If you are thinking about rebuilding the stock carburetor or buying a Weber 32/36 DGV, you might want to consider buying a DCH 340 instead, then rebuilding it.

The downside of the Hitachis are that they are built in three sections held together by screws. The screws can loosen, allowing the gaskets to leak. This can be prevented with care in assembly, and by using the bracket that attaches the air cleaner to the manifold The Hitachis are more difficult to work with if you make engine modifications and have to re-jet the carburetor because you have to remove the carburetor from the manifold to switch main jets. You will have to re-jet even if you just add a larger exhaust pipe and turbo muffler. However, if you know in advance that you are going to make such a change, you can install the new jets when you rebuild the carburetor and save yourself some time and trouble.

One advantage of the larger Hitachis is that they look just like the original carburetor. They should easily fool smog inspectors, especially if your 510 is a ‘72 or ‘73 and is supposed to have an automatic choke. Anyway, a properly functioning automatic choke is quite convenient. I prefer them to a manual, and they are more emissions friendly. With the Hitachis, you won’t have to buy an expensive new air cleaner or adapter kit for the original air cleaner. If you want a bigger filter, you just pick up an air cleaner from an early L20B car. It has all the same features as the ‘72 and ‘73 510 air cleaner, combined with a larger air intake and larger filter. Be sure to get the bracket to attach it to the manifold. You should swap the bimetal hot-idle compensator widget (look in your Haynes manual for details) in the L20B air cleaner with one from a 510 air cleaner. The L20B needed a larger idle-compensator port and it will make an L16 idle rough when hot.

General rebuilding tips.

The right way to rebuild any carburetor is to completely disassemble it and soak all the metal parts in a can of carburetor dip solvent overnight.

Disassembly is the most "dangerous" part of the process, because now is when you will lose track of small parts. I strongly recommend that you study the exploded diagram that comes with the rebuild kit or in a shop manual as you take the carburetor apart. Try to become familiar with each part and its place so you will be able to remember them clearly the next day. A couple of modified blade screwdrivers will be needed for the job, figure 6. A narrow one with the sides ground down to fit in the Hitachi main-jet holes without damaging the threads, and a wide-bladed one with a notch in the middle for removing the full-power valve. The notched one can be used on Hitachi and Webers, but the narrow one is only needed on the Hitachi. I bought some old screwdrivers for 25 cents at garage sales or used tool shops for this purpose. Some parts are so small they may fall through the holes in the dip basket. You can use some metal window screen to bundle these up before you toss them in the basket. Obviously, you shouldn't put anything that contains a rubber diaphragm into the solvent. You can clean diaphragms by hand with a good detergent in water.

After a good soak, all the old varnish can usually be removed by picking the screen dip container out and spraying everything down with aerosol carburetor cleaning solvent. Do not try to clean out jets by sticking wires or anything else through them. Just soak and spray them out. It is nice to have compressed air, but the spray can of solvent is an excellent substitute.

After all the dirty solvent is rinsed off the parts, lay them out on clean paper towels to dry. You may encounter stripped threads in a Hitachi. That can be expensive to fix. The one in the photos has all the threaded holes in the center section (for the screws that hold the top on) helicoiled because I took it apart so many times while I was experimenting with jetting. Be sure to use the correct-length screw in each hole or you risk not using enough threads and stripping them. Also, one of the screws that holds the throttle body onto the bottom of the center section is hollow because it is the vacuum channel that operates the full-power valve. It is shown installed in the correct position nearest to the float chamber in the photos, figure 5. You should check that all the vacuum accessories are working. You can do that using a hand-pumped vacuum source like the Mighty Vac, a water aspirator attached to a faucet or hose (these are sold at scientific supply houses, and plastic ones are used to drain and fill water beds), or you can use the manifold vacuum from an idling car. The choke unloader and secondary-throttle-opening diaphragms only need to hold a vacuum to work. If you want to measure the vacuum that operates a part you will need some plastic or rubber tubing, a vacuum gauge, a couple of tee connectors, and an aquarium air valve. Hook this up so that you can use the air valve on a tee to bleed air into the line from the main vacuum source. Connect the device being checked to the other end of the main line, and connect the gauge to a tee somewhere near this end. You can then use the valve to bleed air into the line to lower the vacuum, to see at what level of vacuum a device begins to operate. This is also handy for calibrating the vacuum advance can on your distributor. If you are measuring the vacuum, you can check the power-valve piston or diaphragm more thoroughly. If this part on a Weber doesn’t work, you just buy a new one; on a Hitachi you have to fix it yourself. In the Hitachis, the piston bore and/or piston surfaces can wear as they get old.

This can result in two problems: 1. The clearance increases and air leaks by more easily, which can slightly change the vacuum level at which the valve activates. 2. The piston can get cocked in the bore and stick, keeping the valve from opening. The power-valve activating piston on a Hitachi should drop when the vacuum falls below about 8” of mercury. It should not drop above that, but it is OK if it drops below that, maybe at 5 or 6 inches. If it doesn’t work at all, you will have to spray some carburetor cleaner down its vacuum passage and try to clean the gunk out. If that doesn't work the only cure is to tear apart another carburetor and try fitting it's power piston into your carburetor’s bore. Piston diameters can vary a bit from carburetor to carburetor and sometimes you get a match which makes up for the wear. They are held in the top section of the carburetor by the metal surrounding the little steel washer the rod comes through being staked in three spots. You can carve the squashed metal away with a sharp knife and some patience. Then you can clean the bore with Q-Tips and solvent, and the piston with a Scotchbrite pad, if necessary. Don’t remove any metal, just gently polish it. You can rub a little moly grease onto the piston and bore, then wipe off the excess to leave just what remains stuck on the surfaces after a good wipe. Then put it back together and stake the washer with a small punch.

When you reassemble the carburetor with the new gaskets, don't overtighten the screws, and try to tighten all the screws on the same section to an even tightness. Use Locktite on the screws which hold the sections of a Hitachi together, but be careful that you don't use to much and block a small passage somewhere. You shouldn’t need Locktite on a Weber, so you can reinstall that carburetor on the car once it's back together. Before installing it, let the Locktite on a Hitachi thoroughly set up over night, if you can. Now that you have a good understanding of how these downdraft carburetors work, it will be easier to understand what kinds of changes you need to make to tune one for a different or custom engine. In the next article, I will cover choosing main fuel and air jets, full power valves, and selecting emulsion tubes (for the Weber). The article will include tables of Hitachi jets, Weber emulsion tubes, small drills for the in-between sizes, and more.

Till then, happy motoring and enjoy the ride.

Copyright 1997 The Dime, Quarterly