INSTALLING and TUNING WEBER 40DCOEs
DATSUN 240Z with L28/F54 ENGINE

DatsunZgarage.com





  • REASONS TO INSTALL TRIPLES

    They really do increase performance
    They look and sound great
    The most fun of all the Z mods to install


  • Without a doubt, one of the most desirable and worthwhile mods you can make to a Datsun Zcar is the addition of a triple set of racing carbs, also known in the muscle-car world as a 6-pack.  While not terribly difficult to install, tuning them properly requires that you understand cause and effect of how a fuel mixture works. If the thought of precisely adjusting the linkage and fuel/air ratios scares you…then triples aren’t for you. But for the tuners among us they are glorious.

    Datsun purists like the Solex/Mikuni triples which are Japanese versions of Webers and I have to admit the Mikunis are seductive. But new ones are not available in the US anymore. And rather than buy a used set and search for scarce parts I decided I would rather have a set of new, identically calibrated Webers with no surprises. 

    For the record…SU carbs provide excellent performance and can be tweaked with different needles, nozzles and oil viscosities and are great on the street and mild track. But there came a point where I felt I had maxxed out my SUs short of physically boring them out…which I didn't want to do. So I decided to go ahead with Project Weber.
       

    WEBER UNIVERSALITY

    Important to understand is that Weber never made DCOE triples for the Datsun 240/260/280Z. The DCOE is simply a universal carb with a large number of interchangeable parts allowing it to be used on nearly any engine. The trick is that in the 1970s enterprising companies and racers began creating 'Conversion kit' intake manifolds and linkages to allow them to be used on L-series Datsuns. This means you have two separate systems to purchase and manage: the carbs themselves, and the manifold/linkage. You can buy the carbs complete with a manifold and linkage or buy them all separately and have fun piecing it all together from scratch...building it from scratch was WAY more fun for me. REDLINE is the official Weber importer for the US...meaning you don't generally find new DCOEs discounted, I suspect everyone gets the same wholesale price. For replacement parts I only use Piercemanifolds.com. They have an amazing inventory of parts and knowledge.

    As you would expect, installing three universal carbs and finding the perfect, stumble-free settings can be tricky. In fact a big surprise to many is that flooring the throttle at low revs can cause a momentary bog as the airflow in the runners drops to zero. Its a characteristic of individual intake runners and after a couple of seconds the engine catches up and its fine, but this doesn't happen with SUs. The result is that I don't like the feel of triples in autocrossing where full throttle at low speeds is required, they are better suited for track and hot street.

    OUT OF THE BOX
    Mine came with 30mm chokes, 115 mains, 200 air, F11 emulsion, 45F9 idle, 40 pump, 50 exhaust which I believe are the default 40DCOE values from the factory.

    To remove the mystery of how these little fellows work do this: put them on a bench and take the main cover off, the venturi and chokes out, and write down all the values of the jets (they all have a number stamped into them). Then put them all back together(!) These were designed to come apart quickly and easily and if you do this on day one you've taken a huge step towards becoming an expert. They look scary at first but once you put a couple back together you'll find them easy from that point on, really.

    TIP: In my new DCOEs I found a surprising amount of brass filings which had I not cleaned them out would have clogged the passages and caused major headaches. This a prime reason to open up and clean out a new set.

    TIP: If yours are used I recommend spending $100 and buying new idle jets as well as accelerator pump jets. The orifices in these are so small that old grunge and age can render them out of spec. Also, the previous owner may have redrilled the main jets to mod them, so replace your main jets if you have any question about their accuracy.

    40DCOE VERSIONS
    Several 40DCOEs versions have existed over the decades such as the #2,18,32 and 151 each with small design differences. The 40DCOE model 18 came out pre-2000 or so and has two progression holes and a fat-taper idle screw. Much of what you read in books and online about tuning DCOEs for L-series motors was written about this model. The newest model 40DCOE 151 comes with 3 progression holes, a new idle air-bleed screw in each barrel, square plastic floats, and a thinner taper idle mixture screw. It’s important to understand there are different tuning concepts between the 18 and 151. This page only deals with the 151.

    MANIFOLDS
    Online I have seen the names of up to a dozen manifolds made for the Datsun L-series. You see them popup on Ebay occasionally but in 2017 the only new ones I know of are one made by Cannon and one made by TWM Induction. In the Fall of 2010 I bought a new TWM version online…a nice solid aluminum design I had to grind away a couple of places on the runners as they hit the downtubes on my 6-1 header.



     

    TWM manifold
     

    MECHANICAL ADVANCE AND TIMING

    There is a good reason not to run any vacuum advance on triple webers which is explained by ADVANCED DISTRIBUTORS of Minnesota:

    “Weber DCOE triples need a specific advance curve to compensate for the increase in air/fuel flow they provide. They do not need the vacuum advance, in fact the manifold vacuum levels can/will drop considerably using DCOEs and the vacuum advance will become very ineffective. Simply don't use it.”

    Also, triples like a lot of initial timing advance to run properly. But because there is no vacuum advance to bump timing up at idle, you'll need to manually set it to 16-18BTDC or the engine will chug and you may get some popping out of the carbs. But just unplugging the vacuum dashpot to "convert to mechanical' and bumping the timing up has its problems. If you increase the idle timing into the mid-teens on a stock Z distributor you can easily push full advance into the upper 30s/low 40s which results in detonation.

    THE MOD: The solution is to have a reputable distributor shop shorten the advance-weight range in your distributor so you can set it at 16BTDC at idle but still max out at around 34 degrees maximum. The rebuilder measures the advance and partially welds shut the slots (epoxy or similar materials are too soft). The top advance plate is then locked in place, the vacuum dashpot removed and the hole plugged.  The result is a full mechanical distributor with the correct curve and modified advance range for Webers.  Elegant and simple. I had my Advanced Distributors recurve my 1983 280ZX distributor and the result is a full-mechanical distributor with integrated electronic ignition…nice. I have since removed the E12-80 module and run the distributor trigger wires directly into my MSD6 ignition. I found this to be slightly more responsive.
     

    Full mechanical 1983 280ZX distributor with E12-80

     

    CARB LINKAGE

    Fairly obvious, but your job is to try and make these three things occur.
    - Identical air flow at idle
    - Identical air flow at part throttle
    - Identical throttle lever “action” from idle to full on all three carbs


    TIP: Never use the linkage rods to adjust airflow or balance. All three should be exactly the same length for the life of the carbs. Use a caliper to preset these and leave them.

    TIP: Once the carbs are synched at idle and part throttle, use the stopper under the gas pedal to fine-tune the linkage range to hit full throttle. Adjust it so the the gas pedal hits the floor stopper 'just' as the carb linkage is at full throttle so as not to bind the butterflies...you should be able to hear the pedal "click" as it hits. To find any slack I hold down the gas pedal with my right foot and use a yardstick to depress the linkage in the engine bay.

    TIP: Don’t assume that the side-of-the-carb throttle levers are universal. The genuine Weber levers were originally designed for cars like Alfas and Lotus, not Datsun. This means they may not fit your custom linkage, you may have to do some detective work to find the proper version online.


    LINKAGE INTERLINK PROBLEM
    My TWM manifold's linkage required that I use what’s known as a two piece ‘interlink’ lever between carbs 2 and 3 instead of a throttle rod (seen below).
     
    weber









    photo courtesy Piercemanifolds.com

    I found that the carbs on a triple 6-cylinder manifold are too far apart for the interlink tang to reach the other lever. The solution was to fabricate a new long-tang lever by taking a throttle cable lever with longer tang and dremeling off half the circlet at the top. I unscrewed the ball-socket from another lever and installed it into the hole...making a long tang lever with ball socket. Why Weber doesn’t make one of these levers is a mystery, but that’s the sort of stumbling block you can run into with universal parts. 

     
    weber









    photo courtesy Piercemanifolds.com

    THE PCV HOSE (Positive Crankcase Ventilation)

    Important to note is that if Positive crankcase pressure from blowby and oil spray is not vented from an engine it can cause seals and gaskets to blow out as well as dirtying up the motor internally. This is why the PVC system was created: to create Negative pressure inside the block and remove fumes. Negative pressure is well known to increase horsepower, especially on high performance engines.

    A little-discussed downside of a negative pressure PCV system that it vents fumes directly into the intake manifold to be burned...constantly polluting the air/fuel mixture going to the cylinders, especially at full throttle. This is where the thick gunk that appears on your intake valves actually comes from. Uninformed people will try to blame deposits on leaky valve seals...but its actually caused by the PCV system...an unfortunate downside of this necessary emission control. Racers and turbo owners will use a "crankcase breather bottle" or can to catch this overspray rather than vent it into the engine.

    Since triple manifolds don’t come with a PVC hose inlet many Weber owners end up sticking a K&N filter on the end of the block's PVC pipe to vent the crankcase into the atmosphere like cars did 50+ years ago. This doesn't create a negative pressure "closed system" and I didn't like the idea of venting my block into my engine compartment…the resulting fumes would probably suffocate me and I had visions of oil dripping onto my steering rack.

    I decided to try an old trick from road racing and use a “crankcase evacuation system”. This involves welding a short ¾” diameter pipe into the exhaust header with a one-way air valve on top. The PCV hose connects to it and the venturi action of the exhaust pulls the crankcase fumes into the exhaust pipe. This creates a functional PCV system but more importantly prevents crankcase fumes from contaminating the air/fuel mixture…allowing a pristine fuel mixture from idle to redline. How did it work? Too well! The exhaust flow from all 6 cylinders into one exhaust pipe created such a strong vacuum that at high revs I pulled oil from my crankcase into the exhaust pipe. I looked in my mirror on my 1st test run at redline and saw oil smoke pouring out of my exhaust(!) 

    I later read on a racing forum to put a restrictor in line with the PCV hose to prevent the exhaust from pulling so strongly. So I took a stock Datsun PVC valve, cut the inlet end off with a hacksaw and pushed it up in the engine end of the PCV hose. Test drives since have resulted in no smoke...problem fixed. BTW, I don’t think your car will pass an emissions test with this mod, heh.


    EXHAUST HEADERS

    Conventional wisdom is that an exhaust header is required to “get the most out of” a set of triple Webers. Little is said about why but the reason has to do with valve overlap caused by the camshaft. When valve overlap occurs each intake/exhaust is open for an instant at the same time. This overlap allows an exhaust pulse to feedback and disrupt the fuel mixture coming down the intake runner from the carb. This backwash is more pronounced on triple Webers which use individual intake runners than on SU carbs which share a common-plenum manifold. The result is that this reversion pulse interferes with combustion...especially if you have a high-performance cam with large amounts of overlap. It has been said you can actually see fuel mist hovering around the carb throats at times under these conditions.

    A header places each exhaust outlet farther away from the head’s exhaust ports. This makes it harder for each exhaust pulse to affect the others flowing from the head, which increases exhaust “scavenging”. So while headers belong on every high performance engine they become especially important if you have triples and a high performance camshaft.

    IGNITION
    While a stock ignition will work with Webers a high performance ignition is highly recommended to manage the extra fuel mixture they supply. If you have a 240Z distributor I suggest replacing the points with a Petronix unit paired with a MSD 6A or similar. If you're lucky enough to have a 1976-83 280Z(ZX) distributor these can be easily wired into a MSD to create a full electronic ignition.

     
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    PCV inlet welded in and a partially drilled hole for the oxygen sensor bung
     

    40DCOE model 151 AIR BLEED SCREWS

    On the 40DCOE #151s you’ll notice an extra air bleed screw on each barrel. One of the online Weber retailers states that they are “not used for idle adjustment or idle quality, the settings for these screws should be closed”. I don’t agree.

    WHAT THEY DO:
    Webers are mass-produced and despite tight tolerances flow-differences between butterflies on the same carb exist. This may not be apparent if you use the older Unisyn flow gauge which has coarse readings. But if you use the newer SK flow meter it’s so accurate you can easily spot flow differences between carbs. That’s the purpose of the air bleeds: to let you synchronize airflow between barrels on the same carb. For a 2nd opinion read page 100 and 158 in the Pat Braden Weber Book. He calls them barrel balancers which are a well-known feature on other Weber models like the IDA…now the DCOEs have them. Read below for how to tune with them.

     



    30mm choke on left - 32mm on right
    Inner diameter may seem similar but performance difference is perceptible

     

    FUEL RATIOS

    Todays oxygenated fuels require a richer mixture than gasoline from before the mid 1990s. 14:7 used to be the standard while 13:1 or so is said to be the more realistic value with RFG and E85.

    Rather than fiddle with reading spark plugs or using a Colortune tool I decided to use the 21st century method: I welded a bung into the collector on my header and installed a Bosch oxygen sensor. This fed into an Innovate LC-1 under my dash and the output went to an air/fuel meter on my dashboard. Even better, this output was sent realtime to my laptop where the Logworks program let me monitor fuel mixture and save it in graph format. It’s exactly the data and readout I got when my car was dynoed…I now had my own built-in version in my dash. Talk about being in control of tuning!


     
      NEW METAL  

     

    TUNING TIPS

    Don't underestimate the importance of the linkage and getting the carbs in perfect synchronization. The slightest misadjustment in flow can cause a shaky idle and can result in cylinders with different power levels. Put your old Uni-syn gauge in storage and don't even think about using the old "rubber hose" method. It may have been fine in the 1950s but not in 2017. Instead buy the highly sensitive "STE SK carburetor syncrometer".

    IDLE MIXTURE JETS
    Initially I found that 2.25 turns out from full-in gave a good idle and an AFR of around 13. It wasn't perfect though and idle was still slightly choppy. I also had the well-known stumble around 1800rpm when I accelerated. My O2 meter showed that when the engine stumbled it would go lean to 19 AFR. Good clue!

    As a test I changed the jets to a next step richer 50F9...and on startup there was an immediate difference. Much smoother idle with evenly spaced pulses heard at the exhaust pipe..it idled "correctly" now. The AFR was now richer and I had to readjust the idle mixture screws down to 1.6 out from full-in to achieve 13AFR. The best part was that on the road the transition stumble was completely gone…no kidding.

    NOTE: twice over the past couple of years I have been driving when suddenly the engine began missing below 2,500rpm but was fine above. That it only happened below 2,5000 was an instant clue that something was amiss with the idle jets. In both cases it was fixed by removing the idle jets and blowing each out with compressed air...even with a good gas filter something had grunged up the jet.

    TUNING TIP:
    Per Weber, the idle mixture screws have NO effect over about 1300rpm. They are strictly for idle and have NO effect over the idle jet transition. In other words, if you're getting stumbling from your idle circuit to the mains don't fiddle with the idle mixture screws. Instead you should swap out the idle jets for other values.

    MAIN JETS-AIR CORRECTORS-PUMP JETS
    Be wary when reading online about tuning DCOEs as most info deals with 4-cylinders or V8s. Those values are not valid for high-revving 6-cylinders.

    The only "authoritative" 6-cylinder settings for the street I've seen are in the Pat Braden Weber book in the conversion kit list at the back. It recommends 28mm chokes, 120 mains, 170 air, F11 emulsion, 50F9 idle, 40 pump, 55 exhaust for the 240Z/260Z (the 280Z is not mentioned). The "Honsowetz" book on modifying Nissan engines deals with full-bore 45 or 50mm DCOEs on large cam race engines...don't fall into the trap of using those values on a street car.

    The function of the Main jets is easy but the Air Correctors can be a bit more mysterious. The Airs have nothing to do with increasing airflow. The simplest explanation is that they supply air into the top of the Emulsion tube as fuel is pulled up from the Main jet at the bottom. The larger they are the more they aerate the fuel mixture (lean it) that flows to the main venturis. "Intuition' suggests when you increase the mains you would increase the airs at the same time...but there's no reason assume that. Check out the Weber recommendations for different motors and you'll see the air corrector values go all over the place. In other words the the old "Mains+50" rule isn't set in stone.


    EXCELLENT TIP BELOW!
    PierceManifolds.com shared this one with me concerning 40 DCOE 151's on Zcars: set the idle speed screws as low as possible to completely shut the butterflies. Then open up the air bleeds in each barrel to set the idle speed up to 1000rpm. Pierce said the 1st progression hole is too close to the butterfly and this method prevents any fuel bleed over from them at idle.

    THE METHOD: Unscrew the progression hole covers-crew and look down the 1st progression hole with a strong flashlight. Adjust the idle screw so it VERY slightly begins to move the butterfly. Start the engine and with an SK meter match all three carbs at the lowest possible setting. Mine idled at about 700rpm. Then start opening the bleed screws to bring the idle up to 1000. It took me about 15 minutes of playing to get the idle smooth and everything matched, about 5-6 turns out on each.

    RESULTS: Astounding, no kidding! Even with the proper idle jets I had always had some stumble off idle and had gotten used to it. But this simple adjustment removed 99% of the stumble, makes the engine rev faster (when blipping it at idle) and simply makes the acceleration feel normal. I can still trick the progression occasionally if I hold the throttle at the transition. But in normal driving the engine is predicatible and smooth from idle all the way up. Wish I knew this trick years ago!

    Below is my 1st dyno run with the Webers using the factory defaults of 30mm choke, 115 mains, 200 air, F11 emulsion, 45F9, 35 ft above sea level. These were stock out of the box, no jet swapping. I had only had them installed for a week and looking back its surprising that the numbers were as good as they were. Horsepower took a hit compared to my SUs which was caused my carb linkage not hitting full throttle (discovered afterwards). Excuses aside....what was really significant was the torque curve. It shot up above 160 ft/lbs at 2500 and flat-lined until about 5000rpm...compared to the bell curve of my SUs. This reveals the secret of using individual runners and DCOEs on a ZCAR...the torque. Once I get the linkage and jetting sorted out to bring the horsepower back up I expect some better HP results.

    So armed with a box of main jets/air correctors and a laptop with the Logworks program I began doing full throttle runs from 3500 to redline in 3rd gear. I settled on values which might seem conservative compared to recommended "high performance" settings. But keep in mind we're talking about a 2.8 liter 6-cylinder...with one barrel per cylinder there's no need for monstrous amounts of fuel on the street.

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    BELOW: street testing with AFR Meter
    30mm choke, 115 mains, 200 air, F11 emulsion, 50F9, 124 ft above sea level.

    The "factory default" setting pulled hard off line with a very powerful midrange from 4-5k, slowing
    a bit around 6000 but still pulling to 7000. I noticed that the mixture began richening at 6000rpm.
    After trying different main/air correction combos with the same result I decided the richness
    at 6000rpm was due to the 30mm chokes limiting airflow.



     

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    Below: 32mm choke, 120 mains, 170 air, F11 emulsion, 50F9, 124 ft above sea level.

    I changed out the chokes to 32mm and dropped the air corrector to 180. If you compare
    the AFR curve they look somewhat similar. But on the road the instant-torque feel
    was reduced, less snappy when I punched the throttle. This combo produced a fairly flat
    02 curve as well as a good combination of power and driveability on the street.


     
     
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    CONCLUSIONS

    A one-step change in a jet in a Weber can make a perceptible difference on the street. At first I found that the more off-base my settings were, the more finicky the engine was, bogging, spitting, backfiring. But once I got everything dialed in correctly things really began to shine. My advice is try not to get stuck in the endless search for the "best" settings...spending every free moment going inside your carbs and doing endless road tests. I have been told that there are over 13 billion combinations possible on a 40DCOE...so good luck if you start playing.

    If you want hard numbers, the factory default values of 30mm choke, 115 main, 200 air, F11 emulsion, 45F9 idle on my L28 gave surprisingly drivable and torquey results. Throttle-response was immediate…punching it was like flipping on a power switch similar to fuel injection...no slight lag like on the SUs. The intake manifold's individual runners contribute to this effect because there is no large air mass to move like inside the stock Datsun intake…each runner now moves the air instantly. I kept flooring it and redlining the engine on my back roads just to feel this sharp surge of power. However even with a good cam I found the 30 chokes began slightly limiting my L28s airflow above 6400rpm, true to the Weber flow-charts. Nothing too serious but perceptable.

    So I changed to larger 32 chokes and found the torque produced by the engine changed...the powerful 'slam' when cracking the throttle was reduced. There was also larger "bog hole" when punching the throttle at low revs which made the idle/main transition more obvious. On the interstate the motor did have a more "long-legged" feel...power seemed biased towards the top end and it revved out better to 7,000...but around town it simply wasn't as satisfying. Unlike the rest of the country here in New England the roads are smaller and shorter and the constant traffic makes it difficult to utilize redline. Pulling away from stop lights and in low speed corners this bog got very annoying after a while. Recently at a stop light on a steep hill I had to use the handbrake to hold the car...as the bog made it hard to generate enough torque off idle to get it moving (it would stall!). That did it.

    In May 2012 I finally switched back to the
    30mm chokes. They provide so much immediate torque at all rev ranges and a perfect transition from the idle jets to the mains that I prefer them to the feel of having a "bit" more top end. If I go on a real track I may switch back to the 32mm chokes (or even 34)...but for the street the 30mms are the way to go. After lots of testing on my L28/P90 with 10:1 compression, 230° cam, and high-ratio rear end I found the values in the conversion tables in the Pat Braden Weber book were correct...I found them the most driveable, pulled the hardest, had the flattest AFR curve and plain just seemed to work best for my driving style. FYI: these settings give my spark plugs the perfect chocolate brown color.


    2017 FINAL JETTING: 30mm chokes, 120 mains, 170 air, F11 emulsion, 50F9 idle (1.6 turns out), 40 pump, 55 exhaust.
    Mechanical advance with 15btdc timing at idle. Idle of 1100rpm set solely with air bleed screws. 3.4 PSI fuel pressure.


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