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 which allows 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 these carbs 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 only official Weber importer for the US which means you don't generally find new DCOEs discounted, I suspect everyone gets the same wholesale price.

As you would expect, installing three universal carbs and finding the perfect, stumble-free settings can be tricky. I’ve read of people spending a small fortune in time and money to erase an occasional stumble at a particular RPM. A big surprise to many is that punching the throttle at low revs can cause a large bog as the airflow velocity drops to zero which doesn't happen with SUs...the solution is learning that the gas pedal needs to be finessed differently. My personal feeling is that an occasional hiccup is the price of owning Webers.

OUT OF THE BOX
Whether you get a set of new or old Webers I have an important suggestion: While using a DCOE carb diagram easily found online...take apart each carb including taking the top cover off...write down the numbers of all the jets..and learn how everything goes back together. Discovering the internals of the carbs on day one removes the mystery and will allow you to understand how these gadgets work.

TIP: One a new carb...while everything is apart use a can of compressed air to blow out all the passages. 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.

40DCOE VERSIONS
Several 40DCOEs versions have existed 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 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 that there are different tuning concepts between the 18 and 151.

MANIFOLDS
Online I have seen the names of up to a dozen manifolds that have been manufactured over the last few decades for the Datsun L-series. You see them popup on Ebay occasionally but in 2011 the only new ones I know of are the popular one made by Cannon and scarcer one made by TWM Induction. In the Fall of 2010 I was lucky enough to find a new TWM version online…a nice solid aluminum design I still 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  & 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.”

In addition, triples like a lot of initial advance to run properly. This means instead of running 8BTDC at idle you should be running about 16-18BTDC or the engine will chug and you may get some popping out of the carbs. But just unplugging the vacuum dashpot and bumping the timing up isn’t the way to go either. If you increase the idle timing to the mid-teens using a stock Z distributor you can easily push full advance into the upper 30s/low 40s which results in pinging.

THE MOD: What you do is 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 modified my 1983 280ZX distributor with E12-80 module and the result is a full-mechanical distributor with integrated electronic ignition…nice.

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

TIP: Once you get your throttle rods all the correct length and carbs synched use the stopper under the gas pedal to fine-tune the linkage range. Adjust it so the the pedal hits the stopper 'just' before 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 throttle levers sold online are as universal as they look. The genuine Weber versions were originally designed for cars like Alfas and Lotus, not Datsun. And just because they fit on a DCOE doesn’t mean they fit your linkage, you may have to do some detective work to find the proper versions that work for you. 

LINKAGE INTERLINK PROBLEM:  My TWM manifold 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).

photo courtesy Piercemanifolds.com

But compared to a 4-cylinder the carbs on my 6-cylinder manifold were slightly farther apart and the interlink didn’t reach…the tang between them was too short. Instead I had to use the cable-throttle lever which comes with a longer tang and Dremel off the throttle-cable clip at the top, put a spacer inside, and install a JPS ball-socket. Why Weber doesn’t make a long-tang ball-socket lever is beyond me, but that’s the sort of stumbling block you can run into with universal parts. 

NOTE: DON’T mix cable and ball-socket levers on triples as it throws off the action. There are minute differences in length and throttle stop position.

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 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 when it vents into the intake manifold it allows the crankcase fumes to mix and pollute the air/fuel mixture going to the cylinders, especially at high revs. Think about it: if you have ring blowby (which exists even on new engines) and oil spray it's combining with your air/fuel mixture when you redline it. Ever wonder where the thick gunk on your intake valves actually comes from? Its not from leaking valve seals as many would have you believe. Since Triple manifolds don’t come with a PVC hose inlet most Weber owners end up sticking a K&N filter on the end of the PVC pipe and vent the crankcase into the atmosphere like cars did 50+ years ago. But despite the popularity of this method I don’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 into my steering rack.

So 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 new 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 mist hovering around the carb throats at times under these conditions.

A header helps prevent this because the individual runners place each outlet farther away from the head’s exhaust ports. The result is that each exhaust pulse has a harder time feeding back up into 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.

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

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 Pertronix unit paired with a MSD 6A or similar. If you're lucky enough to have a 1976-83 280Z(ZX) distributor these can be wired into a MSD to create a full electronic ignition.

40DCOE model 151 AIR BLEED SCREWS
If you have a set of 40DCOE #151s you’ll notice an extra air bleed screw on each barrel. One of the popular online Weber retailers states that these 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 barrels 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 butterflies. 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 apparently a well-known feature on other Weber models like the IDA…now the DCOEs have them.

TO ADJUST:
Its not rocket science: after synchronizing your carbs to each other and setting the idle mixture, simply measure the airflow for both barrels on the same carb at idle. If you see any difference simply open one bleed screw to bring the lower flow barrel up to the higher. I found that two of my carbs were slightly off, one barrel needed about 1/2 turn open while the other about 1 turn open. Synching them really smoothed out my idle and my car doesn’t sneeze occasionally like it used to do. The idle settles faster and doesn’t hunt around as much.

THE IDLE PROGRESSION HOLES
Older DCOEs appear to have come with two progression holes while the 151 has three. Much has been said about not "uncovering" the first progression hole while setting the idle. This is confusing because you can uncover the hole by having the butterfly either too low or too high, so which one does "uncovering" refer to?

TOO HIGH: If the plate is too high (towards the air cleaner) and uncovers the 1st hole at idle then fuel gets pulled through too early. This causes a rich mixture and drivability problems as you accelerate. I can’t imagine anyone needing to turn it in that far (my engine would idle at 2000rpm) unless their timing is set too low…in this case the idle timing should be increased to about 16btdc.

TOO LOW: If the plate is too low (towards the head) and uncovers the hole at idle the result is a very low idle (~600rpm). On the face of it it seems OK but on the road this can prevent the fuel from being pulled in just as you crack the throttle because it doesn’t pass the first hole fully. The effect on mine was when I applied a light pressure on the throttle as I cruised down the road I would get stumbling. If I cracked it open slightly more the engine would catch properly. That told me the plate was set too low to let the fuel in.

40DCOE 151s are rumored to have the 1st hole bored too far forward to adjust the idle and butterfly properly. As a result there is talk of a mod to the 151 to bore a 4th hole closer to the engine to smooth the progression. It would seem to me that an extra hole would pull into too much gas and throw off the mixture. On my engine I have the butterfly biased to cover the rear 90% of the hole while the front 10% remains open. With the correct idle jet (see below) my engine idles at 1000rpm and my idle/progression transition is nearly seamless...I have no interest in drilling into my expensive carbs.

INNOVATE LC-1 (fuel ratio)
Rather than fiddle with reading spark plugs or using a Colortune tool to try to guesstimate my fuel mixture 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 feeds into an Innovate LC-1 under my dash and the output goes to an air/fuel meter on my dashboard. Even better, this output can be sent realtime to my laptop where the Logworks program lets me monitor fuel mixture and save it in graph format. It’s exactly what you see when you get your car dynoed…I now have my own built-in version in my dash...the only thing missing is the ability to measure horsepower/torque. Talk about being in control of tuning!

IDLE JET and SCREW SETTINGS
My 40DCOE 151s came new with 30mm chokes, 115 mains, 200 air, F11 emulsion, 45F9 idle, 40 pump, 50 exhaust. My L28 with P90 head, 230 degree cam, and 10:1 compression provides strong vacuum (about 17 inches). While tuning the carbs 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 depending on the engine's mood. I also had the well-known stumble around 1800rm when I accelerated. Luckily I had my O2 meter installed which 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 to 1.6 out from full-in to achieve 13AFR. The best part was that on the road the 1800rpm stumble was completely gone…no kidding. When I come to a stop the idle settles quickly to 1000.

MAIN JETS-AIR CORRECTORS-PUMP JETS

- A 2.8 liter engine with 4 cylinders has 700cc per cylinder
- A 2.8 liter engine with 6 cylinders has 466cc per cylinder

Be careful when reading online about tuning DCOEs as they deal mostly with 4-cylinders or V8s. The only "authoritative" 6-cylinder settings for the street I've seen are in the Pat Braden Weber book in the conversion list at the back. It recommends 28mm chokes, 120 mains, 170 air, F11 emulsion, 50F9 idle, 40 pump. 50 exhaust for the 240Z/260Z (the 280Z is not mentioned).

While the function of the Main jets is easy, the Air Correctors can be a bit more mysterious. 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 resulting aerated fuel flows through the holes in the side of the emulsion tube out to the carb venturis. 'Intuition' suggests that when you increase the mains you would increase the airs at the same time. But if you look at Weber recommendations the air corrector values can be anything depending on the motor type. In other words the the old "Mains+50" rule isn't set in stone.

The result is that the settings I use seem conservative compared to the ones used for racing. I didn't want to fall into the trap of using race jetting on the street without having a baseline. So armed with several main jets/air correctors and a laptop with Logworks I began doing full throttle runs from 3500 or so to redline in 3rd gear. I can't post all the variations but here are a couple I found significant. You'll notice that on all three graphs below, the curve is pretty much the same shape, despite different chokes and jets.

This combo pulled very hard off line with a very powerful midrange from 4-5k, slowing a bit around 6000 but still pulling to 7000. I noticed that my mixture began richening at 6000rpm which was interesting. After trying different main/air combos with the same result I decided the richness at 6000rpm was due to the smaller 30mm chokes limiting airflow. So I went ahead and upgraded to 32mm chokes while keeping the other settings the same.VERY INTERESTING.

The effect of the larger chokes was what I expected...it extended the working airflow from 6000 to 6500 before richening. The overall mixture became leaner because of the extra air volume, close to the "normal" 14.7 for car engines. On the road the instant-torque response was slightly reduced, less snappy when I punched the throttle.The spikiness around 4000 is due to driving over a road divider...interesting that the jolt appears in the 02 curve.

The 3rd graph above has produced the flattest 02 curve as well as the best combination of power and driveability for my driving style on the street. I'm not going to claim these are the 'right' settings, but for my L28 provide some pretty good results.

Below is my 1st dyno run with the Webers using 30mm choke, 115 mains, 200 air, F11 emulsion, 45F9, 35 ft above sea level. My horsepower took a slight hit compared to my SUs which I expected using baseline jetting for a 240Z. In addition I think my carb linkage was not hitting full throttle which would explain why torque was high but not the top end. Excuses aside....what was really significant was the torque curve. It shot up above 160 ft/lbs at 2500 and flatlined until about 5000rpm...compared to the bell curve on 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 nice results.

CONCLUSIONS
My advice is try not to get stuck in the endless search for the "best" settings...spending every free moment going inside your carbs. The Weber baseline settings actually perform very well even on a modified engine. If you want to fiddle, use a wideband O2 gauge so you can see exactly what effect they have.

The out-of -the-box settings of 30mm chokes, 115 main, 200 air, F11 emulsion, 45F9 idle with a F54 L28, 230@.050” degree cam and 10:1 compression give surprisingly drivable and torquey results. Throttle-response and tire-spin was immediate…punching it was like flipping on a power switch similar to fuel injection...no slight lag like on the SUs. The Webers’ individual intake runners contribute to the effect because there is no large air mass to move like in the stock Datsun intake manifold…each runner now moves the air instantly. I kept flooring it and redlining the engine on my back roads just to feel the sharp surge of power. True to the Weber flow-charts the 30 chokes began slightly limiting airflow at about 6000rpm. Nothing too serious but measurable.

When I changed to 32 chokes the best way to describe it is that the immediacy of the torque was reduced. There was a larger "bog hole" when punching the throttle at low revs and the powerful torque 'edge' was rounded off some. It revved out better to 7,000 with louder carb noise but my mind kept returning to the 30s and their instant torque. 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. So despite the 32 chokes opening the top end a bit, the real road-power the 30s provide is really seductive even if I lose a bit near redline.

My October 2011 setup of 32mm chokes, 120 mains, 180 air, F11 emulsion, 50F9 idle has produced one of the flattest 02 curves yet as well as hardest-pulling to redline...7,000 in 3rd gear is fast and easy. These carbs are flat out GREAT on the road.