Dual 1bbl Carb Setup Tech


Photo opportunity courtesy of my friend Tinlizard (check out his gallery pics for more pictures of this awesome project!).  Click here to see more pictures and information!

This page is aimed at helping you with choice and setup of dual 1bbl carburetors.  This page is a work in progress.

There are a few different common aftermarket dual 1bbl carb setups out there, namely:

Weber 34ICT (Baby Webers)
Solex/Brosol H32/34PDSI (CB Performance kit)
Solex 35PDSIT (Scat kit)
Solex H40/44 EIS (Kadrons)
Dellorto FRD34B (Baby Dells)


Characteristics In common:

Individual characteristics, settings, and issues


Weber 34ICT



Parts diagram (click on it for full size with parts list)



Float settings diagram

  1. Make sure that the needle valve seats correctly (all the way in)
  2. Hold the cover vertically so the float does not compress the spring ball (SF) in needle S
  3. With the cover vertical, and the float tab (LC) lightly touching the spring ball (SF), measure from the float (G) to the cover gasket (GZ). The distance should be about 6mm.
  4. To adjust, you have to bend the tab (LC) a little either way to get the setting right.  You can also slightly bend the spot where the float canister attaches to the flat plate, to try to get it closer without using the tab (LC).

Common problems
  1. Throttle shaft bores wear badly on these causing the seals to flex and leak - these need to be bushed, sometimes even new ones are sloppy.
  2. Throttle shafts tend to have lots of end play, which causes linkage slop and errata. These are correctable by rebushing the carburetors and using special shims to correct end play. End play matters, as far as linkage is concerned as well as potentially causing throttle plate and bore wear, when the plates scrape the sides of the throttle body. Many of these come to me with plate wear, which isn't usually bad enough to need new plates. This is a good thing because throttle plates aren't readily available for them.
  3. Accelerator pump squirters are difficult to remove. The o-ring becomes hard or very dry, causing difficulty removing it. One thing that really helps is to warm up the carburetor body in some very hot tap water, or a little warmer.
    Heat a pot of water up to a temperature of close to 150 degrees or so, that should be enough to get it going. Grab the pump squirter with some needle nose pliers, twist it and pull it up and push it down several times, being careful not to tear up the top of the squirter where you are holding it with the pliers. The squirter should loosen up and pull out.
  4. Emulsion tube under the air jet can be difficult to remove. Make a piece of stiff wire with a short bend in the end and fish it down into the tube. Lock the bent end into one of the holes in the tube, and pull up

Solex/Brosol 32-34PDSI (CB Performance kit)



Common problems:

In the few I have worked with and installed, I have seen a lot of problems with these units, though once the kinks are worked out they do seem to be good units. I think the biggest problem with them is the assembly quality control.
  1. Float pulled out of the needle keeper, which will cause the carburetor to not fill, or to flood. I found the following on the last set I installed.
  2. Left carb choke shaft arm bent (the part that the choke tang attaches to.
  3. Left carb choke fast idle stop bent causing left carb to never fully return throttle.
  4. Right carb choke shaft tight, causing choke to stick in the partially engaged position. Fixed with precision impact device.
  5. Linkage center piece fouled the shroud in a big way, causing us to have to clearance it beyond what is reasonable. The shroud was one of those wonderful aftermarket 36HP shrouds, so that could have been part of that problem.
  6. One of the removable air jets was missing! CB Performance sent a replacement.
  7. Left side float was out of the keeper and had to be reinstalled.

Solex 35PDSIT (Scat kit)




Common problems:
  1. Problematic throttle bodies out of the box.. many times I have seen problems with these. The problems are plate shape errata, plate alignment errors, mixture screw stuck, and sticking shafts. The solutions to these problems can
    range from difficult procedures to replacement throttle bodies. The last time I have tried to get throttle bodies replaced, they've been out of stock, and have never agreed to replace them for free, which is very frustrating.
  2. Older kits had problems with the cover gasket interfering with the float, causing flooding or float sticking closed. This isn't a problem in newer kits.
  3. Difficult fitment on engines with OEM doghouse shrouds due to throttle arm interference on right carb to the fresh air outlet, and due to castings having bosses that foul the shroud on the left side. See my article on installing these for more details on this. The article is on this site, also in the tech articles section. There is a lot of detail there. These are probably the most difficult dual carb set to install in some cases, but if you are aware of the obstacles, tackle them and get these installed, they're pretty good carburetors.

Solex H40/44 EIS (Kadrons)


Common problems:
  1. Problematic throttle bodies out of the box.. many times I have seen problems with these. The problems are sloppy throttle shaft bores, and too much throttle shaft end play.
  2. On these units, the accelerator pump discharge nozzle is long, and has a tendency to have its discharge zone too close to the venturi height. Bending the tubing to raise it up but still shoot straight down helps keep the tube from
    siphoning fuel at higher air speeds through the throttle body. If you bend it enough to raise it above the venturi, this will eliminate the problem.
  3. Many times I see these units missing the tiny check ball under the accelerator pump discharge tube, like the stock Solex units have. Using one of these helps keep the entire accelerator pump circuit full, and ensures a positive
    strong stream of fuel from the accelerator pump.
  4. Mixture screw damage - sometimes overzealous adjustment causes the screw to seat into the body too far, or someone changes the spring out for a heavier unit. This causes the screw to bind the spring before the mixture screw totally
    closes off in the seat. This causes the unit to be unable to be set effectively. Getting the mixture to be able to go lean is part of being able to set the mixture correctly. Using a lighter spring usually helps with this unless the
    throttle body is really damaged. In this case the throttle body needs to be replaced.
  5. The linkage that these are typically equipped with is junk. I tend to recommend the Scat universal linkage for these. Modification of the throttle arms is necessary with this linkage, to make it work right. Problems that exist with
    the standard linkage are :

    6. a. Solid center link style rod does not allow for precise adjustment of linkage for good synchronization.
    b. Snap joint ends tend to get loose and pop off
How to modify throttle arms to be used with the Scat universal linkage:
  1. Remove the throttle arms from the carburetors
  2. Place them in a vice with the linkage ball facing down and between the jaws
  3. Carefully grind the formed rivet end flat and flush with the arm
  4. Use a punch point to make a divot centered in the rivet body, so you can drill through it.
  5. Drill Through the rivet material with a drill slightly larger, until you can see that the drill has cut as far as the rivet margin, into the back of the throttle arm. Stop here, and use a punch and a hammer to drive the ball out.
  6. The bolts that come with the Scat linkage are a little larger than the hole in the throttle arm, and you will need to use a drill bit large enough to open the hole so the bolt can pass through it.

Dellorto FRD34B (Baby Dells)


Parts diagram (click on it for bigger size)




Float settings diagram


Common problems:


Common topics for all dual 1bbl carburetors

Throttle shaft wear and end play problems

With the pushing or pulling force of the throttle cable or linkage, the throttle shaft is pushed against the shaft bore and constantly digs into it. Over time, the hole ovals, often more on the throttle arm side than the accelerator pump side. The slop introduces air leaks, and causes difficulty with the linkage, namely keeping it synchronized. Air leaks cause what is called "false air " into the intake runner, which means air that is not controlled by the existing system. Anything leaking from the throttle shaft is not metered by the throttle plate system, and is not controlled. The false air does not come with the effect of delivering fuel with it, and therefore dilutes the air fuel mixture in a way that cannot be controlled, and which cannot be compensated for in a dependable way.

Fuel leaking from the throttle shaft is usually an indication of fuel pooling on top of the throttle plate. This is usually only apparent after the engine has been shut off, because vacuum at the throttle plate causes air to suck in rather than fuel to leak out during the time the engine is running. This may be an indication that you have excessive fuel pressure, or in inlet valve that is not closing off completely. It may also mean float height issues. If this is the case, you may still see fuel leaking out through the bushing after it is repaired. It may be less, but if the problem is significant, you may still see some. I don't consider fuel leaking from the throttle shaft area a sign of a loose throttle shaft, though looser shafts will leak more. The leak is not caused by the shaft wear itself.

The way to deal with worn throttle shafts is to restore the shaft hole, which is a process in which the hole is opened up and an sleeve or bushing is installed in the hole and the ID of the bushing is sized to accommodate the size of the throttle shaft with close tolerance. Materials range from replacement bushings made of plastic, soft metallic materials like Garlock, or harder metallic materials like bronze. There are certain materials that work better than others. Typically the bushings are sized to a press fit into the body, and a size slightly larger than the throttle shaft. Methods of preparing the body include drilling or milling, using either a drill press or a mill. Then the bushing is sized to fit the bore, and the ID of the bushing is reamed to fit the shaft size. Shafts tend to wear a little, so this should be closely matched using a series of reamers surrounding the shaft size. The closer to the size of the shaft you can make the hole, the better. As the hole size is increasingly larger than the shaft size, the rate of air leak and the rate of wear increases. If you consider the radii of the two matching items, a hole radius much larger than the shaft means that only a tiny portion of the shaft touches the bore, and all force contacts this point and causes more PSI on the wall of the hole, and hence faster wear.

The other issue with throttle shafts is end play. When the throttle shaft has uncontrolled end play, the throttle plate is allowed to slide back and forth in the bore and then closure of the plate forces the plate to become centered again by scraping against one wall of the throttle bore. This repeated wear often causes the plate to wear on the edge, and the throttle bore to wear into an oval shape as well. This also destroys the precision of the throttle plate system, and increases the air flow at any reasonably shallow throttle position. By controlling this end play you can eliminate the cause of wear and greatly increase linkage precision.


Linkage styles

There are a few different styles, including crossbar, center pivot, and most recently added to the arsenal, some cable driven linkage. Opinions range on the best linkage. I tend to lean toward non-crossbar linkage because I feel that crossbar linkage tends to wear quickly, and tends to have enough pivot points that the collective weight, drag, and wear slop will make synchronization more difficult. For asymmetric carburetors where the throttles turn in the same direction, A push/pull style center pivot style is best. For symmetric carburetors where each carburetor rotates in the opposite direction, a push/push or a pull/pull linkage is best, depending on linkage geometry.

  1. Crossbar linkage:



    Crossbar linkage has a horizontal bar that pivots at the carburetors via pivots mounted to either carburetor, the air filter, or sandwich plates under the air cleaner. Close to the ends of the crossbar are arms mounted to the crossbar that raise and lower as the crossbar is rotated. Near the center of the crossbar is the actuating arm which has a cable attached to it. When the cable is pulled, then the arm is moved, rotating the crossbar. Connected to the arms at the ends of the crossbar are down rods, which have joints at the top and bottom. The down rods attach to the arms and then connect to the throttle arms on the carburetors. This kind of linkage can be used easily with symmetric and asymmetric carburetor setups.  The version you are looking at is Tayco.  There are a few other brands, but the idea is the same on all of them.

  2. Push/Pull linkage:



    This linkage has a center tower that pivots, with two arms that protrude from the tower. One arm is connected to the throttle cable, and the other is connected to either a single arm that spans the two carburetors, such as in the case of standard Kadron linkage, or two rods that attach to the arm, with the other ends connecting to the carburetors. As the cable pulls one arm on the pivot, the pivot is rotated, and the other arm moves the rods either left or right, opening and closing the throttles on the carburetors.  The version you are looking at is Scat Universal linkage, which I think is good for the price, and adapts well to Weber 34ICT, Solex 35PDSIT, Kadron (with linkage arm modifications), and Dellorto FRD34B carburetor sets.  You can see examples of it installed, and the process I went through on my 35PDSIT page.

  3. Push/Push or Pull/Pull linkage:



    The principle is the same as the push/pull linkage, and is a two rod design, whose pivot attachments consist of two points on the pivots. These are arms 180 degrees from each other, so that when the pivot is rotated, the rods move in opposite directions, either pushing or pulling the throttle arms on the carburetors.

Fuel pressure

Fuel pressure on most every aftermarket carburetor setup for VW aircooled engines tends to be agreeable around 3 to 3.5 PSI. Not all pumps will deliver this pressure, and some require a regulator. Regulators come in different kinds, with different mechanisms. To me buying a fuel pump and then having to use a regulator is a waste of time and money when you can buy a pump that delivers the pressure you need. The electric carter rotary pump that is offered by Aircooled.net is the best pump I have found. It is rotary, quiet, and delivers pressure until shut off, at which time residual pressure in the line bleeds off. The advantage to an electric pump is that it tops off the carburetors when you turn on the key, not requiring the engine to be cranked to do this. This can result in quicker starts and less cranking time. The quality OEM style mechanical pumps of yesterday are gone, and now the replacements are a crap shoot, requiring testing and calibrating of pressure with gaskets stacked under the pump body to reduce the spring tension applied to the delivery stage. This would be ok, but often it requires reducing that pump actuation so much that at higher demand conditions, the pump is unable to deliver the pressure and volume it needs to. The alternative is to use a regulator with a mechanical pump.

Regulators vary, and some are pure junk. The ones that do a decent job tend to cost more. There are two types of regulators, namely deadhead regulators and bypass regulators.

Deadhead regulators have an inlet for the fuel from the pump, and an outlet or outlets that go to the carburetors, and work on a system of springs inside that reference pressure by varying the spring tension. As long as fuel is moving these work ok. But when there is no movement of fuel, they can allow the pressure at the outlet to increase making them ineffective. Some do better than others and are good enough to use.

Bypass regulators reference pressure dynamically all the time, and fuel pressure above the desired pressure is relieved by bleeding off fuel to a line that returns the fuel to either the tank or the inlet of the pump. This type of regulator is superior in performance, and is commonly used in fuel injection systems where a pressure of fuel in the injector ring is critical.

For my money, you cannot beat the Carter rotary units. They are a simple, reliable, quiet solution to the problem of fuel delivery.

One risk with electric pumps is that the pump could remain running in conditions you would not want it to, such as vehicle accidents, overturns, fires, etc., and for best measures, putting a safeguard system in place is recommended.  The system should be aware of accidents and overturns. This type of system is beyond the scope of this write-up, but you can find examples of these systems on the internet. I don't use this kind of system, but I am a bad example!  Recommending it is the responsible thing for me to do.


Float settings

This is one of the most critical parts of setting up your carburetors. If you are unwilling to set your floats, you are unwilling to tune correctly. Setting floats for the various carburetors is covered individually below.


Balance tubes

Though I discuss balance tubes in the FAQ, I should mention them here too, and say that sizes for these tubes varies from 1/4" to 1/2". The larger the tube the faster the tube can compensate for pressure differences. But the larger the tube, the larger the dynamic plenum space, and the more you have to open the throttle plates at idle to attain the same idle speed. This is an important point, namely because the more the throttle plates are open, the more you expose the progression ports and the vacuum advance port at idle. You do not want ANY vacuum from the vacuum advance port at idle or you will engage the advance on the distributor at idle. One way of dealing with this phenomenon is to drill air bleed pilots in the throttle plates, at a size suitable to allow idle speed with the throttle plates as closed as possible. I'm not going to get into a big discussion about that here, but I will say that if you choose to do it, start small and conservative, because it is harder to close the holes back down if you go too far. Start with 1mm holes and go up in small increments until you get close to what you want with the throttle speed screws engaged only a little bit instead of several turns. If they are engaged about a turn after touch, that gives you some room to go either way, and you can fine tune from there.


Carburetor sizing

There are charts for this that have been used over the years with good success. I have my own calculator for it based on formulas, but the charts follow that pretty well. I'd say stick with the charts. If you are a masochist, you can use this formula to determine a venturi size based on your max RPM and engine displacement.



Where:

        Dv = venturi diameter
        Displacement = engine displacement
        Max RPM = expected RPM where engine will produce maximum power

The formula is a general guideline, not fact, and variables are engine combo, what you want to focus on, and practicality.


The Weber ICT carburetors have a fixed venturi, cast into the body. This limits the application to smaller engines than some of the others. I would not put it on anything larger than a low revving 1835.

The Solex/Brosol H32/34 PDSI -2 and -3 carburetors have a removable venturi, and The largest venturi I know of for them is a 28mm venturi, but if a pair were opened up to 30mm, you could probably feed a 1.9 liter engine with a power
range up to about 5000 RPM.

The Solex 35PDSIT carburetors have a removable venturi, and I know that Aircooled.net sells up to a 30mm venturi for them, which would probably feed a 1.9 liter engine with a power range up to about 5000 RPM.

The Solex/Brosol H40/44 carburetors are more gray area.. depending on what throttle body they have and how big a venturi you can cram in them, you might be able to feed a 2 liter engine to about 5000 RPM. The limiting factor is the venturi size you can put in them. The ratio for venturi size to bore size should be somewhere between about 0.7 and 0.85. A 0.7 ratio means that the venturi is 70% the size of the throttle body. So with that, it seems that with a 44mm throttle body, if you could fit 36mm venturi in it, you could feed a 2 liter engine to about 6000 RPM, or a 2275 engine up to about 5500 RPM. But this sizing is something I have never investigated. The body bore size will affect what size vent you can use, and I have stuck with 40mm throttle bodies, and up to a 32mm venturi, which would feed a 2 liter engine up to about 5500 RPM.

The Dellorto FRD34B carburetor can be outfitted with up to a 30mm venturi. You could probably feed a 1.9 liter engine with a power range up to about 5000 RPM with these.


Intake manifolds

Many times, intake manifolds on used units, and even on new units, are not flat in the gasket areas. Dual port intakes have potential at the carburetor flange and the cylinder head flange as well. Single port intakes tend to have their problems at the carburetor flange only. Checking these for surface flatness is an important part of assembly. If you have access to a belt sander this is a quick and easy way to deal with it. Remove studs from the carburetor flange, surface it, and then reinstall the studs. You can tell if they are approaching flat by looking at the wear area from the sanding. You can also use a flat file if you are skilled with one. Over tightening the carburetors or several installations and removals will cause this. Sometimes the surfacing out of the box isn't good either.

Gasket types

I always recommend non-metal gaskets. They don't conduct heat as well, but that's not as necessary with dual carb setups anyway. Metal gaskets are a must with center mount carb setups if you want the best heat conduction to the end
castings, but with dual carbs, I never use them.

Mounting intakes to cylinder heads:

When you install intakes to cylinder heads, keep in mind that if the intakes are aluminum, the only area that you get clamping force is the end of the nut you use to install it. Many times people need to use smaller nuts due to space considerations. With this, the clamping force is distributed over less area. Using thick large area washers really helps with keeping them tight. When the engine vibrates or shakes, the weight of the carbs pulls on the bases, and the nuts will dig into the flanges, causing loose intakes. If you use washers, you distribute the force from the hard attachment hardware to more area on the soft flange material, and this doesn't happen anymore. Ehen you install washers, often you cannot get them to fit due to the material at the base being too close to the hole. Rather than clearancing the intakes, grind the edge of the washer to make it more like a D shape, and it can clear better and still offer plenty of support. Also make sure that the seating area where the washer mates to the flange is flat and square so that the whole washer sits flat, and the nut sits flat on the washer.

Location pins on heads:

Some older heads might still have these, and they can be a bitch to remove. Two things that help:
  1. Using a hammer and punch, apply some licks to it sideways to loosen its fit to the hole in the head. Grab it with some pliers or a pair of channel locks and pull it out.
  2. Heat - it will expand the head a lot more than the steel pin. Then it will come out easier.

FAQ
  1. Why does my engine idle on #2 and #4 cylinder, and #1 and #3 seem dead or weak?

    This question comes up all the time, and some people say it doesn't happen while some say it does. There are factors that will affect how much this happens. Engine displacement, cam profile, intake manifold design, carburetor model all affect the scale of the effect. If you have a setup like this, try pulling the #1 and/or #3 plug wire loose from the cap, and observe that the idle speed and quality doesn't change much. If you do this with #2 or #4, the engine will probably die.

    So what's happening? Why does it do this?

    BECAUSE.. that's all you need to know. That's why.

    Ha! That's the answer that you will get from some people. Here's what I think.

    Look at one side. Let's pick the 1/2 side. You have an intake manifold topped with a single barrel carburetor. The firing order for these engines being 1-4-3-2 (or 2-1-4-3) means that on that side, one cylinder's (#2) intake valve opens, pulls in the air/fuel charge, and then the adjacent cylinder's (#1) intake valve opens right after. The first cylinder (#2) has sucked in the air and fuel that is in the manifold. Since the throttle plate is closed, or nearly closed at idle, this leaves a partial vacuum condition in the intake manifold. There is not enough time for the intake runner to re-fill with air before the second cylinder's (#1) intake valve opens. The second cylinder (#1) also leaves a vacuum in the intake.

    Now, there are two cycles that occur on the other side while this side sits with nothing happening with the intake valves. The vacuum in the 1/2 intake has time to be relieved by the slow bleed of air that makes it past the throttle plate, which is not completely closed. With this air, fuel is also pulled from the idle circuit. Now, by the time #2 intake is ready to open and pull the air/fuel charge in, the vacuum has diminished and the air is dense and rich with fuel. The #2 cylinder receives a strong charge of air and fuel and leaves the intake manifold pressure low again (partial vacuum). This means that when it is #1's turn right away to get its share, there is not much for it to pull.

    The same thing happens on the 3/4 side. So #2 and #4 are getting dense rich air/fuel mixtures, and 1 and 3 are getting less dense air/fuel mixtures. This happens as a result of the firing order, the intake design, and the time frame that this has to occur. The window of opportunity is right for this to happen.

    So now you crack the throttle, and the engine speeds up. Not only does it speed up, but it has much less tendency to maintain a vacuum in the intake manifold because the throttle plate is open more. So with the RPMs up and the plate open more, not only does the intake suffer less vacuum effect, the time between one side's cycle and the other side's cycle decreases, so that the vacuum is more uniform. Jetting takes care of making sure fuel is delivered at the rate it needs to be for this condition.

  2. Ok, great, so what do you do about this odd firing situation? It's making my engine run rough at idle.

    Well, it shouldn't. If you think about the situation, the #2 and #4 cylinders are equal times apart. The "beat" is even, not like a Harley Davidson, or like a single cylinder engine, or a miss. The engine can idle smooth, and I have set up lots of these types of carburetor setups. The key is that the carburetors are synched properly, and that the mixtures are set correctly. With this, you can achieve a smooth idle that is not shaky.

    You say, "I can't deal with it.. I am some kinda neophyte that MUST have 4 strong cylinders of idling fury!"

    Tough love baby. I can help a little with it, but it will still do it a little.

    You can install a BALANCE TUBE!

  3. What is a BALANCE TUBE?

    Gee, I am glad you asked.

    The 1972-1974 USA made Bay Window Bus was equipped with a dual 1bbl carburetor setup, and so was a span of Type 3 vehicles. These were factory equipped 32-34PDSIT and 32PDSIT carburetors, respectively. VW wasn't going to put up with that odd firing silliness, no sir! So they equipped the engines with a balance tube. This tube is just a line connected between the two intake manifolds, which serves to help equalize pressure and help fix the lean charge cycles by allowing an exchange of air between the two sides.

    The balance tube also serves another purpose, which is to keep the intake manifold average pressures the same on both sides. This is effectively what happens when you synchronize the carburetors to allow the same rate of air bleed past the throttle plates.

    In the case of an engine with the carbs out of synch, the balance tube will equalize the pressure on the two sides by causing a net flow of air from the higher flowing side to the lower flowing side. If the carburetors are synchronized, there is an even exchange of air back and forth with no net flow of air going from one side to the other.

    One effect of the balance tube is that in order to achieve the same idle speed as an engine without the balance tube, you have to open the throttle plates more. This can be observed by pinching the balance tube on an engine that is so equipped, and observing that the idle speed increases.

    If you look at the bus carburetor setups, you will notice that the balance tube runs from the base of each of the non-dominant cylinders. This targets the low pressure zone a little better. Those engines run pretty smooth, all things considered. I won't get into what a pain in the butt those particular systems are to adjust and tune here. Check out AMSKEPTIC's tech library for layout and tuning. No need for me to re-invent the wheel.

  4. This balance tube discussed in #3 is there to improve idle quality and equalize pressure. Does this mean that I don't need to pay attention to carb synch?

    No!!  And I will tell you why.  The balance tube has most effect at idle. As the vacuum in the intakes decreases and the speed of the engine increases, it does less to compensate. So at 1/3 throttle it's probably done, and if you have the carbs out of synch, one WILL get more air and fuel than the other. Synch is ALWAYS important.

  5. Ok I'm convinced of #4, or I will just give you the glazed look and accept it. Now I put my synchrometer on the two sides of my balance tube equipped setup and they read the same.. in fact I can't really make them read differently by adjusting the speed screws. Does that mean they are balanced?

    No.  It means the balance tube is doing its job, but probably more than you want it to. Pinch the tube off so air cannot pass between the two sides. The idle speed will go up, yes, but ignore that for now. Take readings this way, and you will probably see that they are not the same. In this case, disconnect the linkage, adjust the speed screws until they are, and the idle is a little higher than you want. Now adjust the mixture screws for lean best idle. Then release the balance tube. If you are happy with the idle speed, then you are done with the carb synch. If you are not, then pinch it off again and make a minor adjustment and release it again, until you are satisfied. Now reconnect and synchronize the linkage.

  6. How does the dual air jet setup work? What are the implications of changing the removable one, versus carburetors with a single air jet?

    Both of these air jets supply air to the same circuit. In other words both contribute. The equivalent size of the two air jets is the diameter associated with the sum of their two cross sectional areas. In other words, you don't just add the sizes of the two to get the equivalent size. You must calculate their hole areas, add them, and then calculate what the diameter of that hole would be. Think about it. If you had two one inch holes, they would not make a 2 inch hole. Lay the two holes next to each other, and then draw a circle that has the 2 inch diameter. Lots more area in that circle than the two 1 inch circles contain.

    So to calculate the equivalent size of two jets, you would set up the formula like this:

    Area     A of a circle:

    A  = or
         
    There are constants here, and to do the conversion, you can drop those, because you will simply multiply all the squared and summed and rooted values by those and then in reverse, you will divide them out.

    Since we are dealing with diameter, let's use the second formula. Drop the constants out, and use the diameters only. If you want to prove it to yourself that the constants do not need to be there, feel free. They cancel out. At any rate, here we go:

    D1 = diameter of one air jet
    D2 = diameter of other air jet

    Diameter of equivalent SINGLE jet =

    So.. for air jets 140 and 90, you have:

    Diameter of equivalent SINGLE jet =    or about 166

    So the implications of changing the replaceable air jet on a dual air jet carb are that the effective change in air jet size is less than if you had changed the size of an air jet on a carb equipped with one single equivalent air jet equivalent.

    Prove it:

    Change the air jet to 130 and 90.

    Diameter of equivalent SINGLE jet =    or about 158

    It is only 8 less, not 10 less, like it would if you reduced the equivalent single air jet sized at 166 by a 10.

    The effective change of the replaceable jet is related to the ratio of the two.  If the removable was considerably smaller than the fixed, the effect would be less, per size.  If the removable was considerably larger than the fixed, the effective change will be close to the change in that jet.  Play around with a few to see what I mean.

  7. Should I use a distributor with vacuum advance?

    If your camshaft is mild, you should have a strong intake vacuum at low engine speeds and throttle angles, and a vacuum advance distributor will be useful, especially if you tune for mileage at cruise and use a lean mixture. Lean mixtures have slower flame front propagation, and will benefit in efficiency from the earlier spark event. Different distributors have different requirements for vacuum strength, but stock SVDA distributors and the aftermarket SVDA distributors tend to work as long as you don't have a wild cam. Use of a wild cam tends to screw you on cruise anyway, so the benefits of the distributor wouldn't be enough to make it worth it anyway.
That's all I have for now. If you have any suggestions on things you would like to add or see, contact me!


***** TO DO:
***** Setup and tuning considerations