Race Prepping the
Weber 32/36 Carburetor
The Lola Registry DIY Series
Step 1. Getting a stock Weber 32/36. The stock carb that we are looking for is a Weber 32/36 DGV 5A, commonly used on 71-74 Ford Pintos and other cars. This model should be stamped on the base of the carb. For Formula Ford, Formula 2000, and S2, this is the legal carb and should be considered standard equipment. Other Weber32/36's, such as the DFV, DFAV, and DGAV, are the same carb, but differs in the choke mechanism, some being controlled by water, electrical, etc. The DGV is a manual choke, but the mechanism is deleted for our racing purposes. So in theory, all the above mentioned 32/36 carbs should be otherwise the same, but I am not sure about legality. Another legal carb is the Weber licensed Holley 5200. This carb is easily identified by a oval ridge surrounding the throats on the top plate. This may require a different filter base than you are currently using, but I am told, it's the same carb. I personally like the Weber, but the mods in this article will apply to the Holley as well.
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Sourcing these carbs inexpensively are getting a bit difficult, as most production cars using this carb has long been out of production. So I doubt the local junk yard will have many early Ford Pintos laying about. I purchased my carb from e-bay. the prices range from $50 for a core to $400 for a rebuilt or new in the box example. I opted for the $50 core since I will be modifying the carb anyway. As it turned out, the carb was un-molested, un-damaged, and a fine core. Don't be afraid of built up grease and grime. The dirtier it looks, the better the chances are that no one went inside and mucked things up. Plus, left alone, these carbs are predominately trouble free.
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Step 2. Inspection, disassembly, and cleaning. This is the time to see what you have and what is missing. Does it have the correct linkage for your car?, is it missing any screws, bolts, and return springs? Etc. Now is the time to order the missing parts, re-build kit, correct emulsion tubes and jets for your application (these will be addressed later in this article).
Most carb kits from Weber come with an exploded view of the 32/36 carb. Using this as your map, completely disassemble the carb, placing the parts on a tray in the order they came off. This will be especially helpful with the linkages and springs. As you will see, the carb is made up in two primary parts, the top plate holding the choke plates, and the lower half, which houses the fuel bowl, ventures and jets. I do not suggest removing the butterflies nor shaft in the lower half if they are good. These are screwed in at the factory where the screw ends are then deformed so they do not back out. Extremely difficult to remove, and you may damage the shaft. Test the shaft after cleaning with brake clean, by moving it up and down, side to side, and rotating the butterflies. What you are looking for is excessive play or a hard spot when rotating where the butterflies stick open, suggesting a bent shaft. Excessive up and down play means that the shaft bores have worn which will result in vacuum loss as air will be sucked in through the shaft bores. To properly repair this, you will have to remove the dreaded butterflies and shaft, bring the lower body to a machine shop and have them over bore the holes and press in bronze inserts of the correct size for the shaft. If you have to remove the shaft, get a dremel, and grind off the ends of the butterfly screws carefully. Remove much as you can before touching the shaft. You may still need some heat to un-screw the screws, just take you time, they will come off. And since you now have them off, reward yourself by getting a set of button head Allens to replace them. These will improve air flow around the shaft, better than the standard screws, and guaranteed to add another 100 HP (only kidding).
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Disassembly is straight forward. Remove the choke assembly first and toss. Remove everything else except for the jets, emulsion tubes for now, and butterflies if good. There are various ways to clean the disassembled carb. Easiest way is to soak in a cleaning fluid like acetone, thinner, or even gas. Then to blast it with brake clean. Make sure to blow out all the vacuum passages. If you have access to an ultrasonic tub, tossing it in there for a few hours is best. You can also glass bead the parts, but just make sure you clean out all the aggregate.
Step 3 Top Plate modifications. Strip and clean the Top Plate including the Power Valve assembly that is held under the plate by 3 screws that we will be deleting. The factory power valve operates by dumping extra fuel into the secondary emulsion tube well to allow a richer mixture during full throttle and allows the engine to run lean at other times to pass emissions. The problem with this system is that the fuel is basically un-metered, making jet selection difficult for racing engines at full power. Additionally, these power valves typically malfunction allowing the engine to run rich all the time. Another inherent problem with the Pinto carb and manifold is that stock fuel delivery is un-evenly distributed among the cylinders. This problem can be rectified by installing "dump tubes" in both the primary and secondary venturies, which is the primary reason to delete the stock power valve system. By directing the tubes to certain parts of the manifold, a moderately even distribution of fuel can be achieved. Positioning of these tubes will be addressed later in this article.
With the power valve assembly removed, next is to remove the factory dump tube. Take a pair of needle nose pliers and work the tube out, rotating and pulling straight out (see pic with red arrow). Get a 2 part epoxy such as JB Weld or anything else that would be heat and fuel resistant. Locate the 5 vacuum ports that need to be blocked up as shown by the yellow arrows and epoxy shut.
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While the epoxy is drying, mark three slots with a Sharpe around the power valve seat. Outlined in yellow, you will have to grind out all of the marked up areas later. Using a long 3/32" drill, carefully drill a hole through the primary and secondary ports, aiming for the center of the previously marked power seat slot. Since the secondary port already had a tube, just drill through the epoxy to the power valve seat. The Primary is a bit more difficult. Start the hole with a dremel, making a divot in the middle of the flat land. Start drilling on a downward angle aiming for the center of the slot. This should take you through the float bowl wall at the center of the casting mark.
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Once the holes are drilled, get some 3/32" brass tubing from the local hobby store and insert into the holes to check for alignment. These tubes should move freely through the holes and will be epoxied in place later. Make sure both tubes intersect at the middle slot that you had marked off. You can adjust the slot to accommodate the pipes, but the pipes should be able to easily pass through the intended slot with slight bends. Be careful not to grind too close to the vacuum passage on the left.
Using a dremel with a small ball type grinding bit, work away the slots to almost flush to the floor. Also grind the 3 prongs flush with the floor. I used the carbide ball tipped grinder to remove most of the material, switched to a narrow cylindrical stone bit to get everything flush, then finished it off with a wire brush bit. Try to round off any sharp corners and edges as well.
Next step is to gut the old power valve assembly since we will be needing the housing only. Get a pair of pliers and work the bottom washer off. Bending it back and forth until it breaks off. Remove the piston, shaft, and spring. Drill the housing out to allow two 3/32" brass tubes to pass through, a 3/16" drill bit should work fine. Now the fun part, fitting and bending the brass dump tubes. Carefully bend the tubes in nice round arcs. First mark off 1" from the venturi end on both brass tubes with a sharpe marker, you'll need two. Put a slight bend on the venturi end and insert from the float bowl side until the tube sticks out about 1" into the venturi. From the primary side, lay the brass tube into the 6 o'clock slot, route out through the 1 o'clock slot, gently bend around towards the venturi side clockwise, enter back in to the 10 o'clock slot and then bend 90 degrees straight up. This 90 degree bend will allow the tube to be centered into the power valve housing. The secondary is installed in the same way but counter clockwise ending up in the center of the housing as well. Once you massaged the tubes into place, insert the housing over the two vertical tubes and secure with the 3 original screws, watching to ensure that the tubes do not get crimped nor shifted out of position. Once secured, blow through each tube. Air resistance should be about the same in both tubes.
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Using 2 part epoxy, glue the venturi tubes in place from the entry and exit points to the venturi. Slightly curl the tubes in the venturi so they point downwards into the throat of the carb once it is assembled. The vertical tubes coming from the power valve housing will be going down into the fuel bowl and should be trimmed about 1/8" from the bottom. Final step for the Top Plate is to remove the press in fuel inlet tube. SCCA requires that a screwed in inlet tube be used. Using a pair of pliers, grab the inlet tube and twist and turn until the tube pulls out. The remaining hole is then tapped with a 1/8" NPT and fitted with the 1/8" NPT to 5/16" male fitting. This can also be an AN fitting if Areoquip lines are to be used. Change the filter under the big brass plug and the Top Plate is done.
Step 4 Prepping the bottom half. Sorry for the long delay in posting the rest of this article, Yahoo web hosting decided to change their protocols on how to publish websites and I was far below the curve in learning the new HTML codes. No pictures yet, but hope to post soon. But this should get you through the modification. Good luck.
disassemble the bottom half
2. Take note that the throttle valve plates are screwed in
and ends staked at the factory. You will have to carefully dremel off the
staked ends and carefully screw them out. Get a set of new screws and a
4mm x 0.7 tap to chase the threads just in case. If you screw them up (no pun
intended) use a 10-32 tap and screws.
3. Clean and degrease everything, use 600 wet and dry
sand paper if necessary, or bead blast.
4. With the bottom completely disassembled,
insert the cleaned throttle shaft (the one with the 4mm x 0.7 screw holes) and
look for play. Wiggle the shaft 90 degrees from the bore. It ideally
should not move. If it wobbles, then you have an air leak and will need to
machine brass bushings into the bore holes. This is important, depending
on the age of the carb, this can be a high wear area.
5. Reinstall the throttle plates, loosely at
first. With both plates in, make sure they are fully closed and centered
at the closed position. Needless to say, you should not have the idle
screw in at this time. If all good, tighten and Loctite. I normally
will not stake the screws again.
6. At this point, some people believe that connecting
the primary and secondary throttle valves together, so they move in unison and
gives you more power off the turns. I am not of this belief, plus it makes
idling the car almost impossible, so it's up to you. So at this point, set
your secondary idle screw so the secondary throttle valve just clears the first
fuel transfer hole. Since you can only get to this screw from the bottom
of the carb, you may have to pull it off and on more than once. Adjusting
this is easy, with the carb on the warm engine, blip the throttle, if it returns
to idle, you are good, if it stalls or stumbles severely, open it some more.
7. Remove and discard the Power Valve assembly (part
#21, see list above) and plug with epoxy, this is the brass valve on the bottom
center of the float bowl. Be careful not to put too much epoxy as this is
where the brass tubes will nestle into that you had installed on the top half.
Just make sure all the passages are blocked.
8. You should have already removed and plugged the
primary side vacuum supply port on the side of the carb if this was a FF carb,
but if, not do so.
9. The rest of the assembly is just putting back everything
you took off.
10. Now you have to decide on jetting. Start off with 170-180
air jets to compensate for the dump tubes. The larger the number for the air
jets, the less fuel. You should have 145-155 jets before you started.
Just remember, the secondary air jet is one size smaller.
11. Leave the idle jets alone, you should have stock 55/50
Emulsion tubes are also left alone and should be F50/F6
Once you have the carb running, revving up to redline, and otherwise
performing well, your good to go, however…….
This was something I was testing, remember those dump tubes you installed
in the top half? Well they can be adjusted as well. The whole idea
is to get an even delivery of fuel between all the cylinders at the same time.
You can bend those tubes around and get different results. I was
experimenting with a pyrometer and checking each exhaust primary at around 5K
rpm and seeing which ones got hotter and which ones were cooler. Then I
would re direct the tube and try to get it as even as possible. Needless
to say, this is fine tuning and if you just got up to step 13, you are in good
shape already. But just wanted to share my thoughts.