Carburetor synchronizing and balancing devices such as the Uni-Syn and Dwyer Magnehelic Zero Center Gauge can cost $50 or more. Though they work well, you can make a useful indicator for under about $10 that effectively does the same thing. It is not a "gauge," really, but it does easily indicate carburetor imbalance and is particularly helpful in setting the linkage (off-idle adjustment).
How it Works:
The basic assumption is that both carburetors are of the same bore and offer the same resistance to air flow if they are set identically. This is a pretty safe assumption unless your vehicle uses carburetors from different years or has been reworked.
The device uses a wooden dowel "shuttle" in the center of a clear plastic tube. The movement of the shuttle inside this tube indicates draft (suction) imbalances between the carburetors. The device basically follows the GM service recommendation to have both intake manifolds at similar draft levels when the carburetors are in sync.
The shuttle is centered by using the fields of opposing magnets rather than springs, etc. The opposing magnetic fields help center the shuttle with minimal interference with its movement. When you apply a draft signal from one carburetor to one side of the indicator housing, and the draft signal from the other carburetor to the opposing side, the shuttle will stay centered only if the draft signals are the same. If the draft signals are different, the shuttle will be pulled toward the side with the greatest draft.
With the engine running at above 1,000 rpm (off idle), one can remove one of the carburetor links and adjust same so that the shuttle is centered at that rpm. Letting the engine return to idle, one can then fine tune the idle balance using the idle speed screws. Very simple, really.

How to Make It
The accompanying sketch below, shows the indicator's components. Here's what you'll need in materials:
1. A piece of 1-inch inside-diameter clear PVC tubing approximately 4 1/2" long
2. A piece of 1" outside diameter wooden dowel, 1-1/4" long, sanded, if necessary, to slide easily inside the PVC tube above
3. Four (4) 3/4 - by 3/8-inch-thick magnets with 1/4" center holes (available from American Science Supply for about $2)
4. Two (2) 1/8-inch hose barbs for 1/4-inch NPT pipe fitting (plastic or brass will do)
5. Two (2) 1" PVC pipe caps (hopefully flat on the closed end rather than domed)
6. A 1/4-inch NPT pipe tap to thread holes into the caps to fit the hose barbs
7. JB Weld or similar epoxy cement
8. Teflon tape to seal the hose barbs
9. Two 15-inch-long pieces of 1/8-inch vacuum hose
10. One -inch wood drill to drill a centering cup in each end of the dowel
11. A red felt tip marking pen to mark a reference line on the dowel
12. PVC pipe cement and primer.
Various plastics fabricators have scraps of clear PVC pipe. It can also be bought in lengths from suppliers such as Ryan Herco (888-848-1141). A foot of it costs about $2.50 plus shipping with a minimum of 10 feet. A club could easily buy a length and cut it up to the 4-1/2" dimension so everyone can share in the excitement.
The 4- 1/2" length is not critical but is based upon the strength of the magnets. If the pipe is too short, the magnets are too close together and it takes a great imbalance to move the shuttle. Too long, and the shuttle moves too easily. The 4-1/2- to 5-inch dimension is about right for the magnets chosen.
The magnets (item number 33386) are available from American Science and Surplus (847- 982-0870 and www.sciplus.com). I'm sure other locations have similar ones, but they must be powerful and have a center hole. If you use weaker magnets, you must use a shorter clear tube to get the magnets closer together.
You cut the dowel to length and sand it if necessary for a smooth sliding fit in the tube. It must act like a piston. Don't sand too much!
Using the : wood drill, bore a cup in each end of the down to center the magnet. This hole need only be about 1/8" deep.
Epoxy a magnet to each end of this dowel (you could also use brass screws). Arrange the magnets so that you have North on one end and South on the other (in other words, they want to pull together). Using the felt tipped pen, draw a reference line around the complete circumference of the dowel, centered from each end. This will be the reference line.
Drill and tap the center of each end cap to accommodate the1/4" NPT by 1/8" hose barb. Use Teflon tape and thread the hose barb in place.
Set one cap vertically (open end up, hose barb down) and mix and spread epoxy around (but not in!) the hole you just drilled and tapped. The draft signal must come through this hole. Place a magnet arranged so that its North pole is facing outward and epoxy it into the cap, making sure the center hole stays open. This North pole will oppose the North pole on the dowel.
Perform the same procedure on the other cap, but face the South pole of the magnet outward (this will oppose the South pole on the dowel shuttle). Let the epoxy fully cure.
Using PVC primer and cement, cement one end (only) of the clear PVC pipe into one of the caps. (The other end cap is left loose so you can gain access to the dowel or shorted the tube if the magnetic fields lose strength over time).
Slide the shuttle dowel into the tube so that poles oppose. If you goof, use something steel to extricate the shuttle. Turn the shuttle end for end and the poles will now oppose (now you know why we put North on one end and South on the other). Slide the shuttle in. It should "float" magically in the center.
Put the other end cap on. If the cap is loose, you could use some silicone sealant to seal it. They usually resistance-fit just fine. If you shake the whole thing side to side, the shuttle should, well, shuttle back and forth slightly. If it doesn't, you may have to sand it a little.

How to Use It:
Connect the vacuum hose from one hose barb to one carburetor and the other barb to the other. Make sure the indicator is level. I set mine on the engine deck rim or the little front shroud shelf on an Early model.
Start the engine and make sure it is warmed up and the chokes have released. The shuttle should "shudder" back and forth somewhat given the firing order of the engine.
Put a small spacer (a business calling card folded over works well) between the idle speed screw and the throttle arm on the right carb and disconnect the linkage on the left carb to boost the engine speed to over 1,000 rpm. This takes the idle circuit out of the equation for now. Adjust the left throttle link swivel so that, when the link is placed back in its linkage hole, the indicator gauge leaves the shuttle reference line centered.
Blip the throttle a few times to make sure the setting holds. If it does not, readjust the link. The shuttle may shoot slightly to one side or the other first. This is normal.
Remove the speed boosting spacer and drop the engine speed back to idle. Adjust the right idle speed screw first, then the left if needed, to get the shuttle reference line to be slightly to the right of center (favoring the right carb which has the vacuum advance port).
You should now have balanced carburetors at both idle and off idle conditions. If you don't, you could have throttle shaft leakage problems or throttle linkage slop.
Disconnect the gauge and replace the choke pull off hoses when you are done. Roll up the indicator's hoses and put it in your tool box. And remember to wash your hands, and don't leave a mess in the bathroom sink! Ken Schifftner

Editor's note: This article also appears in the current issue of Coyote Tales, the Coyote CC's newsletter. The article is an example of Ken's extensive knowledge and clear writing style. We are pleased and privileged to have Ken in SDCC and his contributions in the Vairmail. If you would like more info you can E-mail Ken at scrubbr@ix.netcom.com Tom Berg

Heater Box Rebuilding
By Ken Schifftner, SDCC Vice President

Sometimes I think that there must have been a prankster on the Corvair heating system design team. More than a bit mischievous, he or she just couldn’t resist designing some features that would drive future owners a bit crazy. And maybe give those owners cold chills at the worst possible time.
“Why don’t we line the heater box with some sort of weird fiber fuzz stuff, impregnate the fuzz with a ugly brown resin that will breakdown over time”, the prankster thought. “Then, when someone turns on the heater, when they least expect it, big clods of fuzz will be ejected out of the defroster and heater outlets!” What fun! “Yeah”, said another, “and why don’t we tuck the heater box over the transaxle so they have to drop the whole drive train out to get at it!” Sick minds work this way. Prankster or not, GM lined the interior of the heater box with a fibrous fuzzy material, most likely glass wool but perhaps not from this Planet. It appears that they used a resin binder to hold the fuzz together. Over time, after untold cold-hot-cold heat cycles and attack by various fumes and moisture, the resin breaks down and the fuzz can be ejected. To add insult to injury, once the liner is lost the insulating property of the liner is lost resulting in reduced heat delivery. Heater box rebuilding was previously covered in the Tech Guide so I had a good idea as to how to separate the two halves of the heater box. I tried to think of ways a modern day non-prankster would design the box liner. The new lining should correct these old problems (exception, the heater box location being an unfortunate “given”). The liner should:

1) Exhibit good insulating properties.
2) Be able to resist temperatures of up to 400 deg. F.
3) Be held together somehow so chunks could not be ejected.
4) Be light weight.
5) Be fire resistant given its proximity to the engine.
6) Be inexpensive.

I earn money for buying Corvair parts (and, to a lesser extent, eating) by designing air pollution control equipment. In our business, devices called fabric filter collectors (or “baghouses”) are used to filter particulate. Some of these collectors operate daily on industrial sources such as boilers at 400+ deg. F. using fiberglass, Nomex (DuPont) or P-84 (Lenzing Co., Austria) felted bag material. The hot gases pass around and through the bag material and the particulate is trapped in the filter media. The felt is pressed and locked into place by a process called “needling”, so the end result is what is called a “needled felt”. Modern filtration bags often receive a slick, breathable Teflon membrane coating so that the dust that is collected can be more easily removed from the media’s surface. This membrane (called GoreTex, W.L. Gore Company) is also used in foul weather gear and shoes. It breathes, not hold much moisture, is resistant to oil attack, and is slippery so dust and dirt doesn’t cling to it easily. Bags made from this material are also dimensionally very stable (don’t easily stretch or sag).
Most recently, P-84 material has been used in the clothing of firefighters since it is highly resistant to heat, light weight, low odor, and does not off-gas harmful vapors until it reaches very high temperatures. It is also used for the clothing of refinery workers since it offers excellent resistance to oils and gasoline. It is technically called a “polyimide” fiber. The scrap used in making bags for these collectors, I thought, could possibly be a source of lining material at low cost. As a lining, the gas does not pass through the material, it passes over the material. The breathable teflon coating would offer a slick surface to resist dirt buildup, but its porosity would allow it to dry out if it got wet. In nearby Carlsbad, CA, we have a friendly maker of such bags, Standard Filter Corporation (760-929-8559). I contacted them and their Mr. Tim Hitchcock agreed to supply some scrap P-84 material with GoreTex membrane for a test rebuild. Standard Filter suggested the use of P-84 given its reduced tendency to possibly release fibers and its use in human contact applications such as clothing. They suggested placing the GoreTex membrane to the air side of the heater box thereby providing the slick, smooth surface over which the heated air could flow. The felted side would go against the metal housing. The lining would have cost about $30-40 if Standard had charged me for it, perhaps even less if I bought the material in bulk.

The heater box I rebuilt was supplied by Mark Aksamit of Southwest Corvair. I did not at the time want to drop my engine for this project, so Mark supplied a rust free heater box from one of his Arizona parts cars.

After cleaning the box’s external surfaces, I passed a vacuum cleaner hose into the interior to draw out the remaining fuzz. Large fuzz chunks easily came out but a few stubbornly remained. With some modifications to the Tech Guide technique, I separated the two halves of the heater box.

To do this, I felt along the flange to detect the depressions left by the factory spot welds. I center punched these welds and drilled them out with using 1/8” drill to help separate the top cover from the bottom and drilled other holes about 2” apart along the flange so the flanges could later be pop-riveted back together. I used a battery powered reciprocating saw equipped with a metal cutting blade to cut away the remaining sections of spot welds. To be able to insert the blade, I sharpened a small, but wide (about ½”) chisel (see photo) and hammered it into place where the flanges had separated. I then used a 2” wide putty knife to gently separate the seams. Though I tried to avoid it, I over-cut a few spots but filled them later with JB Weld.

A sabre saw was used to saw the welds apart. I used a fine tooth reciprocating blade to make the cutting job easier. A broken hacksaw blade in a handle might also work but expect slow going. Make sure you hold the heater box securely so that the full action of the blade is used. (illustration #2)

Along the flanges you’ll find embossed sections that allow you to insert the chisel. These “bumps” are far enough apart to allow you to separate the flanges enough to insert the cutting blade. I put WD-40 on the blade to reduce its tendency to stick to the flanges (since the blade is pinched between them as it cuts). (illustration #3)

The most difficult area is where the flange is curved. Three or four spot welds are used in that area and I had to be very careful with the saw. OK, I was not careful enough. A little more JB Weld was used. Once separated, I could see the retraining straps that GM used to hold the fuzz blanket in place. These would be retained to hold the P-84 material in place as well. (illustration #4)

I cleaned out the interior and Frank Siebenborn of the San Diego Corvair Club brush blasted it for me using the Club’s blasting cabinet. Pressing the flanges on the anvil portion of a vise, I hammered the flanges as flat as I could using a body panel hammer. I painted the interior with rust preventative primer and used a high temperature black (minimum heavy metal filler) paint as the interior coating.

The “Inlet” half is at the top and the “Outlet” half is at the bottom, (illustration #5). The inlet half has a single square opening through which the blower forces the air or hot air mixture. The Outlet half has two outlets that fit against rubber seals in the seat-back bulkhead. These outlets feed hot air to the distribution ducts behind the seat.

I made paper patterns of the interior surfaces of each half and cut pieces of the P-84 material using those patterns. The material can easily be cut with a good scissors (not the good sewing scissors, please!). I put dabs of high temperature silicone sealant/adhesive every three or four inches on the interior surface to hold the material and passed the material under the restraining straps then pressed the material down. I used a light bead of silicone sealant near the hot air outlets (rectangular) to help hold the material to the curved surface and restrict the air from flowing under the new liner.

The metal restraining straps under which you pass the media are shown in the center of the photograph. (illustration #6)

The photo to the left (illustration #7) shows the paper pattern installed under the straps to provide an approximate size to cut the media. I cut the media to the height of the flange (no overlap) but it appears that a 1/8” to ¼” overlap could be used to help seal in the flange area.

I thought about folding the material out and around the air ports so that a smooth curvature resulted. I did not do this though you might want to try it. It seemed at first too complicated for me. The material is very flexible, however. Wrapping it towards the outside of the heater box and gluing it there with weather-strip adhesive would allow the rubber gaskets and assembly bolts on the heater box to help hold the gasket in place when the box is installed on the car. In my case, I simply used high temperature silicone sealant to seal the edges to the air flow openings. (illustration #8)

I placed a film of silicone sealant along the housing flange and, using the holes I had drilled previously, pop riveted the flanges back together using a washer on the rivet’s expanding side to distribute the sealing pressure.

In the corners, you may have to hem the edges. I put a dab of silicone sealant/adhesive behind the fold to hold it in place. I buffed away the GoreTex coating first to allow fiber to seal with fiber. A pop-rivet through the heater box and large thin washer can hold the material in place if you don’t trust the silicone sealant/adhesive.

The two chamber halves were then pop riveted back together after placing a bead of silicone sealant on the flange faces.

After masking the air inlets and outlets, I sprayed the outside with high temperature paint. I purchased new heater box gaskets and a boot from one of our reliable vendors and installed same to complete the conversion.

I’d guess the whole project took about 5-6 hours, not including the time waiting for the paint to dry.

The new lining material provides a slick, oil and moisture resistant insulating lining that restores the heater box to better than new operation. Now, if only GM had this stuff back in the ‘60’s!

If you would like more info you can E-mail Ken at scrubbr@ix.netcom.com