Salsbury Restorers Society
Carburetor Service of the Marvel Schebler VH-26 - Thanks to Jim Caudill!
Servicing the carburetor can be divided into three steps: Disassembly, Cleaning & Inspection, and Reassembly.
Disassembly: Many carburetors have been damaged due to careless/improper disassembly techniques. It is imperative that the screwdriver blades are an exact fit when unscrewing the nozzle and inlet float valve. Both of these components are made of brass and have very little engagement area for the screwdriver blade. The slots are cut straight and traditional screwdrivers are ground with a wedge shape. Use of standard screwdrivers can damage the slots, or worse yet, result in the destruction of those parts during removal. I use custom ground bits that are ground with straight, parallel sides with the width and thickness to match the part being removed. These tools are used for nothing else except carburetor work.
Begin by using a 6-point wrench to loosen the main nozzle nut (aka bowl nut). This nut contains the main jet for the carburetor and feeds the fuel from the bowl up through the main nozzle. Usually, these are rather large passages and don’t become blocked. Should you see evidence of blockage here, you are likely to have many other areas that will need careful cleaning and inspection. It would be helpful to place each of the parts in a designated container to keep the parts from becoming separated or lost. Remove the bowl and inspect for any pinholes or evidence of sediment and corrosion. Next, use needle nose pliers to pull the float pin and remove the float. If the carburetor had gasoline in it immediately prior to disassembly, shake the float and listen for any evidence of gas leaking inside the float. I once discovered a small hole in a float that allowed gas to seep in and prevent the float from rising up and shutting off the gas flow. This resulted in a flooded engine and led to the scooter being abandoned for over 60 years! By that time, the engine was seized and corroded, all because of a pinhole in the float. I was able to clean and solder the hole and reuse the float.
Next we want to remove the small needle that comprises half of the valve assembly. Inspect it for a worn ring around the conical tip where it has been in contact with the seat. The original needles were all metal, and I prefer them to the newer rubber-tipped needles that are included in the rebuild kits today. So, if the needle is in good condition, you may want to consider reusing it. Next, use a good fitting screwdriver blade to remove the inlet fuel valve seat. It is easier to use the correct tool than to have to drill and use “easy-outs” later. All carb rebuilding kits include a new “needle & seat”, so damage to the valve is not the end of the world.
Now we move to the most critical part of disassembly: removal of the main nozzle. If things went well during removal of the inlet fuel valve seat, then you have a good understanding of what is necessary to remove the nozzle. The tool must fit as precisely as possible when you go to unscrew the nozzle. If you don’t feel comfortable after your first attempt, STOP! Take your carburetor to someone who has the experience and tooling to properly remove the nozzle. Ruining the nozzle turns your carburetor into a “parts donor” for someone else! Unfortunately, the nozzle must be removed to properly clean and inspect the carburetor. When the nozzle is removed, there is a small gasket/washer at the tip that usually will remain inside the main body. You can use some small diameter mechanic’s wire with a hook bent on the end to reach up inside and dislodge the gasket. I can sometimes remove it using a machinist’s scribe and a little spray brake/carburetor cleaner.
Moving to the top of the carburetor body, remove the fuel inlet hose barb and elbow (fig. 1). Unscrew the idle mixture screw and friction spring. Disassembly of the choke and throttle shaft assemblies is usually not required. If the throttle shaft seems excessively loose, then disassembly & inspection is warranted.
Should the throttle shaft assembly exhibit loose and worn movement, it may be easier to consider carburetor replacement rather than attempting to repair it. If wear seems to be primarily in the shaft, either a new shaft can be machined or it may be possible to purchase one. Some shafts are available, and cost around $25.
If the body is worn, the holes can be reamed oversize and a custom shaft machined and fitted. This is highly-skilled and advanced work, which can make repair of the carburetor not cost effective. Looseness of the choke shaft does not usually impair performance and can be overlooked.
Cleaning: There are several ways to clean the carburetor body and the components: There are one gallon “dip-clean” systems that contain a parts basket and are sold at many auto supply houses. These are convenient as they are pretty much “plug & play” and are easy to store when not in use (fig. 2). A second way is to use an ultrasonic cleaning tank. These have become more accessible due to affordable versions being made by low-cost suppliers (Harbor Freight). Costing less than $80, purchasing one for the sole purpose of cleaning your carburetor doesn’t make much sense. Having access to one of these tanks, you then have to select a chemical solvent. I have used the “Gunk” brand carb cleaner (from the one gallon dip-cleaner) as well as cleaners like “Greased Lightning” and “Simple Green”. Usually a couple of hours are all that is required in the ultrasonic cleaning tank. If using the dip-cleaner system, the parts may need to soak for a couple of days. Once the parts come out of the cleaning solution, rinse them thoroughly in a hot water bath. Finally, there is the traditional and dangerous method of carb cleaning using a container of gasoline and aerosol carburetor cleaner. Next: final cleaning & inspection.
Inspection: During the final cleaning & Inspection, I use the aerosol cans of brake/carburetor cleaner, with the plastic tube attached, to direct a spray through all the passageways and orifices. When I am satisfied the part is clean and clear, I place it in a parts container to await assembly. Begin with the nozzle nut (bowl nut). Check for clear passages from the side (large holes) and carefully inspect the small orifice in the top surface – that is your main jet (fig. 3). Next, pickup and examine the main nozzle. The longitudinal passage is rather large and should not be difficult to clean and inspect. However, just above the threads is a single, horizontal hole of approximately .030” in diameter (fig. 4). I use stranded copper wire, which I unravel, to yield single strands of soft copper that I use to probe and clear small passages. This verifies that the passage is clear when you can see the copper wire in the middle of the nozzle tube. Moving on up the nozzle about ½” you will see two orifices. This is actually one, cross-drilled, hole (about .035”) through a groove in the nozzle body. Again, probe and verify everything is clear. Moving on to the main body casting, use the spray brake/carburetor cleaner tube to blast and probe all the various passages. Some of these are vents for the float chamber and also serve as overflow passages should the float valve stick open. Probably the most critical of these passages, is the idle feed circuit. Looking down the throat of the carburetor near the throttle plate, there are three holes that are fed by the idle circuit (fig. 5). The hole nearest the opening is the largest, and is easily seen. Deeper inside the throat are two very small holes: one on either side of where the throttle plate rests when closed. I use a small strand of copper wire, held by forceps, to probe those openings (fig. 6). I direct brake cleaner spray backwards through those openings and also through the opening where the idle mixture screw seats. Once I am satisfied that all orifices, openings, and passageways are clean and clear, I get ready for assembly.
Reassembly: Begin the assembly process by installing a new gasket/washer on the tip of the main nozzle. Hold the carburetor body in its normal position and then screw the nozzle vertically up into the body (fig. 7). This prevents the fiber washer from falling off. Turn the body over and use the proper screwdriver blade to snug the nozzle in place (fig 8). Use just enough pressure to compress the sealing washer and prevent the nozzle from loosening.
The next step is to install the fuel inlet valve (fig. 9). If you are using a new assembly, it will have a Viton-tipped needle. Spray both the needle and seat with carburetor cleaner and blow them dry. It seems as though the manufacturing process tends to leave a residue on these parts that may cause the needle to stick in the closed position. Some mechanics suggest placing the needle in the seat and gently tapping the needle with the plastic handle of a screwdriver a couple of times. The theory is that this forms a “seat” or “set” on the conical tip that makes it less likely to stick or leak.
From experience, I feel it is advisable to spray the Viton tip with a shot of silicone spray. Rumors that ethanol blends of gasoline attack the Viton tip and produce swelling are unfounded. While there is no doubt that the new fuel blends can affect various rubber products, there is no evidence that it affects Viton seals.
Place the float in position and slide the float pin in place. With the carburetor body held in its normal position, raise and lower the float and ensure the float moves freely. Turn the carburetor upside down and hold it so that the bowl flange is flat and level. The float should rest in a level position as well, parallel to the bowl flange. Purists may want to measure verify there is ½” height difference between the casting surface of the nozzle and the float. Place the bowl gasket in the groove of the bowl flange and install the fuel bowl. Place the gasket on the main nozzle nut (bowl nut) and tighten using a box-end wrench.
Turn the carburetor over and place the friction spring on the idle mixture screw. Screw the adjusting screw home until the needle bottoms-out. Do not use anything other than finger force while seating the needle or damage may result. Note the position of the screw and back it out approximately ¾ of a turn. Some new mixture screws have a “cross-bar” instead of a screw slot with the knurled head. It may be necessary to remove the “cross-bar” in order to install the screw. The “cross-bar” can be replaced once the screw has been installed.
Apply sealant to the male threads of the elbow and hose barb. Note the approximate angle that is required for the fuel hose to align with the carburetor and position the elbow accordingly. Then, install the hose barb (fig. 1).
Probably the best source for a carburetor kit is McDonald Carb & Ignition (www.mcdonaldcarb.com) where you can buy the kit (#778-516) for about $20 online plus $7 shipping. They are also available from NAPA (#CRB 21405) for about $37 including shipping and tax, but the NAPA kit does NOT include a new idle mixture screw. Carbs Unlimited sells a kit (# KT-5365) for about $47 including shipping (fig 10). However, the Carbs Unlimited kit appears to be a re-labeled version of the McDonald Carb & Ignition kit at a much higher price. McDonald can also sell you gaskets and parts for the Fairbanks-Morse FM-J1 B78 magneto!