VM38-SS - VM38-9 Comparison
VM38-SS on left, VM38-9 on right
Mikuni builds carbs for manufacturers (OEM) and the general public (aftermarket). The OEM carbs are engineered to fit a particular OEM product, while aftermarket carbs can be adapted to fit many different motorcycles. Normally Mikuni negotiates a license for the OEM carbs. The OEM carbs, as well as the parts for these carbs, can only be obtained through an OEM dealer. Aftermarket carbs can be obtained through distributors like Niche Cycle.
This is a comparison of the OEM VM38-SS and the aftermarket VM38-9. But this comparison could just as easily be between the VM36-SS and the VM36- 4 as all these carburetors use a very similar body and many of the same parts.
The VM38-SS was used by Mikuni in a number of OEM bikes. Not all VM38-SS were identical from manufacturer to manufacturer. Usually the differences were minor, like the location of the idle screw. The measurements provided below are from a VM38SS used on a Yamaha WR500ZD (seen in the picture to the right). These measurements may not match all OEM VM38-SS carbs. New parts for this carburetor have only ever been available through OEM dealers and are probably no longer being manufactured by Mikuni. Some new old stock parts are available and can occasionally be found on eBay.
The aftermarket, VM38-9 is a very similar carburetor to the 38-SS. Several parts from aftermarket carb will fit the OEM carb and in most cases, it’s not too difficult to swap the carbs out entirely. Below is a comparison between an OEM, Yamaha carburetor used on the WR500ZD and an aftermarket VM38-4. This comparison was originally made because a customer wanted to trade out a VM38-SS for a VM38-9.
This is a comparison of the OEM VM38-SS and the aftermarket VM38-9. But this comparison could just as easily be between the VM36-SS and the VM36- 4 as all these carburetors use a very similar body and many of the same parts.
The VM38-SS was used by Mikuni in a number of OEM bikes. Not all VM38-SS were identical from manufacturer to manufacturer. Usually the differences were minor, like the location of the idle screw. The measurements provided below are from a VM38SS used on a Yamaha WR500ZD (seen in the picture to the right). These measurements may not match all OEM VM38-SS carbs. New parts for this carburetor have only ever been available through OEM dealers and are probably no longer being manufactured by Mikuni. Some new old stock parts are available and can occasionally be found on eBay.
The aftermarket, VM38-9 is a very similar carburetor to the 38-SS. Several parts from aftermarket carb will fit the OEM carb and in most cases, it’s not too difficult to swap the carbs out entirely. Below is a comparison between an OEM, Yamaha carburetor used on the WR500ZD and an aftermarket VM38-4. This comparison was originally made because a customer wanted to trade out a VM38-SS for a VM38-9.
The differences in the length of the intake bells can easily be seen in this image
Carburetor fittings:
Both carbs share the same outside diameter (OD) at the manifold spigot (43mm). The intake bell has the same OD (62mm), but the bell is much shorter on the VM38SS (12.14mm) than the VM38-4 (30.66mm).
Overall dimensions:
Height is measured from the base of float bowl drain to top of mixing chamber cap (excluding cable adjuster) The width is measured from edge of intake bell to edge of spigot.
VM-38-9 VM38-SS
Height: 154mm (bottom of float bowl plug to top of cap) 154mm
Width: 116mm (end of spigot to end of intake bell) 97mm
NOTE: The VM38-9 comes with an extended intake bell which can be trimmed down to make the width of both carbs close, if not exactly the same. The extended intake bell on the VM38-9 can be seen in the picture to the right.
The intake bell can be trimmed with a Dremel tool with a diamond cutting wheel, a hacksaw with a carbide toothed blade, a band saw with a metal blade or you could have a machine shop could trim it down using a mill.
It is critical to block all passages on the carburetor before cutting the bell. Use good quality masking tape to tape off the spigot and around the top of the cap. Also make sure you plug the venturi tightly with a paper towel or cloth. Be sure that the air jet and orifices at the base of the intake bell are taped up or covered. Finally when cutting is complete, it is best to blow off the carb with compressed air and to inspect to make sure no cuttings were left behind.
Both carbs share the same outside diameter (OD) at the manifold spigot (43mm). The intake bell has the same OD (62mm), but the bell is much shorter on the VM38SS (12.14mm) than the VM38-4 (30.66mm).
Overall dimensions:
Height is measured from the base of float bowl drain to top of mixing chamber cap (excluding cable adjuster) The width is measured from edge of intake bell to edge of spigot.
VM-38-9 VM38-SS
Height: 154mm (bottom of float bowl plug to top of cap) 154mm
Width: 116mm (end of spigot to end of intake bell) 97mm
NOTE: The VM38-9 comes with an extended intake bell which can be trimmed down to make the width of both carbs close, if not exactly the same. The extended intake bell on the VM38-9 can be seen in the picture to the right.
The intake bell can be trimmed with a Dremel tool with a diamond cutting wheel, a hacksaw with a carbide toothed blade, a band saw with a metal blade or you could have a machine shop could trim it down using a mill.
It is critical to block all passages on the carburetor before cutting the bell. Use good quality masking tape to tape off the spigot and around the top of the cap. Also make sure you plug the venturi tightly with a paper towel or cloth. Be sure that the air jet and orifices at the base of the intake bell are taped up or covered. Finally when cutting is complete, it is best to blow off the carb with compressed air and to inspect to make sure no cuttings were left behind.
|
External comparison:
While the air screws are in the same location (right rear looking down intake bell), the idle screw is on the right next to the air screw on the VM38 and on the left on the VM38-SS. The air screws on both models appear to be the same. This is also the case with the idle screws; however, the idle screw on the 38-SS uses a lock-nut to hole the screw in place, while the 38-9 uses a spring. The choke mechanism is similar in design for both models, but not the same. The plunger on the 38-SS is longer and is of a different design. None of the parts of the choke are interchangeable other than the starter lever spring plate. That being said, the plungers have the same OD and share the same threads. The 38-9 mechanism puts an extra spacer under the spring plate, so if you compare the throw distance from the base of the spacer on the 38-9 compared to the bottom of the nut head on the 38-SS, it is very similar and the complete VM38 choke assembly will fit in the VM38-SS. However, because we haven’t replaced one on a working bike yet, we do not know if the choke circuit will be completely effective with this swap. The throttle body caps are different and may vary due to OEM models. With the Yamaha-specific 38-SS, the cap is made of plastic and is designed so the cable will drop right in and is held in place with a clip. The aftermarket 38-9 has a cap with a threaded-hole and a cable adjuster. If needed, Niche Cycle Supply manufactures a billet aluminum cap that can replace the Yamaha cap. The fuel nipple is a bit shorter on the 38-SS, but they both have the same OD so both carbs can use the same size fuel line (8mm). The float bowl is similarly sized between both carbs. The 38-SS has an overflow exiting directly from the float bowl. The 38-9 has no provision for this. There are also guides for overflow lines cast into the body of the 38-SS. The aftermarket 38 has small, stamped guides that attach to the float bowl screws. The float blow drain plug is interchangeable between both carbs. Internal comparison:
The big differences between the carbs are internal. The throttle slides are different between both models and cannot be interchanged. The 38-SS differs in shape and the throttle cable does not attach the same way as the 38-9. There is also no spring seat plate. However, both carbs use the same throttle spring and the same jet needle could be used in both. The float bowls are not the same between the carbs, but they do use the same float bowl gasket and screws. The float needles are similar in design and a new, 38-4 float needle and seat will fit into the older 38-SS carb, but the correct size for the SS may not be available. While not identical to the 38-SS, the aftermarket, 38-9 baffle plate will fit into the SS, and the small, round gaskets used between the float needle assembly and baffle are identical in both carbs. As mentioned before, the float systems are completely different in both carbs. The 38-SS uses a copper float with two separate float elements soldered to a single beam with an adjustable tab in the center. The aftermarket carb uses two independent, plastic floats suspended on pillars running up the side of the float bowl. These floats lift the float arm which has an adjustable tab. The main jets are dimensionally identical and use the same threads, so new aftermarket main jets for the 38-9 will fit the SS. However, the standard main for the 38-9 is not the same size as the 38-SS. The needle jets are very similar in design, except the SS version is a bit longer with a boss added to the top of the jet where the main jet screws in. On the aftermarket carbs, there is a screw-in main jet extender that takes place of the boss. This is where the main jet screws in on the aftermarket carb. The needle jet/main jet assembly (needle jet, jet extender and main jet) from the aftermarket carb will fit in the OEM carb, but the individual parts are not interchangeable except for the main jet. The pilot jet on the OEM we are using as a sample has been damaged and cannot be removed. An unsuccessful attempt was made to drill out the remains of jet by the previous owner, so interchangeability of the pilot jet cannot be determined. However, the pilot jets on both carbs are in the same location and have the same size opening, so the pilot jets are probably interchangeable if the correct size is available in an aftermarket part. What we've learned...
Many of Mikuni original manufacturer (OEM) carbs are very similar to the aftermarket carbs. Aftermarket parts can often be used to rebuild old OE carbs and, it the carb is too far gone, the aftermarket carb can often be swapped for the original equipment carb. |
The bright, new screws on the outside edges are from the VM38-9
The VM38-SS choke plunger can be seen on the right
The plastic cap on the right is on VM38-SS. Not all models had plastic caps
Similarities between these carbs can easily be seen looking through the venturi
Castings between the carbs are almost identical
The VM38-4 has a longer nipple for the fuel line
The camera angle makes it appear as if the bowls have a different shape, but they are really very similar in size
|
