How to rebuild Mikuni SBN 38 Triple Carbs

[BlueFishCrisis]

This thread will describe how to rebuild the Mikuni SBN 38 Triple carbs found on all Polaris Fuji Engines. Note that the 650, 750, and 780 versions are all the same, with a few minor variations I will point out.

Carb kits can be obtained from many different sources. Ensure that only Genuine Mikuni parts are used. For this thread, the carb kit was obtained from John Zigler at Watcon.com. At the time of this post, the kit was $150 USD plus shipping.

The parts included in this kit are 3 ea o rings for the high and low speed adjusters, 3 ea needles and seats (size 2.0), 3 ea springs (size 80 gram), 3 ea clear check valves, 3 ea filters, and 3 ea diaphragms.

Here is a layout of typical tools and supplies needed for the rebuild:



Carb cleaner, multipurpose spray lubricant, small and large phillips, small and large flat heads, scraper, 2 stroke oil, and paper towels. I also used various small pics and brushes as you will see throughout the photos.

[BlueFishCrisis]

Start by unbolting the carbs from the intake manifold.



Once you have the carbs off, remove the screws holding the alignment plate to the 3 individual carbs.



You will also need to loosen the spring loaded linkages between the carbs connecting the throttle plates and the choke plates.

[BlueFishCrisis]

Next, remove the fuel inlet cover. Pay attention to which cover comes off of each of the various locations ( MAG, CEN, PTO). The covers are different for each location, and may vary based on year and motor size. All, however, are interchangeable and will work on any version.



Remove the fuel filter, and discard.



Remove the black rubber gasket. Keep this as long as it is in good condition. The kit does not come with a replacement.

[BlueFishCrisis]

Before removing the adjuster needles, you can count how many turns it takes to screw each of them in so they are fully seated. Record this measurement for each carb, each screw. When it its time to Re-assemble, you now have documentation on where they were. Or, you can reset to factory settings.

Next remove the low speed adjuster. Note the o ring. If the adjuster comes out without one attached, you will need to reach in with a small pick and retrieve it. Often the washer stays in place covering the o ring as it is painted after it is installed.



Remove the blue cap on the high speed adjuster with a pair of needle nose pliers. I choose to discard these and not re-install after the rebuild.



Remove the high speed adjuster, keeping an eye out for the o ring.

[BlueFishCrisis]

Flip the carb body over, and remove the diaphragm cover with 4 phillips head screws. Note these have lock washers on them that need to be retained.



Remove old diaphragm.

[BlueFishCrisis]

Next, remove the screw holding the lever arm pin. This may be corroded and tough to remove. Use lots of downward pressure when removing. You can also use an impact screwdriver to assist in removal of any of the screws in the carb. It is essential to remove the fuel inlet cover first, so the carb will lay flat on the bench when trying to remove these screws.



Next, remove the lever arm, pin, and needle.



Note the spring that remains when the arm is removed. This spring will need to be retained for 780 carbs, as the replacement provided is for the 650 and 750 models. If you are rebuilding a 650 or 750 set, discard the springs.

[BlueFishCrisis]

Here we will remove the seat retaining screw. This compresses a small plate the holds the seat firmly in place.



With those removed, use a pair of needle nose pliers to grip and remove the seat. Pull straight up with a decent amount of force. The more corroded your carbs, the more difficult it will be to remove. PB Blaster or similar may help in removal.

[BlueFishCrisis]

Next, remove the screws holding the valve body assembly in place. Lift off the valve body assembly with pliers. You may need to wiggle it a bit due to corrosion.



Remove the rubber valve body gasket and retain for reinstallation.

[BlueFishCrisis]

Remove the low speed jet using the small flat head screwdriver. Find a screw driver that just fits inside the hole for the jet. Remember the jets are brass and therefore a softer metal. The larger the blade you use, the better chance you will NOT cause damage to the jet.



Remove the high speed jet.



Retain the jets for re-installation. The size of the jets are imprinted on top of the jet next to the screwdriver slot for the high speed, and on the side of the jet for the lows. Ensure you have the proper jets for your ski, especially if you bought the carbs separate from the ski. Reference your owners manual for the correct sizing.

[BlueFishCrisis]

Now that the old carb is completely disassembled, it is time to clean it. Be sure to use spray carb cleaner and compressed air to clean it out. Others prefer a full carb bath and small gauge wire to clean out the passages. Ensure every nook and cranny is clean and corrosion free, including the jets themselves.



You may need to use a wire brush and or picks to clean out corrosion.

[BlueFishCrisis]

After the carb body is cleaned and prepped, take the valve body assembly and flip it over. There is a small phillips screw and a retaining plate. Look closely and you will see a small clear plastic check valve. These need to be removed and discarded as new check valves are included in the kit.



Finally, remove and discard the old o rings from the high and low speed adjusters. These too are included in the kit.

[BlueFishCrisis]

Now that everything is clean and prepped, I next like to layout all my parts and screws to make sure I have everything needed for reassembly. This is the time I take inventory, and decide to replace any parts that are beyond recognition, badly corroded, or had to be destroyed to be removed. Most of the parts were in good condition, so I am re-using them here.



At this time I also like to take the "old" rubber parts and stick them is a small zip lock bag with some 2 stroke oil to prep them for re-assembly.

[BlueFishCrisis]

Let's start re-assembly with the Valve body assembly. Locate the new clear check valves in your kit. Be careful when opening, as they are very light and easily will disappear into your surroundings.....



Install the new check valve with the retaining plate and screw. Note the check valve has two holes. One for the screw, and one for the locator pin on the plate. This keeps the valve in the proper position once installed.

[BlueFishCrisis]

Pay close attention, as this valve CAN be installed backwards.

Correctly installed:



Incorrectly installed:



All three complete......

[BlueFishCrisis]

Next let's install the jets and the valve body gasket.

High speed jet:



Low speed jet:



Valve body gasket:



Note the gasket is wet with oil from the zip lock bag.....