How to rebuild Mikuni SBN 38 Triple Carbs

[BlueFishCrisis]

Install the valve body assemblies with two phillips screws.



At this point, I open the balance of the parts from the kit and install the needle and seat. In this kit, the needles are packaged with the seats with a clear dome keeping them together.



Needle and seat:



I like to put a bit of 2 stroke oil on the o ring on the seat to help it into the carb body.

[BlueFishCrisis]

Push the seat into place, then install the plate retainer and screw.



Next install the new spring. Re-use the springs from a 780 sized motor.

[BlueFishCrisis]

Before installing the new needle, I like to spray a bit of multipurpose lubricant like WD40 into the seat.



Install the lever, pin, and needle assembly.



When you set this assembly in place, make sure the spring is properly seated on the raised "dimple" in the lever arm.

Not properly seated:



Properly seated:

[BlueFishCrisis]

Next install the screw which retains the lever pin. I use my left index finger to hold the assembly in place while installing the screw. Of course, I couldn't take a pic at the same time!! You get the idea.....



Next I like to apply a bit of oil to the groove where the diaphragm rests.

[BlueFishCrisis]

Install the diaphragm and cover.

[BlueFishCrisis]

Flip the carb back over to the inlet side.



Install new filter.



Install gasket, now wet with oil.



Locate the correct inlet cover for the carb you are rebuilding (MAG, CEN, or PTO), and install.

[BlueFishCrisis]

Next install the new o rings on the high and low speed adjusters. If you look closely, you will notice that there are two different sizes. The smaller o rings go on the high speed adjusters, while the larger goes with the lows....



Install the high speed adjuster, and lightly seat it. When you are ready to install on the ski, open up each of the adjusters the number of turns specified in the factory settings section of the manual.



Install the low speed adjuster, and lightly seat it.



The assembly is now complete. Perform it two more times, then move on to the next step.

[BlueFishCrisis]

Lay out the three completed carbs, and apply the backer plate. Install the 4 screws that hold the plate to the carb bodies loosely without tightening them. (Unfortunately the plate is upside down in these two pics, as you will see in the next set of photos.)



Next install the set onto an intake manifold, and secure the carbs to the manifold without gaskets. This will align the mating surface of the three carb bodies with the manifold.



Once securely attached to the intake manifold, tighten the screws holding the plate to the carb bodies.

[BlueFishCrisis]

Once the screws are tightened, remove the intake manifold for adjustment of the linkages. Tighten the spring loaded screws to connect the carb linkages together.



Adjust these screws until all three choke plates are full closed, and open fully when the choke arm is operated.

[BlueFishCrisis]

Aligned so they operate together:

[BlueFishCrisis]

Next, let's go through a mechanical approach to synchronizing the carbs. Some of the older intake manifolds have enough vacuum fitting to use a manometer to Sync, but most I have found do not.

Close the choke plates, and set the carb rack on the bench with the choke facing down.



Throttle plates are now closed.



Open the idle speed screw so the throttle plates open up about 1/4".



Adjust the linkage between the MAG and CEN carbs to "eye ball" the same gap.

[BlueFishCrisis]

The MAG and CEN appear aligned...



Next adjust the CEN to the PTO



All three now appear aligned.



Now, take a 1/16" drill bit or similar gauge wire or feeler gauge, insert in between the throttle plate and the carb throat, and slowly close the idle set screw until it starts to grip the bit. Adjust until you can slide the bit in and out of the gap.



Now move to the CEN and do the same thing. Adjust the linkage as necessary.



You will need to perform this operation over several iterations moving from carb to carb as you go. Every adjustment at each linkage affects the throttle plates in each of the carbs. Take your time until all three are identical.

[BlueFishCrisis]

Once all three carbs are sync'd, close idle stop screw to the point when the throttle plates touch the side of the carb throat, or are "fully closed". Open them just a hair so the tension of the spring is on the idle stop screw, not on the side of the carb throats.



Finally, some PTO carbs will have a matching stop screw. I like to set this at the same fully closed position that I just referenced. Adjust this screw so it just touches the tab on the linkage. This will provide the throttle plate linkage two points of contact when fully closed.



Of course, your actual idle speed will be set on the water by adjust the idle set screw. This will negate any functionality of the second stop screw if you wind up opening the carbs more to raise idle speed.



This completes the rebuild of the Mikuni SBN 38 carbs. Comments / questions welcome!!

[ripcuda]

Bravo! This is an awesome pictorial "how to". Thank you for putting it together... I know how time consuming that part can be.

I nominate this thread for a sticky!

I started going thru the carbs on my new SLT750 last night... couple things I'd mention

I like to count the turns as I gently seat both Low and High speed adjusters and record their values... so I know what they were originally set at.

And for first time carb rebuilders... doing one carb at a time will allow you to look at one that's still assembled for reference. Although... with your very thorough pictures and descriptions... this shouldn't be necessary.

Lastly... I've had luck just pulling the carbs apart (after removing the backing plate) without backing off any of the spring loaded throttle and choke linkages. Then when I put them back together... I've just gently pried the springs back to slide them back together. Though this can be tricky with only 2 hands since you have to finagle both the throttle and choke linkages while sliding them back together. Your way looks easier!

Cheers!

[BlueFishCrisis]

You can test pop off pressure utilizing this inexpensive tester....