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JBM Industries - Mikuni BST40 Flat Slide

Diaphragms
M-81

$29.00 USD

Mikuni BST40 Flat Slide Carburetor Diaphragm replacement instructions.

Installation Steps

Step 1

Remove the old plastic retainer from the throttle slide with sharp knife as shown. Plastic retainers warp and leak with age.

Step 2

Use a 1/8" drill to open the holes slightly to fit the rivets. Air filter side of slide shown. Use aluminum rivets only.

Step 3

Rib presses into the rubber and forms a seal when rivets are set.

Step 4

Insert both rivets in diaphragm and slide before setting to make sure alignment is accurate.

Step 5

Set the rivets with a Pop Rivet Gun, making sure to press down for an air tight seal.

Step 6

A set rivet must be drilled out to be removed.

Step 7

Roll the diaphragm to install. (A mark of paint can make
it easy to get the slide back in facing correctly in the future.)

Step 8

Be sure the spring guide stud is not bent and the spring does not rub on the hole in the diaphragm or slide.

Finishing Notes:

Install the top cover and check for smooth operation.
A smoothly moving slide is needed as only a very light return spring returns the slide.
A small hole in the throttle slide is what allows the venturi vacuum to operate the slide. This constant velocity carburetor prevents stalling and gives smoother running than other direct acting carburetors.
Some folks want to use Super Glue adhesive on the slide, and this will not harm anything, but be sure not to get adhesive on the sides of the slide. (This prevents rotation or removal.)
Note: Never use carb cleaner or compressed air.

M-73 Diaphragm

The "Hard Rubber Tire" is what holds the diaphragm to the slide.

BS34SS CV Carb has a 70 mm diaphragm

S34SS CV Carburetor Diaphragm is shown above and below. The outer diameter of this diaphragm is 70 mm. (Part M-70)

Roll before spring and cover Installation. (Part M-70)

Click here to see how to remove and restore a damaged slide.

Yamaha Venture Notes: The 73 mm Mikuni Diaphragm fits the Venture models even with plastic slides. Remove the short metal tube from the plastic slide if loose, and remove the rings. Place the JBM Industries diaphragm on the metal tube, and then push tube into the plastic slide. Use Super Glue on metal tube. If metal insert tube is not loose, just cut the rings off plastic slide with side cutters as shown.

Click here for more info on MIKUNI CARBURETORS and Yamaha 650 information.