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Old 06-30-2011, 10:51 PM
Toolmaker Tim Toolmaker Tim is offline
XSive Maximus
Join Date: May 2008
Location: South Bend, Indiana
Posts: 671
Carb Diaphragm Replacement Option

Carb Diaphragm Replacement Option
by Toolmaker Tim

Everyone who rides a XS1100 knows that sooner or later their carb diaphragms are going to resemble one of those astronomy displays when you hold it up to a bright light. Pinholes can be seen everywhere. The slides in our carbs work off of vacuum, so the pinholes affect throttle response, smoothness and horsepower. Anyone who has priced a NOS, aftermarket, or used slide has suffered sticker shock. There have been several threads about replacing the diaphragm itself, but they involve cutting the retaining rings and then using the same rings to retain your new diaphragms, or CAREFULLY stretching the new diaphragm and reseating it between the retaining rings. Doing that involves the risk of tearing your new diaphragms during the installation, and the replacement diaphragms are not cheap either. JBM Industries has a replacement diaphragm for the Mikuni 34MM CV cards that takes the risk out of replacing this vital part.

added by T.C.
These are the early model 78-79 model carbs with the smaller vacuum slides. The 80-81/82 series carbs use 1mm larger diameter slides and both the slides/diaphragms are obtainable as a single replacement from several sources:
www.mikesxs.net Part #20-6028 $80.00
www.partsnmore.com Part #20-6028 $75.00

Further investigation on the JBM site, reveals that folks with LATER model/styled BS34 carbs ie, 81 SR250 which uses the same diaphragm assembly as the 81 XS11 were able to install these replacement diaphragms onto the larger slide. The later slides are 79mm where as the early slides are 78mm, but their INNER diameter is the same, and so these replacement diaphragms may be a slightly snugger fit ONTO the larger slides, but their outer groove diameter of 73mm is the same!
The following pictures show how simple the replacement is;

Here you see the relationship of the vent hole to the tab sticking off the flange of the stock diaphragm.

The first step is to cut the top retaining ring off the slide using a pair of wire cutting pliers.

After cutting the ring it comes off easily and the old diaphragm can be removed.

When the bottom ring is cut it comes off the same way as the top, leaving a bare slide.

Your new diaphragm, ready to be mounted.

You can see the identifying marks on the new part clearly. The 73 at the top refers to the diameter of the outside sealing ring, the BS34 to the carb it fits in, and the small dot on the bottom lines up with the vent hole to give you a reference point to properly position the vent towards the engine side of the carb.

The side view shows the rubber ring that replaces the 2 plastic rings on the stock slide.

Start on one side of the slide and work your way around the slide body like mounting a tire.

Your full assembled slide with a new diaphragm.

The dot circled in red is your refence point for aligning the vent hole toward the engine side of the carb. It needs to be closely to lined up with your vent.

The dot again shown facing the engine side of the carbs.

The old diaphragm with the alignment tab showing the vent in relation to the engine side of the carb. If anything, the dot makes it easier to make the dot-vent-engine line up easier. Some stock slides have the vent offset one way or the other after they were assembled. You were stuck with whatever you got from the factory.

The outside seal ring fits nicely in the groove and doesn't try to squirm out when you put the carb tops on. Before you put your new diaphragms in, it's a good idea to make sure there are no sharp edges where the diaphragm fits over the inside edges of the carbs. I used a red Scotchbrite pad to smooth things down before assembly.

After you are done assembling the tops on the carbs, you can check for a good seal by blocking the diaphragm vent hole with your thumb after pushing the slide up. If the seal is good, the slides won't come down on their own until you remove your thumb. I did notice the slides will set lower to the bottom of the carb throat than the stock diaphragms did with their plastic rings.

The diaphragms can be gotten from www.jbmindustries.com . You can order on line with PayPal or call the owner at the phone number listed. They offer 2 diaphragms for the Mikuni CV34 carbs. For our purposes we need the BS34 with a 73mm groove. The diaphragms for the BS34II or the BS38 won't fit.

As of the date of this tech posting, their prices were $19.50 each, with $3.00 TOTAL shipping fees PRIORITY MAIL.

Last edited by Ken Talbot; 07-15-2011 at 12:03 AM. Reason: minor typo "no sharp edges", not "now sharp edges"

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