Yamaha XJ550 diaphragm replacement tutorial - Mikuni BS-28

Ravivos said:

Hi,

Since the diaphragms on my Yamaha XJ550 carbs were gone, i thought about replacing them, this will be a quick tutorial on the how-to, since this information is nowhere to be found across the web.
unfortunately, these diaphragms cant be replaced without replacing the slider as well, which sets the price to over $100 per slider/diaphragm set.
looking for a replacement, i came across a company called JBM industries which manufacure different rubber products for many different models and applications.
i contacted mr Don James regarding the diaphragms for the BS-28 but he said they never designed one for it... he stated that they are not motorcyclist and their knowledge is limited to whatever there is on their website (a lot, but no Mikuni BS-28).

so i took some measurements and decided to order a set of four T-63 diaphragms, hoping they will fit.
Don set the diaphragms very quickly, and once they arrived i started working.
hope the following will be informative and helpful for those who need to replace their worn out diaphragms on their carbs... there is a lot of information over their website, so it is recommended to take a few minutes and have a look, you might find something that you need (like diaphragms or rubber boots for your KZ400 carbs).

first, take off the carb's top and take out the slider, you can tare away the old worn diaphragm.

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using a cutter, cut the plastic ring and metal ring that holds the old diaphragm into place

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make sure the new diaphragm is clean and with no holes in it, also de-burr the edge of the slider to avoid tearing the new diaphragm.

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slide the new diaphragm over the slider, it will be a tight fit, make sure that the thick rubber boot of the diaphragm is seated into the slider's groove.

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put the slider/diaphragm back into the carb, make sure the hole at the bottom of the slider is facing the engine.
make sure the outside lip of the diaphragm is seated into the carb's top groove.

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close the carb's top cover (dont forget the spring), and slide the slider upwards with your finger... when releasing it, is should slide back down slow, approx 0.5-1 sec to get from top to bottom, this is an indication that the diaphragm is mounted correctly and sealed properly.

that is it, rather easy...

when i mounted mine, i made sure the diaphragm isn't binding or interfering the slider's movement... so the slider remain fully functional with its full dynamic range of movement.
hope this information will help someone get his bike back on the road.

Regards.
Ravivos.

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