Source of new inexpensive diaphragms for CV Pistons!/?

VidGameKing

VidGameKing

Just need a couple more days.
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Joined
Dec 26, 2024
Messages
52
Age
42
Location
Sacramento, CA
Bike
2001 ST1100
Okay… I’m gonna give these guys a try. It seems to me that this could save folks some moola.

JBM "How to Measure" the motorcycle CV carburetor diaphragm goove.

How to measure the carburetor body to determine what size CV diaphragm you need and if JBM Industries has a part that might fit your vintage motorcycle. A chart showing the dimensions of the diaphragms we manufacture and notes on what motorcycles they fit.
jbmindustries.com

 
Okay ordered a set. Also removed the rubber from my #4 carb’s piston. This had the most deformation of all the diaphragms and so was the one I thought made the most sense to replace. Amazing how impactful ethanol is on our carbs…

The plastic is actually glass fiber Bakelite or some other non-thermo plastic. One must be very careful with chipping away at the top ring to remove it. Biting too much can eat into the piston cylinder itself. There is no bottom ring on our CV pistons since they are molded plastic.

The new ones are reportedly TSR latex-based rubber so they will still suffer the fate of the original ones. In theory if 100K+ miles are how long original ones last then these will need to be replaced in another 20-years.

Still I’m out $90-ish USD shipped for a set of four. That’s half the cost of one replacement piston. Good value prop, even if they only last 50K miles.

Wish I could run non-ethanol gas, but in California… lol nope. Not a chance.
 
Thanks, ‘King. So, what was the measurement of the ST1100 carb body, and what is the JBM part number…?

Please keep us poSTed. I’m quite interested in this aftermarket source. FirST I’ve heard of an alternative to the $$$ OEM.

Regards, John
 
Alright, the new CV rubbers came in. for the ST1100 I went with their model "K72-S" which fits very snuggly on the plastic piston. Plus I splurged and bought loctite super glue. the one in the red and black bottle. "ultra liquid control". total out of pocket including shipping is about $95-ish. give or take some pennies.

JBM Diaphragm for Honda VFR400R, VFR400RR, NC-30, VTR250, PC800

Instructions on the installation of the K-72-S diaphragm for your vintage motorcycle.
jbmindustries.com
 

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VidGameKing said:
Alright, the new CV rubbers came in. for the ST1100 I went with their model "K72-S" which fits very snuggly on the plastic piston.
Click to expand...
Nicely done, ‘King. Thank you for the documentation! Looking forward to the report when the ST is running again.

Regards, John
 
ST1100Y said:
Positive that the vapors from the cianacrylate won't harm the rubber? :unsure:
Click to expand...
The company recommended super glue from dollar store. Same primary chemical, but spending a buck more has me feel better… in this situation the vapors and any residue are local to the area they (JBM) refer to as the “tire” or the 3mm thick hunk of rubber that replaces the washer I cut out. This part is not intended to flex like the rest of the diaphragm is. The glue I chose very specifically stated it would bond to Bakelite as well as rubber. It was marketed as safe for fixing these two materials together.

www.loctiteproducts.com

Loctite Super Glue Ultra Liquid Control

Fast bonding of common materials. Pin-point control squeeze-grip applicator. Dishwasher safe!
www.loctiteproducts.com www.loctiteproducts.com

That being said one must be cautious to not get any on the slide itself, or the thin film diaphragm membrane. On the slide it would cause stiction and on the thin film it would restrict flexibility and produce a stiff area that would cause uneven friction. Even if one was to wipe the glue from the rubber, it would still have caused damage.

So what are we trying to say here?

  • these are cheap, $20/pop.
  • rubber was already on its way out
  • cost of rubber versus new part is silly.
  • effort to swap out in the event that there is an issue down the road is minimal
  • let’s say they don’t last as long as stock. Maybe they last 100K or 75K or hek, 50K miles. For me that would be five to six years from now, up to ten to twelve years. If I have to swap out again I’m out another hundred bucks and an evening.
Time will tell, but also time has told. This is not a new company they have been around since the late 70’s. And a google search for them brings a cornucopia of positive references. And you know you cannot lie on the internet.

Has anyone used JBM replacement carb diaphragms ? | ST1100-Carb Talk

I am looking at just buying replacement rubber diaphragms and glueing them to the existing piston slides, the procedure to cut the diaphragm mounting rings off , is shown here : http://www.jbmindustries.com/K-72-S.html Was wondering if anyone has been down this path before ? Thankyou to...
www.st-owners.com www.st-owners.com

JBM diaphragms and slide rings - V4MuscleBike.com

JBM diaphragms and slide rings 1st Gen Honda Magna and Sabre (1982-1986)
v4musclebike.com

Still I’m not trying to sell things here, or maybe I’m trying to sell it to myself. But I like spending $90 and not $1000 for carb parts…
 
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