CV Carb Diaphragm Replacement

Has anyone ever replaced the diaphragms on a set of BS34 Mikuni carb slides? There are a few different diaphragm options on the market including JBM, and some companies in the the UK and China. The original Mikuni parts use a fabric reinforced rubber material but many (or maybe all) of the aftermarket options are all rubber.

Reason for asking is because I recently picked up a rebuild project and the idiot previous owner used ultra black RTV to seal the diaphragm's to the carb bodies. Such a waste. The diaphragm's themselves look to be in good shape but the RTV is holding fast and I can't figure out how to get it off.

Ed, I have not done so but have read posts from others that have had good things to say about JBM. As to removal of the RTV there are products that claim to do so. Permatex and 3M (there are others) have products that claim to remove silicone based RTV. I have not tried either and have no idea what they might or might not do to the diaphragms. But would think it has to come off, and short of attempting to scrape it off I would think some type of remover to help soften it would be of use.

Just my 2 cents.

I replaced the diaphragms on two sets of BS34s within the last three years (using the JBM replacements). I don’t have enough miles on them to say for sure how successful the process was. The main concern I had was that the groove into which the rubber collar fits was significantly wider than the collar. It wasn’t clear if it should be installed in the middle, top, or bottom of the groove. I eventually decided to bias it towards the bottom. A secondary concern I had was that they seem a fair amount stiffer than the original items. Again - I don’t really know if my concerns are legitimate or not. The bike seemed to run OK after installation and re-syncing.

[hr]I recently replaced the diaphragms on my xs650 and 750GPz carbs. I used the JBM brand diaphragms and they went on easy peasy no problems. I've ridden the XS650 and all seems good.

I'll give the JBM parts a try but first, I'm going to try to get the RTV off. Mineral spirits = no good. Isopropyl Alcohol = no good. Acetone = no good. 3M Adhesive Remover = half way decent. Soak for a minute or so then scrape RTV with fingernail. This stuff has Xylene in it which I think is the main active ingredient. It doesn't seem to be harming the diaphragm material itself which is a concern with strong chemicals. Fingers crossed there aren't any tears under all the goop.

I hope you are scraping with your fingernail through a glove. Xylene is pretty bad stuff.

TexasKZ wrote: I hope you are scraping with your fingernail through a glove. Xylene is pretty bad stuff.

I should be dead 10x over from all the crap my body has ingested over the years. Regarding the xylene, thank's for the reminder. I'll try to find a way to scrape off the junk without killing myself once again.

Ha! Me, too.
In my previous career, I worked with a guy who would routinely stick his bare hands and forearms into a vat of sodium cyanide solution to retrieve parts that were soaking.

Got most of the RTV off other than some very thin residue on 3 of 4 slides. The last slide is beyond saving though; found tears under the goop. How come the tear had to show up on the last slide instead of the first? Dang.

Ed, I feel ya. I never find my keys in the first place i look. I think its a mechanics curse.

Took on the diaphragm replacement project. Couldn't stomach the $100 cost for JMB's so got some from China for $12 shipped and taxed. The quality looks excellent. Used my lathe and bored down into the slide a cm or so and cut a recess in the top plastic cap. Then made some T shaped sleeves to hold it all together using 3M DP420 Black epoxy. Last step was to dress down the aluminum insert smooth with the slide on the inside as well as the top. Good fun.:lol:

Of course the best laid plans are not without potholes in the way...

My elcheapie Chinese mini lathe decided to break a belt while working on this project. I've got the 8" version mini lathe which is more beefy than the typical 7" jobbie which proliferated like crazy since they hit the scene. Anyway, the build quality on these units is suspect, which I was trying to ignore until this happened. An investigation as to why the belt broke showed that the headstock bearings are binding so now I'm into a project to replace those. Can't say "good fun" on this one though.