CV Carb Diaphragm Tips

By Bill Denton


Hi all,

A few points to make here.

First and most importantly, you should not, under any circumstances, use sealant or adhesive on CV carburetor diaphragms to hold them into the groove in the body.  Not only is it not necessary, but it will also adversely affect your ability to R&R the next time you need to go in there for whatever reason.

Second, petroleum jelly does not attack the elastomers commonly used in CV carb diaphragms.  OTOH, PJ is not necessarily very good for latex rubber, the kind used in the bedroom, so perhaps this myth began near the bed and radiated outward from there <g>.

Mini-tech session - There are two basic reasons why synthetic rubber CV diaphragms don't stay in place very well upon carburetor reassembly.

Reason #1 - the top rim diameter has gotten larger.
Reason #2 - the top rim diameter has gotten smaller.

Both events mystify the mind <g>, but there is an explanation and a cure.

The explanation is solvent gain/loss.  Rubber swells or shrinks when it gains or loses interstitial solvent.  A new piece of rubber contains a certain amount of solvent from the manufacturing process.  Lose some solvent through heat and age and the rubber will "shrink".  Gain some solvent through contact with certain hydrocarbon mixtures (like gasoline/additives blends) and the rubber will "swell".

In the case of the swollen rubber diaphragm, the cure is a simple one. Here's what you do.  Send your wife to the mall, then set your kitchen oven to 175 - 200F and pop the offending swollen CV diaphragm in there for about 30 minutes on a piece of tin foil or small pan.  Cool and refit.  In most cases, this will have cured the swelling condition by driving off the excess solvent. In some cases, you may have to treat it for a longer period of time, but in no case should you have to go above 200F in oven temp.

In the case of the shrunken rubber diaphragm, you'll have to find a suitable solvent to reabsorb into the rubber to re-swell it to normal size.  I have used several different items commonly found on my chemical shelf in the mc garage, but for liability reasons, I hesitate to mention any of them here. In addition, if I told you what I have successfully used in the past, many of you would scoff at the idea of using (insert the name of a solvent borne carb or brake cleaner here).  In mild cases of shrinkage, a good dousing of Hondaline (or equivalent) silicone spray and hand kneading will make the old rubber pliable enough to fit into the groove long enough to get the spring and cap back in place.  As a matter of fact, I routinely treat the CV rubbers with silicone spray whenever I have them out of a carburetor for any reason.

Enough for now.... good luck and happy carb rebuilding!

Bill in Yardley, PA
wkdenton@comcast.net