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
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
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
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 175º - 200ºF 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 200ºF 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