With the country going to ethanol in the gasoline, what do I do, if anything, about my SU carburetor diaphrams? I've always been told NO ALCOHOL of any kind. Indeed, some years ago I got some alcohol in the tank unawares and had to rebuild a gooey mess. Thanks.

Submitted by SC20-30420CJ on Wed, 01/30/2008 - 10:30

Joe Curto's response is very interesting. I do know that maintenance on antique aircraft with rubber bladder fuel tanks usually involves putting a few drops of a small molecule oil such as Marvel Mystery Oil into each tank of fuel. Modern fuels contain chemicals such as tolulene which dry out rubber and apparently the oil helps. Some of these aircraft are over 50 years old and still using their original rubber tank bladders.

John Testrake
Jaguar Association of Greater St. Louis
74 XJ12L rhd, 85 XJ6

Submitted by SC38-21185J on Thu, 01/24/2008 - 18:08

Here is the response from Joe Curto:

Patrick I have nightmares about HD diaphragms, there are many
diaphragms out there , 2 greens, 1 blue and a few black. The greens are
pretty old after market, the Blue is a currant one , I make and use a
black one sold in my kits "British Superior" So far we have had very
little problems. I am involved with a fuel systems study sponsored by
Haggerty Old Car Insurance specifically studying Ethanol fuel and old
car fuel systems . The other thing about this new fuel is that there
are regional differences in the fuel and in some places no problem
other places same product but with problems. I am pretty much a
sounding board for SU and Zenith problems and I can count on 1 hand the
problems with my diaphragms and I have made about 10,000 . I am not
sure if I answered your question, but hopefully the Haggerty study will
give us some better direction. Even old original diaphragms which were
rubberized fabric got hard as a potato chip and snapped when the choke
was actuated.

Submitted by pharmon@jcna.com on Thu, 01/24/2008 - 09:15

I agree with the ethanol issue not being a problem with the diaphrams. In a similar vein, though, I have had problems with octane booster products such as Moroso. The stuff attacked my carburator gaskets to the point they were partially blocking the throat.

Submitted by SC38-21185J on Thu, 01/24/2008 - 09:11

This is the first I've ever heard of Ethanol/alcohol damaging or melting SU diaphragms! I have a hard time believing that they can't handle it, and I'm a bit skeptical. If that were the case, there would have been a widespread sellout of SU repair kits before now.

But what do I know, I'm not a chemist. I put this question about Ethanol to Joe Curto who has forgotten more about SU carbs that I will ever hope to know. When I get a response, hopefully sometime today, I will post his reply.

Patrick McLoad
JC Houston