Ethanol- Resistant Carburetor Accelerator Pump Diaphragms Are Available

John Bissell

Contributing Member
Each year I rebuild a handful of carburetors for people and yesterday I was asked to identify the source of gasoline leaking from a Holley four barrel carburetor on a fellow’s classic car in McLean, VA. The fuel leak was quickly located coming out the bottom of the carburetor from its accelerator pump assembly, caused by an apparent crack (split) in that pump’s rubber diaphragm.

This is an easy problem to fix, just remove the carburetor, turn it upside down to drain the fuel out, unscrew the four screws that attach the fuel pump assembly to the bottom of the carburetor’s fuel bowl, and replace the bad diaphragm with a new one. Then put everything back together – altogether about a 20-30 minute job.

I didn’t have a new diaphragm to put in the carb, so last night I went on line to to buy a few diaphragms so I’d have some spares also. The original fuel pump diaphragms on Holley carbs are made of rubber and I wasn’t certain whether that rubber was the kind that could be damaged by the ethanol (ethyl alcohol) that’s now in modern gasolines. Notably, Barry Grant, Demon, and Quick Fuel carburetors also use the same accelerator pump diaphragm as Holleys.

Summit Racing’s web-site did list the rubber diaphragms, Holley #135-5 for the common 30cc pump size ($3.28), and #135-7 for the larger 50cc pump found on some of the racing carburetors ($16.53). These are listed as suitable for “gasoline”, with no mention of “alcohol”.

But I also found a series of pump diaphragms that were listed as compatible with both “alcohol and gasoline”. They’re made of GFTL (one of the family of Viton materials) instead of rubber. The part numbers for these are Holley #135-10 for the 30cc size ($9.89 each) and #135-9 for the 50cc size ($16.53 each), and Summit Racing’s own line #SUM-200250 for the 30cc size ($7.97) and #SUM-200254 for the 50cc size ($11.97). I bought five of Summit Racing’s own 30cc diaphragms and two of their 50cc versions.

These GFTL “alcohol” compatible diaphragms, although the more common 30cc size is more expensive than the same size rubber version, should eliminate any possibility of problems due to today’s ethanol gasolines.

So I pass this info on for everyone who, like myself, wasn’t aware that there are better carburetor accelerator pump diaphragms available should you experience this fuel leak on your Holley-type carburetors.

Call or text me at 703-509-7611 if you have any questions.

John Bissell