This is a relatively recent annoyance that we have battled more than once in the last year. It generally happens this way. Our Holley carb-equipped small-block El Camino sits for several months. Then when we start the car, it runs for about 20 seconds, belches black exhaust smoke, burbles, and dies.
A quick investigation of the carburetor reveals the primary (or secondary) bowl is overflowing and drowning the engine in fuel. At first, we thought it was just dirt in the needle and seat, but that was not the cause. What we discovered is that the O-ring installed around the needle and seat assembly tends to get brittle and shrink and eventually will not seal properly. When we tried to remove the O-ring from the needle and seat cartridge, it split in two.
The next problem became finding the right O-ring to replace it. Holley does not sell this O-ring separately but we did find two companies – Allstate Carburetors (allstatecarbs.com) and Bo Laws Products (blp.com) that offer this O-ring for sale. The BLP part number is 9020-008. Both companies sell individual O-rings and we’d recommend buying a handful of them as you might find your friends will soon be in need of these as well.
A standard 008-style Buna-N O-ring will work in a pinch, but we’ve found that these are slightly undersized and will not withstand the nasty chemicals in today’s gasoline. Everybody wants to blame the alcohol in gasoline for these problems, but we’ve tested these O-rings in both pure rubbing alcohol and E85 nothing happens. But if you soak a Buna-N O-ring in a gasoline additive like toluene the rubber will quickly deteriorate. What kills these O-rings is the additives called aromatics which are benzene, toluene, ethylbenzene, and xylene – abbreviated to BTEX. These chemicals, which can represent anywhere from 18 to 25 percent of the volume of pump gasoline, are the real culprits that destroy carburetor parts like O-rings, accelerator pump diaphragms, and power valves.
The best solution for this problem is a Viton rubber O-ring that is resistant to these nasty chemicals. Holley, for example, now sells a green accelerator pump diaphragm that is made of Viton and likely an O-ring is probably not far in the future. But for now, you may have to replace these O-rings every year to avoid issues. A little prevention might go a long way toward heading off future problems.