What does it mean when fuel is leaking from this hole located behind the float bowl?
This hole is the relief, or indicator hole, coming from behind the accelerator pump diaphragm.
1. The diaphragm could be ruptured allowing gas to get behind the accelerator pump diaphragm.
2. The metering block that covers the diaphragm could be warped (so many of them are because of the way they are tightened down), allowing fluid to get past the block and into the dry area of the diaphragm socket.
The solution to #1 is easy, rebuild the carburetor, replacing the diaphragm.
While #2 isn’t quite as easy, it’s still doable. Use a flat faced grinder or flat file to flatten the block surface. As a last resort, you can apply a small amount of Permatex anaerobic gasket sealer around the edges of the diaphragm. Longterm you’ll want to look for a replacement.
Tip: When you tighten the metering block down, start from the middle and work your way out, tightening the screws a little at a time.
If you just rebuilt the carburetor and need to replace the diaphragm, keep in mind there are several different diaphragms available for the Holley 1920 and 1904 carbs. The differences include stem length, stem shape, and the diaphragm layout.
Holley 1904 pumps are available for purchase here
Holley 1920 pumps are available for purchase here
Need help identifying your 1904 or 1920 carburetor? See our tech article here for help locating and deciphering the carburetor number.