You are standing in the shed with a fuel connector in your hand and no idea which of the three it is. The badge on the cowling only gets you part of the way, because Mercury changed the design partway through, and fittings get swapped over the life of a boat by people who are long gone.
The good news is that the whole answer is in two measurements. A dual barb connector has a fuel barb and a locking barb, and the diameters of those two barbs are what separate the three common types. Once you know the numbers you can sort a connector by eye in about ten seconds, and confirm it with a vernier if it matters.
The three types, and the numbers that tell them apart
| Outboard | Fuel barb | Locking barb | Looks like |
|---|---|---|---|
| Evinrude and Johnson | 6mm | 5.5mm | Two thin barbs |
| Yamaha and Mercury, pre-1999 | 8.2mm | 5.6mm | One thick, one thin |
| Mercury, 1999 and later | 8.2mm | 6.7mm | Two thick barbs |
The fuel barb is the one the line from the tank goes onto. The locking barb is the smaller second barb on the same fitting.
Sorting one by eye
- Two thin barbs. Evinrude or Johnson. The fuel barb is 6mm, so the whole fitting looks slight next to the others.
- One thick, one thin. Yamaha, or a Mercury built before 1999. The 8.2mm fuel barb next to a 5.6mm locking barb is an obvious step in diameter.
- Two thick barbs. Mercury, 1999 and later. Both barbs are substantial, 8.2mm and 6.7mm.
The one measurement worth taking
If the fuel barb is 8.2mm you are looking at either a pre-1999 Yamaha or Mercury, or a Mercury from 1999 on. The only thing separating them is the locking barb, 5.6mm against 6.7mm. That is 1.1mm, which is comfortably inside the range where people guess wrong. Measure the small barb rather than eyeballing it, and you have your answer.
Why the year matters as much as the badge
Mercury changed the connector design in 1999. A Mercury fitting is not simply a Mercury fitting, and two of them sitting on a bench can be different parts. If you are ordering by brand alone, you are ordering on half the information.
Fitting a replacement
Stick to the same brand for both halves where you can. Connectors from different manufacturers are often close enough to push together and not close enough to seat properly, which is the worst of both outcomes, because it feels connected. An audible click is a good sign you have it home.
These three cover the great majority of what turns up on Australian boats, but your outboard's service manual is still the authority on part numbers and configuration for your particular engine. Worth checking before you order, not after.
While the fitting is off
- Use marine grade fuel line. Not garden hose, not general purpose rubber. Fuel attacks the wrong material from the inside, so it looks fine right up until it does not.
- Look at the line and the fittings properly, not just the one you are replacing. Cracking, hardening, chafe where the line passes an edge.
- Replace anything cracked, brittle or loose straight away. A fuel connector that weeps under a cowl or in a bilge is a different class of problem to a fitting that simply will not connect.
The short version
Two thin barbs is Evinrude or Johnson, 6mm and 5.5mm. One thick and one thin is Yamaha or a pre-1999 Mercury, 8.2mm and 5.6mm. Two thick is a Mercury from 1999 on, 8.2mm and 6.7mm. If you are stuck between the last two, measure the small barb. Match brands, listen for the click, and use fuel line that is meant for fuel.