Aux fuel system on FJ-09

[KeithU]

I added an auxiliary fuel system to my 2015 Yamaha FJ-09. I am entered in the Butt Lite X Rally, which was originally scheduled for July 2020 but is currently postponed to September. We'll see if that actually happens. Butt Lite is a six day endurance rally, and I knew the stock fuel range on my FJ-09 would be a hindrance. OEM capacity is 4.8 US gallons, and with a big barn door windshield I only get 40-42mpg at highway speeds. This gives a max range of ~200 miles and a safe range in the 160-180 mile range. Not enough.

The plan was to add a four gallon aux fuel cell. I figured this would increase my range to ~320-350 miles. I bought a four gallon cell from RCI designed for race cars, and plumbed it into the main tank using a gravity feed system. The cell would mount on the tail section in about the same place as a trunk.

The first step was to build a rack. I used aluminum T-slot tubing from 80/20. We use this stuff a lot at my work to prototype, modify, and construct manufacturing machines. The T-slot tubing has excellent strength-to-weight ratio, and it allows you to build strong, complex structures without any welding. Nuts slide into the T-slot channels for infinitely variable, no-drill attachment points.

The rack bolts to the FJ-09 subframe using threaded hard mounts for the OEM grab handles and luggage rack accessories. Rack photos below.

[Sport-Touring]

[KeithU]

There are three methods people use to plumb auxiliary fuel on a EFI motorcycle:

1. T into the EFI return line and use an aux pump to fill the main tank. Very complex, and also not an option on the FJ-09 because it has a returnless EFI system.

2. Use the main tank vent as the feed line. Main pump vacuum draws fuel from the aux tank, and the aux tank vent becomes the main tank vent. Easily reversible, but it might also strain the main fuel pump and eventually lead to failure.

3. Drill the main tank and gravity feed aux fuel through a bulkhead fitting. Scary, but this is the method I chose. I've done this before on a Triumph TT600.

Packaging on the FJ-09 is tight, so it was difficult to find a good spot for the bulkhead fitting. I tried to just use the nylon gaskets supplied with the fitting but it leaked, so I sealed the bulkhead fitting with JB Weld. It is now permanently part of the tank.

Our dog Lily was skeptical about this whole project.

[KeithU]

I made my own brackets to attach the tank to the 80/20 tubing using flat aluminum stock. I attached strips of vinyl and dense rubber foam under the tank and on the mounting straps to protect the tank from vibration. The mount is pretty solid and the tank is basically part of the bike.

The line from the aux tank to the main tank has an inline fuel filter and also a quick disconnect fitting for leak-free removal as needed.

So far I have not been able to get more than 3.25 gallons into this "4 gallon" RCI tank. During the last test run I was able to go 300 miles before the low fuel warning started to flash, so this is definitely an improvement over stock. It's not as much range as I would have liked, but it is way more usable for LD riding.

There is a lot of anti-slosh foam stuffed into the RCI tank, and I'm sure that displaces some capacity. I removed one big chunk of the foam and we'll see if I can get more fuel into it next time.

Some people have asked about the weight. Yes, when the tank is full I notice a slight difference in handling. It's just a hair less flickable, and I can feel it when moving the bike around in the garage. But I think it looks worse than it is. The tank is under five pounds, the rack hardware is less than four pounds, and 3.25 gallons of gasoline is 19.5 pounds. So grand total this whole thing adds less than 30 pounds when full. This is roughly equivalent to a GIVI trunk bag with a few things in it. But notice that the tank is positioned further forward than a typical trunk rack, so it's closer to the bike's COG. So yes, I can notice the weight, but barely. It doesn't fundamentally change the handling of the bike.

[Blue is Best]

Good job. And as you say the drilling on your stock fuel tank is a little scary.

[KeithU]

Good job. And as you say the drilling on your stock fuel tank is a little scary.

Nothing a little JB Weld won't fix.

[kver]

I wonder how much anti-slosh foam you need in there?   could a pair of very thin baffles replace 80% of the foam?  Just leave foam in the bottom 20%?  or 2 baffles with small holes in them and foam in the middle 1/3 only between the 2 baffles?

[Blue is Best]

I wonder how much anti-slosh foam you need in there?   could a pair of very thin baffles replace 80% of the foam?  Just leave foam in the bottom 20%?  or 2 baffles with small holes in them and foam in the middle 1/3 only between the 2 baffles?

I hadn't even thought of that. Is it really necessary? We're only talking about four gallons?