Fuel system modifications

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Fuel tank Fuel dot Oil Gas can

Fuel tank

The fuel lines on the SBach were starting to harden and had to be changed. While I was at it I decided to switch to a water bottle fuel tank. The Fiji 500 mL bottle is a popular choice and widely available, so I went with that. To the shop!

Plan the fuel system

The airplane came equipped with a two-line fuel system: One dedicated vent line and a T-fitting to join the fill line to the carburetor supply line. I decided to stick with this approach. After much planning and consultation, this is the plan I came up with:

Two-line gas engine setup

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Assemble the parts

500 mL Fiji water bottle (available in 6-packs at grocery stores):

Fiji water bottles 6 x 500 mL

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Fiji water bottle cap:

New parts - Fiji water bottle cap

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Single barb vent:

New parts - Single barb vent

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T-fittings (only one is required):

New parts - Fuel tees

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Tygon fuel tubing (several feet):

New parts - Tygon

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Viton fuel tubing:

New parts - 1' Viton tubing

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Walbro felt clunk:

New parts - Walbro felt clunk

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Inline fuel filter:

New part - fuel filter

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Components of the fuel filter: Mounting clip, barrel end (female), filter (with o-ring), spring, barrel end (male)

Fuel filter components

Not shown: small zip ties

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Remove the old tank and prep for the new

Here's a look at the old tank in place:

Old tank with vent line on top

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And there it was, gone:

Old tank removed

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Create a base for the new tank

As shown in the previous photo, the old tank with its more rigid structure just sat atop some blocks in the bottom of the fuselage. I felt the water bottle would need more support so I added a piece of thin ply to spread the load. I could have just removed the blocks but with the exhaust canister directly below I figured some air circulation would be a good idea:

Fiji bottle platform in place

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Then, a little foam to cushion the tank:

Foam in place

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And a quick check to see how it looks:

Fiji bottle mockup

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Plumb the water bottle

There is a single barb for the vent line. I started by marking the hole - it needed to be high enough on the bottle to allow for complete filling and low enough not to intrude into the cockpit:

Vent barb hole marked

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Next step: drill the hole. When drilling through thin plastic I prefer to use a brad point bit turned with finger power:

Hand drilled with brad point bit

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The spurs on the bit scribe a nice clean circle:

Scribes a nice line

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A nice clean hole is the result:

There's the hole

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The barb installs from the inside:

Installing the vent barb - 01

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A piece of wire or a push rod is handy for lining it up:

Installing the vent barb - 02

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Once the barb is in place, the nylon washer is added:

The barb in place

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And the nut is added with a little thread lock:

A little thread lock

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There is an interal hex socket inside the vent so that the barb can be secured while the nut is tightened. The vent, installed:

Barb vent installed

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Next step is to thread the cap onto the bottle:

Barb and cap

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Then comes a pressure test, which consisted of (a) attaching a length of fresh fuel line to the cap; (b) blocking the vent barb; (c) immersing the tank in water, and (d) blowing into the fuel line. In fact, the first time I did this test, the bottle failed because for one reason or another the cap would not go on all the way. I had to prep a second bottle, which passed. Now, unlike glow engines, gas engines do not require pressurized tanks... so this test is just to ensure there are no leaks:

Pressure test time

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Lastly, remove the cap and install the internal line and the clunk. Ensure that the clunk does not touch the punt in the bottom of the bottle in any orientation:

Tank with clunk installed

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Connect the external fuel lines

The fuel supply is connected to the carburetor:

Supply line to carb - zip tie installed

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The other end of the supply line is connected to the filter, then the T-fitting. In this photo there's a short length of fuel line with an old fuel dot blocking the end; that's just to keep dust and dirt out. Note also that I found those wire fuel clamps to be a pain in the butt and in the end I tossed them and went with wire ties:

Filter, tee, fill line and stub supply line

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The fill line is connected to the last open barb on the T-fitting, routed out of the fuel dot collar and connected to the fuel dot. Here you get an out-of-sequence view of the new fuel dot:

McFueler connected

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The vent line is connected to the vent barb on the bottle and the bottle is strapped in. The vent line is looped around the back and side of the bottle to prevent undue leakage. Those fuel line keeps are made out of some spare rigid foam; they're glued onto the strap:

Fuel tank installation complete

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On this plane there is some copper tubing to which the vent line connects. Once connected, the installation is complete:

New vent line tubing

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