Adding a fuel cell to 1978 GS1000

[Guest]

I like to ride long distance Iron Butt style, for no reason other than to do it. The stock tank has a 225 km range, about 150 miles, before reserve, or every 2 hours between gas stops. Reducing the frequency of gas stops is my goal. I use about 5.33 litres per 100 km, which is about 50 miles per gallon using the Imperial system. I estimate the auxiliary tank will allow about 35 litres total fuel capacity, an improvement over the stock 20 litre-5 gallon-stock tank.

I am in the process of deciding how best to connect a 4 gallon Jaz brand auxiliary fuel cell mounted on the passenger seat.

I have a very strong mounting arrangement using the luggage rack's former slide in back rest mounting. I used the tubing to make a flat frame for an aluminum plate, or deck , the size of the fuel cell. The plate screws to the frame with countersunk screws. The fuel cell straps to this using heavy duty large stainless hose clamp like straps, normally used by linesmen to attach weather boxes to utility poles.

Now, the platform for the fuel tank telescopes in to the luggage rack just as the back rest used to do. It sits half an inch above the passenger seat. It is strong enough to shake the bike by it without anything moving. A thrift store soft carry on bag camouflages the whole arrangement so its not noticeable. So far so good.

Here is a description of the plumbing so far:

The fuel line leaves the AN fitting at the back left side of of the auxiliary tank, through a fuel filter, then crosses over to the right side, then through a quick release fitting, under the seat down the inside right side of the bike frame. It comes out the right side of the seat under my knee on the side of the airbox, where it is zip tied securely so as not to move. A fuel valve is mounted at this point. The line then goes over the top of the number 4 carb intake boot and down between number 3 and 4 carb intake boots, where it joins the line from the stock petcock with a Y fitting, and into the fuel tube at the bottom of the carbs as normal. The fuel line is held firmly with zip ties around the frame. The internal diameter of the fuel line I used is 6 mm, the same as the stock petcock, so fuel flow will not be restricted.

Gravity fed, the auxiliary tank sits high above the stock tank on the rear rack. It is fitted with a vent on the top right back edge, which is plumbed with a hose to a small catch can for any drips, open to the atmosphere to allow venting to prevent vapor lock.

I routed the fuel line so as not to have any connections out of sight, to watch for leaks on the road, as under the seat of the bike will not be accessible with the auxiliary tank installed. (Murphy loves dripping gasoline.) The right side gave plenty of room, the left side could not fit the line past the stock petcock or allow a logical connection to the main fuel line.

The whole tank is easily removeable by disconnecting the quick release, un doing the strap I added to hold the tank firmly back against the luggage rack, and sliding it out forward over the seat. The fuel line remains in place, and the male end of the quick release tucks away under the back of the seat. The only thing visible when the tank is off is the very small fuel valve and 3 inches of fuel line visible under the right edge of the tank and on the airbox.

Questions to answer :

My assumptions are untested. Please ruthlessly critique them!

Petcock mods:
I used the hand operated fuel valve and retaining ring from my old petcock to make a new modified version. I filed off the little nub at the top of the retaining ring to allow the fuel valve on the stock petcock to rotate to sit straight up. I shortened the handle of the fuel valve by cutting off the little round end, and reducing the width of the ring so it will clear the edge of the stock gas tank when in a vertical position. I can modify the installed petcock by changing the retaining ring and fuel valve, and change it back again to stock, at will.

My assumption is that this change will allow an "off" position, pointing straight up, which as far as I can tell from looking at it will shut off fuel from the main tank.

This is a vacuum operated petcock-Will this work as a shutoff? Will shutting it off cause any problem or harm? The vacuum line will remain connected.

First hypothetical way to run this: (preferred)

The plan is to run the auxiliary tank first, to get the weight off the back of the bike , then switch to the main tank when the auxiliary runs out. Open the main tank by turning the stock fuel valve to the straight down "fuel" position, thereby allowing it to function as normal. Shut off the auxiliary tank valve. Carry on to the next fuel stop. This seems the best way, as it gives control over fuel flow.

Second hypothetical way less favored)

Leave the auxiliary tank shut off, until the main tank is running low. Then, turn the stock tank to the prime position, open the auxiliary tank, and let gravity refill the main tank.

Would this work?

Third way,
Leave the auxiliary tank open, and the main tank on prime, and run it as one big tank of fuel. The assumption being that this will work to feed the engine and won't flood out the carbs. I like this idea least of all and don't think it will work.

I hope my description is clear. Comments and suggestions are most welcome!

[Rob S.]

You must have an awfully comfy seat (plus an awfully young spine!) to be able to ride hours on end without needing a stretch.

Before you do the plumbing, maybe you should just strap the filled auxiliary tank in place and go for a ride. Just to see how the changed weight and center of gravity feels.

Happy trails!

PS Is that a "Skunk"? If so, we'll need pics! On second thought, pics no matter what model she is.

[MAC10]

Thanks Rob. I have already load tested it with heavy luggage. No problem carrying the weight, and balance is much easier than a passenger. They flop around too much anyway. None of them are equally crazy , er, dedicated to do an iron butt ride anyway.

The tank is foam filled to stop fuel sloshing around.

I use a good reupholstered seat, a sheepskin, an Airhawk pad, highway pegs, bicycle shorts, a back belt, and earplugs. And sometimes Craig Vetter's seat board contraption. Oh, and aspirin. Lots of aspirin. Not so much a young spine anymore.

Plus, the tank will make a good backrest. I have an old very nice looking pad that came with a fancy sling I needed for a shoulder rebuild which fits perfectly in the small of my back in front of the tank.

I will get some photos up. My focus has been on functional perfection and ergonomics, not making it pretty just yet. It's a C model, the first one, minus the spokes which I am rehabbing right now.

[Steve]

Sounds like it should work, if the petcock actually works as you hope.

I like the first scenario, too. One of the advantages is, like you said, you get the weight away from the high, rear-ward position first. When it runs out, your 'regular' fuel tank becomes an "intermediate" tank that has a fuel gauge AND a reserve.

.

[MAC10]

A complication has occurred. I installed the altered face plate that will allow the fuel valve to be set in a straight up position. The number 3 carb started to overflow down the drain tube. Touchy things these petcocks!

I am not worried about oil contamination into the crankcase because 78 models with VM 29 smooth bore carbs have such drain tubes to prevent it. Thanks bwringer for this observation! I have the drain tube run into a small catch can to confine any further leakage while I sort this out.

I suspect that the seal from the o ring around the fuel valve got disarranged, and the spring that puts pressure on the valve needs tweaking. There is a smaller chance that the float needle got stuck. I will reassemble everything carefully, and replace the fuel valve o ring.

How exactly do I give number 3 carb a whack to try to knock it loose? It wouldn't hurt to try, gently. Where should I nudge it?

Am I missing any other possibilities? The carbs are clean, and the problem is confined to number 3 carb, which runs a vacuum line to the petcock.

[Spyderman]

I am not worried about oil contamination into the crankcase because 78 models with VM 29 smooth bore carbs have such drain tubes to prevent it. Thanks bwringer for this observation! I have the drain tube run into a small catch can to confine any further leakage while I sort this out.

Mikuni VM29mm Smoothbore never came on any GS 1000 they were only an add on for better performance
if you have a set of VM 29 smoothbores with drain tubes and a vacuum port on #3 for the petcock
then if you do then they are a one of a kind set.
I have had several sets over the years and currently have 2 sets for my skunk and have never seen or heard of a set with those options
as far as i know all the 29mm smoothies have no drain tubes (29s' do have 17mm drain plugs in place of) and there is no vaccuum port on #3
the petcock must be run in the prime position unless you have an after market petcock
i have put an inline ball valve on mine to prevent fuel from possibly draining into the engine because i have a stock petcock and have to run it in the prime position

here is a pic of my skunk
DSC_33411.jpg