I am thinking of making my G/SPD+ look more like the bike on the right.
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When I bought the bike new, it came with the stock fuel tank, which has some plumbing weirdness that I do not understand.
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People familiar with this model know that it was BMW's early experimentation with emissions technology. In addition to some exhaust plumbing, there appears in the fiche to be an evaporative capture or fuel flow control system of some kind... and it appears to fit under the starter cover. The whole thing is called a "16_0241 Shed Fuel Supply" on the fiche.
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There is an un-numbered mystery hose coming from beneath the tank that goes into a T on the rear of the evaporative gadget and then maybe to the airbox or maybe behind the airbox to the right carb (?) Nothing is numbered.
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The underside of the tank has two tubes coming from the tank in addition to the petcock. The larger diameter tube goes up to a vent hole in the filler neck (to drain any water that may collect there) and the smaller tube goes into the tank and I cannot seem to fish a wire through to the filler neck.
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Here's what the filler neck and cap look like....
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Ideally I will use the single petcock with a splitter directly to two lines to the carburetors (with the PD tank the fuel line to the right carburetor goes through a gap at the lower front of the airbox), and just block off that smaller tube coming out of the tank.... but 1) is that going to be enough fuel to the engine? (my PD tank has two petcocks), and 2) will there be a vacuum problem inside the tank as the fuel drains?
On the MOA Forum, someone suggested that I may not be looking at the right parts picture, but I pretty sure I am.
This is the other tank shown on the site, which is shaped like mine... but notice that the tank is vented to the atmosphere. This was a big no-no in California back then.
When I bought the bike in Atlanta, the PD tank (also vented to atmosphere) was already installed and there was no emissions stuff included with the stock tank. Did not get any of the stuff that goes under the starter cover, either.
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Thanks for any advice...
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Ian
Go soothingly through the grease mud as there lurks the skid demon.
I'm no expert on the G/S, but I can help fill in some of the gaps on the fuel system.
* The tube from top of the filler neck area to bottom is to help prevent spillage of raw fuel. In a wreck, the bike will lay on its side and raw fuel may slip (or be pushed... if the tank gets crushed) past the screw-cap seal allowing raw fuel on the ground (an environmental hazard) and complicating the rescuer's job if the rider is in a pool of fuel.
The original plumbing did nothing as you were riding down the road. It simply routes any raw fuel that passes the first seal to a storage area where the fumes can be burned off. I've never seen that bit actually plumbed with all the hoses shown. It may in fact be a dealer installed item when selling the bike in CA.
* Using a single fuel cock will trap about a quart of fuel on the opposite side of the "hump", making consumption of ALL the fuel in the tank nearly impossible AND turning a situation where you run low on fuel into a far more exasperating experience than it ever needed to be. Using 2 petcocks actually gives you 2 reserve sources, as you can turn each cock to Reserve one at a time.
* If you have issues with the cross-over fuel line going under the air box, there is another way to route the fuel line which uses no additional hardware, where the cross-over is a simple loop to the rear of the air box...
* The other 2 big lumps under that cover are emergency vacuum/pressure operated fuel valves. What you see in the diagram is plumbing for fuel fuel IN and fuel OUT, plus hoses to route vacuum/pressure from the engine crankcase to operate the 2 hidden valves. Again this is all about turning OFF the fuel in case of an emergency... with the direct intent of keeping raw fuel from pooling on the roadway or running off the road into streams and rivers.Â
Similar type valves appear on newer riding lawn mowers and fail all the time. And like the lawn mowers, there is no way to predict their demise... you simply get stuck when it does. Unlike lawn mowers... on a motorcycle you could be 200 miles from the nearest toolbox that would have enough diagnostic gear to allow you to diagnose the issue when it does happen.
All reasonable people want to preserve all natural habitats, but the far, far greater probability is that you will be walking home rather than saving some snail or minnow. Especially when you consider that none of the special molded hoses are still available so that maintenance can be performed on this system. Therefore, most reasonable people completely remove this "rats nest" and the "ticking time bomb" it represents to overall Airhead reliability, as has been done with yours. And remember too, there are multiple environments within the USA where YOU will become the endangered species if you find that you need to walk 100 miles to get help.Â
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Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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