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gravity-fed fuel systems suck (or don't)
feylike
a week or so ago, i finally got the Bandit repaired after it had been sitting parked and broken for months. total repair cost was more than 90% of the insurance valuation. damage summary: astoundingly bent handlebars from multiple people parking badly and knocking the bike over (the shop said that everyone was amazed that the handlebars hadn't broken given how badly kinked they were), dead battery, badly varnished carburetors, seized front brake...

today, i found out the hard way where the fuel tank reserve level is, by stalling on Fresh Pond Parkway. it took a while to diagnose, because there are so many other things that habitually go wrong with this bike that simply running out of fuel was not near the top of the list. there's no fuel gauge or warning light either.


what this bike does have is a gravity-fed fuel system with a vacuum petcock on the tank. this means that engine vacuum pulls on a diaphragm that opens the valve that feeds the fuel line. the petcock has two manual settings: "on" and "reserve". the "on" setting uses an intake that draws from most of the fuel tank except for a small reserve portion at the bottom. ideally, when you run out of gas at the "on" setting, you flip the petcock to the "reserve" position and find a gas station really soon. running on reserve for extended periods of time isn't such a great idea either; the reserve intake will pull from the very bottom of the fuel tank, which will have accumulated water, heavy particulates, and other stuff you don't want in your engine.

the thing is, if you run out of fuel on the "on" setting badly enough to run the carburetors nearly dry, flipping the petcock to "reserve" gives ambiguous results. starting will be difficult until the carburetor bowls refill, and they won't refill unless the engine is turning and drawing enough vacuum to open the petcock. this means running the starter for extended periods of time, which can be nerve-wracking if the electrical system is being dodgy or you have doubts about the amount of charge left on the battery. some more modern bikes have a vacuum petcock with a third "prime" position that overrides the vacuum diaphragm and forces the valve open, so you can refill the carburetor bowls without cranking the engine. maybe i can get a retrofit of one of those some day. (pulling the vacuum hose off the engine and sucking on it to open the petcock seems many kinds of wrong. handheld vacuum pumps exist, but i don't carry one in my back pocket.)

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Aircraft have little push-petcocks at the very lowest points in the fuel system in part so you can make sure the tanks have gasoline not diesel, but also in part because that way you can dump all the accumulated water/particulates/crap. If/when you re-valve the thing, that might be a neat addition. (We drain into something like a test tube, and then flip it off onto the tarmac, thus adding measurably to hydrocarbon contamination of the groundwater.)

Well, some of us filter and pour back into the fuel tanks. And some airports provide cans and take the accumulated fuel to hazmat disposal. Not me, mind you, but some. :)

the lowest points in the fuel system would be the carburetor drain screws, so that would kind of work if i had a petcock with a priming position. crud in the tank would still contaminate the float needles and bowls, but it might be better than some alternatives.

i recently read that supposedly, removing a single screw on the face of the petcock enables an unmarked priming position. i've confirmed that i can turn the petcock an additional 90 degrees once the screw is removed, but haven't had a chance yet to check whether it's actually a priming position.

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