Motorboat fuel tanks have a reserve as well. It's just a raised area that splits the bottom of the tank into 2 separate concave areas. One of the concave areas contains the end of the fuel line, and the other doesn't. When you run out of gas, you tip the tank up to dump the remaining gas from the other basin into the main one, and then you restart the engine (or keep it from stopping at all if you're quick enough on the draw) and head for the docks.
Old SCUBA tanks didn't have gauges, they had a reserve tank with enough air to get you to the surface. You'd realize you were running low (which I'm sure was terrifying) then hit the switch and slowly surface (you don't want to surface quickly when diving).
Yeah, my dad had a tank like that. I dove with it exactly once - never again, yikes. It was coated inside and out so, despite being a steel tank, it was in excellent shape.
The bikes I've had that have had reserve tanks have also been old enough to raise the disconcerting follow-on question, which is: "is the reserve gas also full of sludgey crap that's settled in the tank and hasn't been disturbed really in a year, and am i about to run that through my poor carbs?"
My friend had a truck with a reserve tank, but it was the same size as the main tank, so he would just flip the switch at every fill up to make sure they both got used.
Had this in a 70s F150. A "Main - Aux" switch on the dash, right above the 8-track player. I used to let the main tank sputter out on fumes and then triumphantly shout "Rerouting auxiliary power to engine!" while sliding the switch. Letting them empty out alternately would have been a lot smarter.
My father drove a '95 F-150 for years that had the dual tanks. Shortly after highschool I got in accident that ended up totaling my vehicle and got a couple months I was using his truck (he runs an Auto repair shop from a garage behind the house so he almost always had something available to drive) and I ended up using it to go out on a date with someone I had met at work.
I noticed on the way to pick them up that the truck was running on empty in the main tank but I checked and the aux tank was full. Then I remembered the first time my dad let the tank run down and start sputtering down the road and decided to keep going on the empty tank.
Make it to pick them up and start heading down the highway(where we were it was a good 3-4 miles to the nearest gas station) and then the truck finally started to sputter. I proceed to play along with it pretending to panic for a good 20 seconds and then I turned and saw the look on their face and couldn't help but start laughing. Switched to the aux tank and when the truck started running again I turned and and the look I was getting indicated I was being mentally murdered. Then they punched the crap outta my arm and started laughing and calling me not so nice things.
Ended up being an awesome night out with someone I'd end up being friends with for a long time. It's weird how this kind of random conversation in an unrelated internet post can drag you way back down memory lane.
This is typically used for agricultural/off-road fuel which is not priced with road taxes and as a result much cheaper. Off road fuel is dyed red in the US. If you get caught running dyed diesel on road you will be fined. Thus the switch on the dash, when you leave the highway to drive on your farm you flip over to dyed fuel to save $$.
Oh, fascinating! My first vehicle was the family's 3/4-ton Diesel '84 Chevy Pickup from the farm, and I'd forgotten it had an Aux fuel tank! This makes a lot of sense.
the two-tube design of the tank on my 1975 honda CB meant that there was about an inch and a half of tank that sat below the primary fuel port. Tank crud (steel tank, theoretically passivated, 40 years old) settles faster than I ran through a tank of gas, so the bottom layer had sediment in it fairly regularly.
I kept spare inline fuel filters in a tool roll just in case after a while.
always fun when you're barreling down the highway and the engine starts to lean out, prompting you to hurriedly locate and switch the petcock over before the engine stalls completely.
suppose then that you go fill up and forget to set the petcock back to normal. 8ball says: "I see a long walk in your future."
out of years of riding it's only happened to me a couple times.
one time i was eastbound on the bay bridge when my bike started to sputter. i'd just reassembled the tank and had left the screw-style reserve fuel valve open, so there was no reserve fuel to be had. a very kind lady put her blinkers on behind me and followed as i coasted the last few hundred yards toward yerba buena island.
i pushed my bike up the ramp and looked in the tank to assess. it's a dirtbike, so the tank has two distinct "lobes" to accomodate the top tube of the frame. I had a few ounces in the tank but they were not in the lobe with the fuel pickup, so i dumped the bike on its side to get the fuel to slosh over to where i wanted it.
i got back on the highway and, going quite slowly and gently, managed to get to the gas station at west oakland bart, the engine leaning out and sputtering right as i rolled into their lot.
I think that driving on those last few ounces of fuel is a completely different feeling.
Normally you take for granted that the engine works for hours at at time.
When you've come to a stop and found those last few ounces of fuel, it's such a relief that the engine can run again, and you know it won't run for very long, but every minute that it continues running saves you many minutes of walking or pushing. You appreciate every minute that the engine produces that amazing amount of power (compared to your own power when you're pushing a 300+ pound bike)
Typically there aren't two separate tanks - In one tank there are two tubes at different heights. As the fuel level falls below the height of the "main" tube the engine sputters, then turning the petcock engages the lower down "reserve" tube which is still below the fuel level. It's more of a warning than a true reserve, and most bikes with an actual fuel gauge don't have a reserve.
On bikes like that, there's a reserve-reserve trick sometimes. Sometimes, the tank is an inverted U shape so when the pickup runs dry there's still a little more fuel on the other side of the U. If the bike is light enough, you can stop the bike and lean it way over to pour that last bit over to the pickup side. Might get you another couple miles.
Fuel injected motorcycles don’t have reserve (at least, none that I’ve seen.) instead they have low fuel lights or full fuel gauges. I’m guessing it’s because the fuel pumps are in the tank and the fuel injection system needs high pressure.
Fuel injectors require filtered gas because even small particles can clog them, and said filter is more likely to be clogged or even compromised by sucking up the last drops of fuel (and scale and debris) in the tank, so the low-fuel warning is required.
Carb jets can get clogged, too, but are wider since they're not under as much pressure. Also, since they're a wear item they're a lot easier to clean and/or replace.
I think grandparent commenter had it right: it's because the pump is in the tank. There's just no good way to have an external petcock determine where a tank-internal pump gets its fuel from.
Many new bikes come with a lot of rider aids for safety (ABS, TCS) as well as all kinds of electronics (fuel maps), so this is changing. But of course manual transmission won't go away until bike are electric.
I am one of those who likes things old school. My bike still has a carburetor, has no fuel light or tachometer, and I have certainly had some practice reaching down to turn the fuel petcock to reserve while sputtering on the highway. If they didn't intend for me to do that, why did they put it on the left side? :)
Goldwings also have a reverse gear.
Even more remarkable: I used to have an Aprilia scooter that had a remote release button (on the key fob) for the under-seat storage area. I think I used it once just to see if it works.
Some newer bikes, like mine, don't have a reserve petcock. They have a low fuel light. No forgetting about the petcock and an obvious warning light instead of sputtering.
Some older bikes, like my '99 Ducati Monster, don't have a petcock. It has a low fuel light that first failed in around 2002, and for which that part that fails (the in-tank float switch) stopped being available in about 2015 or so. No petcock _or_ warning light. (And that trip where the speedo cable fails so I couldn't even use thew trip meter to estimate fuel requirements was a fun one...)
Can you find someone who can adapt a float switch from a different bike? It seems like a very useful thing to have, even if it's not the original factory part.
I've just gotten used to it. I'm fairly reliable about always resetting the trip meter when I fill it up (and always fill it to full). I know it'll get 200km easy, maybe only 180 if I'm having _way_ too much fun. That's always about time I want to stop and stretch my legs anyway. It doesn't bother me enough to "solve the problem".
Most motorcycles with a manual petcock are very manual in nature. Often this is to minimize the number of moving parts that could die on you if you take it into rural areas. An automatic petcock adds more complexity that could cause a malfunction.
It is a shame that motorcycles have moved away from this model. My last bike had a manual petcock with a reserve setting. It was problematic because I’d forget to turn it from off to on, take off on what’s left in the carburetor bowl, and the engine would start sputtering just down the road. But I also never got stranded.
New bike has a vacuum-actuated fuel valve, no reserve. It does have a fuel gauge but since the tank is not a nice simple rectangle and the angle makes a difference the gauge is basically untrustworthy. So I go by the mileage and hope I don’t get it wrong. How hard would it be for them to add a reserve setting so it could just be between On and Reserve so I could just flip between them as needed?
In the Honda CBF125 group on Facebook, a fellow Indian shared a photo of his bike. A British guy asked what's the switch, he's never seen one before. Same bike, same country of origin, but only certain markets get the switch and the recessed panel.
It is extremely thick plastic. I wouldn't be surprised if it dislodged from the frame before it burst. In any event, in any collision violent enough to rupture the tank, the rider will have already been thrown a hundred feet away (and be dead...)
