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  1. #1
    Senior Member RaZed1's Avatar
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    Question/ Thought Experiment

    So here's a little brain teaser that has spurred a fairly heated debate on another forum. Airplane on a treadmill sort of thing.

    The question came about with a motorcycle vapor-locking during high speed riding in very hot weather. This is a sort of known issue with the Valkyrie (like if you want to cruise at 100mph through death valley- not something you'd routinely encounter by any means), and several people noted that flipping over to reserve (even if the tank was still mostly full) solved the problem. The proffered explanation was that by drawing from the bottom of the tank rather than a couple inches above, the fuel pressure (gravity feed system, no fuel pump, same as TW) would be slightly higher and could help overcome the vapor buildup and push fuel through to the carbs.

    Someone piped up and said this was not true, and that fuel pressure is solely governed by the fuel level in the tank, not by where it's drawn from, so that explanation could not be true. As long as fuel was above the "normal" pickup, fuel pressure (at the carburetor) would be identical whether the petcock was on normal or reserve. This went on to the point someone did an experiment where they attached a clear hose to the petcock and secured it parallel to the fuel tank, noting that the level in the hose did not change whether the petcock was on normal or reserve (it was always level with the fuel level in the tank), and thus supposedly proved pressure does not change whether you are on normal or reserve.

    To me, the answer seems very clear, but to avoid tainting the waters I'm not going to give my thoughts on it.
    Last edited by RaZed1; 09-06-2019 at 07:15 AM.

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    Senior Member TW-Brian's Avatar
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    "Someone piped up and said this was not true, and that fuel pressure is solely governed by the fuel level in the tank, not by where it's drawn from, so that explanation could not be true. As long as fuel was above the "normal" pickup, fuel pressure (at the carburetor) would be identical whether the petcock was on normal or reserve. This went on to the point someone did an "experiment" where they attached a clear hose to the petcock and secured it parallel to the fuel tank, noting that the level in the hose did not change whether the petcock was on normal or reserve (it was always level with the fuel level in the tank), and thus "proved" pressure does not change whether you are on normal or reserve. "

    Assuming that end of the clear hose was open to the atmosphere, this test will always only indicate the level of fuel in the tank and proves nothing about the pressure of the fuel in the hose. To detect a pressure change, the end of the hose would need to be closed with a means of either measuring any change in pressure of the trapped air in the hose when the petcock is switched to reserve, or observing any rise in the fuel level in the hose which would correspond to an increase in the pressure of the fuel exiting the petcock.



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    Senior Member Apolloha's Avatar
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    What's the question?
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  5. #4
    GOF
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    In any column of liquid pressure will be higher at the bottom. Gravity pulls down on each molecule the same. But those at the bottom have to support not only itself but but those above it. Look at the oceans. Divers who got deep need to take special precautions because of the pressure involved. However, I don't know that the difference would be significant enough in a small fuel tank to make a difference.
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    Senior Member Rickhem's Avatar
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    In short, the pressure of a column of liquid is determined solely on the height of that column of liquid.

    This includes some things that add detail and specifics to the question, but in short, your answers are here presented in a very easy to understand format. I like this guy's stuff.
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    Senior Member ejfranz's Avatar
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    I doubt the difference in pressure would be all that much and at 100 mph that Valkyrie is burning a lot of fuel and the pressure caused by the column of fuel is dropping rapidly. Stock tank is 5.3 gallons (internet search) At 100 mph, I doubt it will even be getting 20 mpg ( my neighbour claimed he got 25 mpg and he had a heavy hand). Death Valley road is 72 miles long, so 72miles/(20mpgX5.3gal) or 68% of the fuel would be burnt. The trip will take about 42 minutes. In 10 minutes you will have burnt 0.86 gallons. Reserve is 1.1 gallons. If you are following my logic every 13 minutes the pressure variance is the same assuming a uniform tank shape.

    So the real question is will you have enough fuel to provide the pressure to prevent the vapour lock?
    Also 100 mph on a Valkyrie seems a bit pedestrian. My neighbours wife mentioned that she was often reminding her husband to slow down. 180 kmph is not a leisurely pace.
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    Senior Member CJ7Pilot's Avatar
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    The fuel "column" would be measured vertically from the top of the fuel level in the tank, down to the float valve in the carburetor.

    If the fuel is higher than the "on" intake, then it doesn't matter if the selector is on "on," or "res," the pressure at the carb will be the same.

    However, I've noticed that my fuel petcock flows noticeably more freely on "res," than it does on "on." This is probably because the "res" pickup is not screened, and maybe the path through the petcock is more clear.

    Maybe there's a similar situation with the Valkyrie fuel selector... and at that speed, maybe the engine is drawing as much or more fuel than the "on" position can flow.
    Last edited by CJ7Pilot; 09-06-2019 at 08:09 AM.
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    Senior Member RaZed1's Avatar
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    Quote Originally Posted by ejfranz View Post
    I doubt the difference in pressure would be all that much and at 100 mph that Valkyrie is burning a lot of fuel and the pressure caused by the column of fuel is dropping rapidly. Stock tank is 5.3 gallons (internet search) At 100 mph, I doubt it will even be getting 20 mpg ( my neighbour claimed he got 25 mpg and he had a heavy hand). Death Valley road is 72 miles long, so 72miles/(20mpgX5.3gal) or 68% of the fuel would be burnt. The trip will take about 42 minutes. In 10 minutes you will have burnt 0.86 gallons. Reserve is 1.1 gallons. If you are following my logic every 13 minutes the pressure variance is the same assuming a uniform tank shape.

    So the real question is will you have enough fuel to provide the pressure to prevent the vapour lock?
    Also 100 mph on a Valkyrie seems a bit pedestrian. My neighbours wife mentioned that she was often reminding her husband to slow down. 180 kmph is not a leisurely pace.
    It wasn't really if you could make it through, that just arose as one example where someone noticed it. I experienced the same issue (cylinders dropping out, eventually it was running only on one bank of cylinders) riding from Utah to Vegas also in about 105* heat, holding around 80-85 on the freeway. It only occurs generally when ambient temps are 100+, and you've been riding on the highway at a somewhat elevated pace for at least 20-30 minutes. However, I did find flipping the fuel to reserve, vs normal, almost immediately fixed the problem, and so did others.

    My buddy was riding a BMW GS310, which realistically topped out around 90 and asking it to hold more than 80 or so continuously felt abusive, so I generally kept the speed around 80. Even at 80mph, I was lucky to keep MPG's north of 30 and I definitely had a couple fill ups in the high 20's with a stiff headwind. Valk's were never known to be fuel sippers. I think the absolute best I ever saw was about 40, and that was all like 55mph side streets.

  10. #9
    Senior Member FIRE UP's Avatar
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    Not to mention, just exactly WHERE in Death Valley can you do 100 mph for ANY length? I mean, we've only been there once or twice and all I can remember is curves. And, not only that but, there was a fair amount of traffic here and there.
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    Senior Member elime's Avatar
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    I had a Buell where the plastic fuel filter turned to mush (gasohol) and I replaced it with a big metal filter from O'Reilly's. It worked OK until one warm day after 15 or 20 miles on the freeway the engine died -- vapor lock. Switching to reserve didn't help. What did work was removing the gas line from the carb and letting fuel flow through and reattaching it to the carb and repeating said procedure every few miles until I got home.
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