Thanks for your insight, TT!
Let me see if I can respond (kindly, I hope) to some of your points.
About this part:
The head pressure present at the fuel petcock outlet is a function of the total fuel depth in the tank. An argument can be made that the head pressure at the top of the stand pipe is less than that at the bottom of the tank where the reserve feed is, as there is less fuel stacked in a column above the entrance orifice. However, both the reserve orifice and the main orifice in the petcock have the same exit hole diameter and cross sectional area, and it exists at the same depth in the fuel tank. They will have the same head pressure as they have identical columns of fuel above them being pulled by gravity. Things change, of course, when the fuel level drops to the stand pipe tip.
I was trying to explain about that, maybe I didn't make real clear. The hydrostatic head, at the bottom of the petcock bowl, when the fuel is at least at the top of the RUN tube, is about 1.5 oz. When the fuel falls to the top of the RESERVE side of the petcock, it has dropped to about .12 oz. This lowering pressure (force) is where the problems originate: I'll try to elaborate below.
When the fuel level falls to stand pipe depth, the head pressure falls as the tube empties. Then the engine runs lean when the supply can't meet the demand. Switching to reserve immediately restores a head pressure equivalent of the remaining fuel level in the tank. Longer lines become more difficult to fill as the head pressure reduces, however. This may lead to a reduced level of fuel in the carb bowls farthest from the source of supply, assuming other fuel line impediments do not exist. When the float bowls run low, the slow jet starts sucking air well below the lower main and the cylinder(s) goes lean, and hot. This can happen as fuel levels approach the standpipe tip level if there is a minimum of sloshing in the tank (steady speed cruise on a smooth road for example).
Sure, that's why it sputters when we start to "run out" on MAIN. The things I'm attempting to explain are: what happens when the system's pressures fall to this lower (RESERVE) level.
These seem to be pretty low velocities and it's very hard to imagine such bends would have much of an impact on flow rates.
The pressure drop in a bend is related to the angle of the bend and the rate of fluid flow, minus a factor for laminar flow (i.e., the fluid nearest the pipe itself, which will hardly even move because of friction and small eddy forces). While the drops are very small in absolute number in a "drain system" like this fuel system, please remember that the head pressure on RESERVE is also very small to begin with: a bend-induced drop of .005 oz is equal to over 4% of what you have left. It really adds up in the last quart of gas.
(P.S., I think your math about fuel flow is good.)
Personally, I feel that leaning on reserve or main setting is more a function of the slow jet starving before the main at cruise as the float bowl fuel level drops.. I agree that a carb farthest from the fuel petcock may starve before others. But, I don't see any physics that support the petcock itself from contributing to that starvation.
I used to agree with you, until I learned something from my fellow Production Racers in the early 1970s. One of them was Don Vesco, the guy famous for making Yamaha's TD350 go faster than Yosh could. I learned lots about these carbs from him: one of those things is about the pilot jet's action once the ram tube in the inlet side of the carb starts receiving significant velocities. I'll get to that part below, too.
What is not in your picture is a view of the outlet spigots. This amounts to a horizontal tube supplied from one end with two downward outlet tubes along it's length. The fuel flows from the supply inlet toward the end and falls into the first outlet tube it encounters Think of how a small ball bearing would travel in this arrangement. When the first tube is filled, flow then continues on to the next outlet. During periods of fuel supply starvation, the first tube is always the one supplied first, and the second tube can easily be more starved than the first one. It is a matter of gravity in a gravity feed system. One carburetor supply tube will always go dry before the other with this arrangement. Personally, I'd like to see the longer fuel line at the far end of the petcock log. The reasoning is that longer lines store more fuel than shorter one. And there is a better chance that ALL carbs will starve together to give a drastic indication rather than just two cylinders behaving slightly different during idle jet starvation.
This gets bantered around a lot. The problem with the flow, when on RESERVE, is the very low head force: the moving bubble in the longer fuel line causes momentary compressions in the flow paths, which disrupts the flow completely for a while. I once discussed this with a fellow who insisted it be "tested" by removing the float bolws and watching the flow, which completely changes the physics of the whole system...
(this is about the float vent tubes into the airbox...)
Whoa, this one is real trouble, I think. If I understand what you are describing, these hoses are the vent lines for the float bowls. The pressure on the surface of the fuel in the float bowl relative to the reduced pressure in the carb throat is what pushes the fuel through the fuel jets. The pressure in the airbox is separated from atmospheric by the filter membrane, and is lower than general atmospheric. If the carb bowl vents are tapped into this lower pressure (reflected from the carb throat) the jets will all flow LESS than when a higher differential is applied across the borders of the individual fuel jets. The only saving grace might be the carb overflow stand pipes. These are also atmospheric vents when not occupied with fuel overflow. And, hopefully they will negate any suction placed on the bowl vent tubes.
Actually, this part all started with two projects: Production Racing at 130+ MPH and drag racing with a turbo CB750. First, a bit of explanation to the novice: Production Racing, in the early 1970s, consisted of "stock" bikes. You could not change so much as a coil, carb, or shock on your bike. You could alter jetting, timing, oils, tires and pressures, but little else: it was "stock bike" racing. We found, on fast oval tracks, that the engines would go flat lean at speeds over 120. Problem was, if you simply "jetted fatter", then the midrange was so wet you could not dig out of a turn because it too too long to clean out, regardless of needle settings or calibrations. After hearing Mr. Vesco one day in a discussion of pumping out crankcase air ("positive venting" on 4-strokes) to improve RPM by reducing air friction, as he was doing at Bonneville, I wondered what other things vacuum could do. I tried this business of "positively venting" the float bowls to see if it would raise the float levels and improve the top end speed. Our plugs went from white to light tan, and the top end increased (we had no speedo, only a tach) 220 RPM or so. This allowed leaner mainjetting, which really helped on non-oval tracks, because we could control midrange better.
On the turbo bike, the owner had drilled out all the passages because he thought flow was the issue and bigger tubes would help. He was "running out of fuel" at about 2/3 of the track, you could hear it go flat lean. I raised his tank to ABOVE the front rubber mounts and set the back end on top of his seat to raise the head pressure: he went almost 10 MPH faster. (Then we installed a Gold Wing fuel pump instead.
) It was just a matter of increasing the pressure to feed the fuel fast enough.
About 1997 or so, I obtained one of Motorola's tiny 0-2 PSI pressure transducers (like you find in the air sensing systems of today's cars). Using a separate 9 volt battery and stabilized power supply, I could see (on a voltmeter, while riding), a positive PSI reading in the vent tube (on cylinders 1&2) at high RPM. This reading would go away as soon as decel starts, or when the throttle setting was less than about 1/3 open. Vesco had explained that this positive PSI (which comes partly from your explanation of bowl pressure vs. venturi) pushes fuel up the emulsifier tubes in the carbs and this chills the area around the idle jet's inlet, raising air density at that spot and pushing back down the idle jet's throat, so to speak, which is one of the 2 reasons why the idle jet "fades out" about 1/4 throttle (the other reason is reduced venturi velocity over the jet). In these carbs, the tube that goes from the inlet flange down into the mainjet's emulsifier tube accelerates this process as airflow increases with throttle. At higher throttle settings, it begins to also push back DOWN into the fuel in the bowl, which creates some pressure in the bowl, slowing the fuel feed. That's where the high-speed starvation comes from. At really high RPM, Vesco hinted, the tube looks like it's boiling air DOWN into the float bowl!
So, I teed into the hole on my airbox (not inside the filter, just the box) to see what the reading did while connected to the carbs: it was a negative reading on the order of .1-.25 PSI (measuring in reverse, because it's not a vacuum sensor) when the RPM was higher. This confirmed Vesco's tips and helped explain my successful 1970s experiments that raised the float level, richening the mix, because it lowered the extra bowl pressure at higher RPM, letting fuel drain in a little easier. It seems that the richer mixture overcomes the slighter tendency to resist fuel flow up the emulsifier tubes with a higher vacuum in the bowl, but at 1/3 throttle, the venturi vacuum is on the order of 8 inches or more, while the float bowl (with the mod) only sees a fraction of an inch. I think that's probably why it works in touring, too.
Feel free to tell me where I failed the physics lesson.
Failed, hell! You just cheered me on. I haven't had a good workout like this in a long time, and this is what makes men's minds GREAT! Keep it coming: we can all figure it out, but not alone! ("A single log on the hearth will go out, but together they make a great fire.")
I hope this doesn't sound self-important: I'm just trying to spread around the things I've learned or observed.
