Topic: Fuel pressure question

[krksquared]

I have recently been thinking about carbureted motorcycle fuel delivery. No particular reason. I realize that with no fuel pump, gravity is all that provides pressure to fill the carbs. As the pressure is dependent on the height of a fluid (gas), it would seem that fuel pressure to the carb(s) is a function of how much gas is in the tank. In cars, too low a fuel pressure can result in a lean mixture. I got to wondering if this could be true on bikes. Using a standard equation, I calculated fuel pressure for a hypothetical tank (see below). Perhaps the Honda engineers somehow took this into account. I don't mean to claim I'm smarter than them. Just wondering. I think most fuel delivery points on tanks are toward the rear. Perhaps they are counting on acceleration to increase pressure? I guess the best/only way to test for this would be to install an O2 sensor on my exhaust. That ain't gonna happen on my perfect CB400F headers! I assume the newer bikes with EFI use a pump. This doesn't seem to manifest itself as a real problem but it does make me wonder if the mixture varies with the amount of fuel in the tank and if so, how much?

[PeWe]

My car has a fuel pump delivering rather high pressure to the injector rail, 4BAR. But is has fuel injection, no carbs with float bowls which work as a reservoirs keeping fuel enough for the engines need.
Too high level it will overflow at idle running way too rich killing plugs within minutes (black velvet).
Too low level it will lean out at full speed or even earlier.

[Bodi]

Fuel pressure doesn't matter as long as it's withing a very wide range. You just need to keep the carb bowls filled. Too low fuel pressure - meaning very low indeed - will flow less fuel volume than the engine is using, and the carb bowls will empty causing lean-out. Too much - and this is much higher than can possibly happen on a gravity fed motorcycle carb - will force fuel past the "closed" float valves and overflow the bowls. Plus burst the fuel lines.
It's worth considering, adding fuel filters and hotrodding the engine will reduce fuel flow and increase the amount required. I'm doubtful that any 400 will be using more fuel than the stock system can deliver.
Almost all cars (and a few motorcycles) with carbs use fuel pumps because the fuel tank level is below the carburetor level. They still just fill carb bowls. Fuel injection always needs a fuel pump to supply a higher and regulated fuel pressure.

[brewsky]

  it does make me wonder if the mixture varies with the amount of fuel in the tank and if so, how much?

The mixture does not vary with fuel tank level changes since the float valve disconnects the tank pressure from the fuel in the bowl, which is at atmospheric due to the bowl vent.

IF, as stated above, there are other issues, there can be fuel supply or demand problems, then there can be mixture problems, but they won't be due to the tank level.

[flybox1]

Fuel pressure doesn't matter as long as it's withing a very wide range. You just need to keep the carb bowls filled. Too low fuel pressure - meaning very low indeed - will flow less fuel volume than the engine is using, and the carb bowls will empty causing lean-out. Too much - and this is much higher than can possibly happen on a gravity fed motorcycle carb - will force fuel past the "closed" float valves and overflow the bowls.

This ^^
maximum fuel pressure cannot exceed that which keeps the float valve closed (float buoyancy pressure upward on the float valve)

[krksquared]

Actually I wasn't concerned with pressure being too high. It seems intuitively obvious (i.e. I made no actual measurements) that pressure gets a bit low when the tank gets low. I can imagine that it could be possible to run a little lean if hard on the throttle with a low tank. Maybe not. I doubt if anyone here has tested this. Maybe yes 

[HondaMan]

The pressure is a little higher than yours calcs: you omitted the hydrostatic head height, which is almost as much as the weight of the fuel(!). This is the added distance from the float valves to the bottom of the tank, and is constant. This is why some of the modified cafe' bikes struggle with good fuel flow when they make 'lobes' over the sides of the tank, close to the engine, for a lower looking tank: it loses that hydro head pressure. Acceleration briefly (and unevenly) does feed a bit more to the carbs if the petcock is far enough to the rear, which helps those jackrabbit starts, and it is marginal enough that you can actually feel the difference in the 1974-1976 CB750K bikes, especially those with the -086a carbs, because they were set so lean with lowered float levels (for a variety of reasons at the time, including improved MPG for ads) that overfill slightly about the time you're hitting a hard 2nd gear.

[American Locomotive]

What you're missing, is that the fuel tank is not directly feeding the jets of the carb. The fuel fills the bowls, which are kept at a constant level by the floats. Everything is sized such that gravity can provide enough fuel flow through the float needle valves to keep the bowls filled.