Fuel pump on carb'd bike - why?

Why do you want to remove it?
Is it kaput?

Interesting question posed by wobbly. I'm intrigued. My own guess was similar to adilmon's in that ultimate power has increased and therefore fuel flow rate. So, the need to maintain a required pressure in the float chamber at diminishing fuel tank levels.

One thing to bear in mind however, is that pressure is determined by head (the distance in height). So, yes, as the amount of fuel decreases, the pressure will fall. This is because the head is reduced but is only indirectly a result of the amount of fuel in there. You could have nearly nothing, but as long as the head is the same, so will be the pressure. It is to do with the weight of a liquid, or more accurately, the mass, but not the volume. The volume will determine how long the required head can be sustained.

From (distant!) memory I think the figure for water is 0.43 psi per foot. The mass of petrol is something like 0.85 that of water so (if I'm right!) we can establish what the pressure is. I'm pretty sure gravity comes into the equation and that varies depending whereabouts in the world you are, although the difference is negligible.

Coming back to the original question, the size of pipe will influence the flow rate, as fluid flowing down a pipe produces friction and so pressure loss. The length of pipe concerned, the viscosity and the flow rate are all low so the pressure loss will be low.

BMW got around the problem by putting 2 taps in the tank, although I think this is more to do with the shape of (older) bike petrol tanks being like panniers, straddling the frame.

Maybe that's part of the problem in the case of the fazer?

I'm going back a bit, so I reserve the right to be wrong with any of the above!

Hats off for doing the testing. I would still be cautious before consigning the fuel pump to the black hole that is e bay. There may still be a good reason for it that we've not yet thought of.

As you and Adilmon say, it may be under full throttle acceleration, with a low fuel level, that the float chambers can't fill up quickly enough to match the amount being "sucked" into the engine.

Few more little thoughts: Is your bike the one where the cylinders lean forward quite a lot? If so, that would position the float chambers quite high, reducing the fuel head to almost nil... ? With a pumped system there will be a by pass return to tank for the excess fuel I would guess. Long shot, but maybe they're using this to keep the fuel in the float chambers cooler? Also, on a general note, I wonder if float chambers are subjected to a negative pressure? I think the fuel line must be once the float needle drops away from it's seat. If so, then this would increase the effective head, working in your favour.

Anyone out there know the answer to that last question on a more general technical level?

Definition of expert:

X is the unknown quantity

Spurt is a drip under pressure.

This all poses the question of how the hell did we manage on bikes with carbs and no fuel pumps for years?

I don't recall any of my old nails ever having fuelling issues apart from when they really just ran dry - which happened quite a lot but no pump on the planet could have turned air into a suitable hydro carbon.

What about inclines and knee down or tight bends?
Where will the low tank of fuel be in relation to the carbs?

Very good point from roundincircles.

I think it's brilliant that a seemingly innocuous remark about fuel pumps has opened up this debate. It's not so much questioning the fact that Yamaha saw fit to put a pump in there, but more WHY they have. To understand fully why, we have to prise away, as usual, at the fundamentals of motor vehicle technology. Which reminds me, I could do with digging out that book again!

I think the vent will perform two duties (at least). Mainly to keep the pressure in the float chamber at atmospheric. This will ensure the depression in the inlet will always work with a steady reference, even if that reference is slightly different on a day to day basis. Secondly it will act as a safety device, protecting the engine from flooding, should something go wrong with the float/valve/ attitude of the bike!

The pumped pressure supplying fuel can't pressurise the float chamber because it's vented. I think the float and valve effectively "throttle" down or regulate the supply to match demand. So there will be a pressure drop across the seat of the valve whenever fuel is flowing.

Feel free to shoot me down in flames!

It's important to keep the float chamber topped up rapidly to ensure there are no/fewer intances of the engine running lean. The last thing you want is a bike that runs lean only when you give it some revs. Especially since it would be difficult to diagnose.

Thanks adilmon

Are we in a position to use your comment as a summary to give wobbly a direct answer to his question? (which I think is a very reasonable one to ask)

Is it because gravity alone is unable to satisfy that requirement?

Everyone will say: Why? It has done for eons before. What has changed?

Even if Yamaha had carefully altered the tank shape, increased the pipe size, done everything they could to keep the outlet covered, even up inclines, around tight slow bends, etc? If it comes down to scavenging the last half litre, why not just leave it in there? ! The tank has to be empty sometime!

Incidentally,I thought roudincircles' earlier comment about knee down (its effect on the machine) was brilliant.

Thomas Crapper’s water tank:

The crapper’s tank works the same way as a carbs float bowel. Both have the same result. The level of the fluid in the tank or bowl remains constant.

The carb's bowl acts basically like a very small gas tank who’s fuel level always remains constant.

As mentioned earlier by pittsy, it is not the volume in the gas tank that determines the pressure at the fuel tanks outlet, but the vertical height or distance between the tank’s fuel outlet and level of the fuel in the tank.

Carbs must have a constant fuel level in the bowl to work correctly. So a carb's bowl is simply a very small fuel tank who’s fuel level always remains constant.

As the level of the fuel drops in the tank, the pressure at the tank’s fuel outlet drops. Plus the fuel pressure at the carb's fuel inlet drops.

The manufacurer would decide what the ideal pressure should be at the carb's fuel inlet. If that ideal pressure becomes less as the tank's fuel level drops they could add a small low pressure fuel pump.

JAG

P.S. Crapper - what a great word - glad to get a chance to use it.

Thoughts from the throne,

You are right pittsy, for many years bike carbs worked apparently just fine without fuel pumps. Many still do.

So what has changed on some bikes?

I think it comes down to precision.

The carbs on many of the later carbureted engines were far more precious instruments than older designed carburetors. They were far more accurate in their fuel metering delivery than many of the older designs.

To achieve this higher level of control they needed a far more controlled or constant working environment.

You are correct when you indicated that when the engine is running the needle valve in the carb doesn’t completely close but reaches a balance point to maintain that important constant float bowl fuel height. The more fuel leaving the carb into the engine the more the needle valve is open.

Think of the slightly open needle valve as a very small orifice. The flow through this small orifice is dependent on the size of the orifice and the pressure difference between one side of the orifice and the other side. To achieve good control of the rate of flow through the orifice, the pressure differences on either side of the orifice must remain constant.

The greater the pressure difference, the greater the rate of flow will be. So conversely, the less the difference the less the rate of flow will be.

Fuel flow through the carb changes with engine load and RPM so the size of this orifice must change. If the changes happen rapidly you would want the pressure difference on either side of the orifice to still remain constant and at that idea pressure difference. The carb designer wants to be able to predict exactly what the volume of flow through the orifice will be under any operating condition regardless of the level of fuel in the gas tank and still keep the float bowl fuel level constant.

That was a really long one.

JAG