Carb float level

How much difference (if any) would the height of the fuel supply over the carb rack make on carb float levels?

How about on the bike with a full tank versus almost empty? Is there any change?

It seems that more pressure would make the float level higher and less would lower it. Maybe it does but it's insignificant. Maybe someone has already checked into this?

 

It might affect how fast the bowls fill up, but even then a very small difference. The gas still has to go through the 1mm hole in the float needle seat, and no matter the pressure, the floats are going to stop the bowls from filling at just the right spot.

 

Your intuition is correct. Physics dictates that there must be a change in bowl level for a change in fuel pressure because of the way the valve works, but your intuition is also correct again in that it's insignificant.

The mechanical advantage of the float lever would completely swamp the change in fuel force pushing against the small orifice in the needle valve.

We need an ME to weigh in here, but I believe the force is the weight of a column of the diameter of the needle valve opening, right? So, the difference between the weight of that diameter column of empty tank height, and that same diameter column diameter of a full tank height.

In other words... Pretty much nothing.

Glad to see that I'm not the only one that thinks about that kind of stuff. :lol:

 

Oh my is it raining where you guys live?

 

Didn't see this till just now, but to answer the question, the height of the fuel tank does indeed set the fuel line pressure, assuming that the flow rate is small (it is). It's hard to say how much pressure the float generates to oppose this force, but I agree that it must be very large compared to the pressure that's in the lines, considering the length of the lever on the float, its large volume, and the tiny hole in the float valve.

I also worried about this in the past, when placing my tank for testing things. I think if everything is properly cleaned and set up, it shouldn't matter. The floats have to be set properly regardless, and the height wouldn't be affected by the pressure in the fuel line.

 

ME here ... interesting debate but I have to disagree.

The float closes the valve against whatever line pressure there is and ensures that the variations in tank level don't affect the fuel level in the bowl. If it didn't the varying fuel levels would cause unpredictable performance outcomes as the tank level changed.

If you are suggesting that the valve has to close a bit more to hold against a higher line pressure and therefore requires the float to be a little higher then that's also a false assumption. Once the valve is closed it's closed, and this will always occur at the same position. And while open it will still allow fuel to flow in order to fill the bowl, pushing it to the closed position.

The principle of operation is that the fuel bowl is supplied many times more fuel than it needs even at full load, so that it is always kept full, ensuring a constant head pressure to the jets and therefore a predictable performance outcome under all load conditions.

To get an idea of the relative magnitude of fuel in/fuel out, read Len Chacal's post "The Secret Life of Carburetors" which describes just how small the amount of fuel being consumed actually is. Then with the bowl off and the line connected lift a float to open the valve and watch the fuel gush out. It has to be this way for the carburetor to work predictably.

 

No, they're right.

The weight (pressure) of the fuel on the needle has the effect of increasing the weight of the float. If the float is heavier it needs to displace more fluid to "float", so the fuel level on the float itself will be higher with a tall fuel column.

Since the float has a lot of leverage at the valve, and the valve is so small, this is all a theoretical discussion. Any reasonable height to the fuel source will be just fine.

 

It turns out it's a very extremely theoretical discussion (as several suspected). I happened to be setting my float levels last night, and just to test I lifted my small gas can to varying heights above the carbs on the bench. I think everyone will be happy to know that no matter how high I lifted the tank (from a few inches above the carbs to several feet), it did not affect the fuel level in the carbs one bit.

I know other people have their small tanks in various places in their shops, so I'm sure others have observed this.

Also, we're not suggesting that the valve has to close "a bit more," we're saying the fuel level has to be higher to make the valve close if there's a ton of pressure in the line. But it looks like the pressure doesn't matter at all.

 

That's because the pressure is approx 1/3 PSI. Gasoline exerts a pound of pressure at 27 inches, but the level in the tank is about 10 inches above the needle.

Now - divide the square inch by the "area" of the little Viton rubber tip - specifically JUST the portion of the tip that seals off the fuel flow, not the whole diameter of the black part. It's about 2 MM; do the whole "pies are squared" thing to get the area, and it's a really tiny fraction of a square inch. That is the exact weight being added to the float.

 

I'm going to put my carbs back together and go riding, this thread is doing my head in

 

@Time to ride; You forgot to calculate in the major leverage that the float has on that needle (I'd guess about 20:1) so it's even less weight! :wink: :roll: