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Old 03-26-2013, 11:49 AM   #1
sigpe57 OP
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Bing Flow Timing

This is from BMW's factory manual. It stated when the flow is parallel with with edge of the carburetor, the fuel will start to flow. Just want to confirm, is this right?

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sigpe57 screwed with this post 03-26-2013 at 11:59 AM
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Old 03-26-2013, 12:16 PM   #2
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The manual makes sense if you understand it?
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Old 03-26-2013, 12:16 PM   #3
disston
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Fuel level in carburetors has a bearing on the mixture. Too high a fuel level will make a rich mixture and too low will make a lean mixture. This is a general rule for all carburetors. In other than the Bing Carbs we use there is sometimes a float level height to be measured with a ruler or special gauge.

The float has a parting line. This is the seam where the two parts of the mold met when the float was made. I use this when setting float height. I find the line "base of the float" to be confusing. Of course the edge of the carb body is easy to see.

With new floats and new float needles the carb bodies are held up side down and the float allowed to rest on the float needle. If the parting line and the carb body are even, you got it.

The same thing can be done with the carbs on the bike by holding the float up with your finger. It will be instructional to flip a petcock on when doing this and see at what point the gas flow stops. The parting line should be level when the gas stops flowing.

With the carbs on the bike and the petcocks on, then the petcocks turned off and the float bowls removed you can see how high the gas level is. This is not exact because the lines have a little gas in them and when the bowl is dropped this gas gets into the bowl. The two bowls should have about the same amount of gas in them and be a little over half way filled.

Understanding this operation you should then see how floats that are not floating can effect operation. Or float needles that don't seat will over fill the bowl causing sometimes leaks and sometimes rich running mixtures.
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Old 03-26-2013, 01:06 PM   #4
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I used to remove the bowls quickly and target for a fuel level of 22mm (32mm carbs) until I calculated that there was enough fuel in the lines to raise the level by 3mm. That wouldn't matter so much if it drained equally into both float bowls but it doesn't. So you really need to clamp the lines or remove them to get an accurate result.

I think an equal level both sides is more important than the exact level being accurate to 1 mm. I suspect that unless the lines are removed or clamped it's more likely that the parallel float method will give equal results both sides.
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Old 03-26-2013, 01:18 PM   #5
Plaka
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Quote:
Originally Posted by disston View Post
/snip
This is a general rule for all carburetors. /snip


Understanding this operation you should then see how floats that are not floating can effect operation. /snip
Not quite. lemme tell you about the carb on my rototiller...

The bit about the floats not floating is critical. So you back it up by checking the level of the gas in the bowl when the needle shuts off.

Also the float needle can stick and fail to up and shut the gas off. There is a slender brass overflow tube in the float bowl. (the one that sticks way up. provided it isn't plugged, (under the bowl usually) if the gas gets too high it goes out this overflow and all over your boot. Usually caused by sticking float or needle. Pretty common on airheads. Rap on the bowl to free things up. Turn petcocks off when stopped.
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Old 03-26-2013, 01:19 PM   #6
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The amount of fuel in the float bowls can be measured. This is done by a). opening the fuel petcock to fill the bowls, b). closing the fuel petcock to stop fuel flow, c). removing the bail wire below the float bowl and carefully removing the bowl. Any 'extra' fuel in the fuel line will remain provided the petcocks don't leak due to a vacuum forming in the line- like liquid does not run out of a straw removed from a glass of liquid when the end is covered with your finger.

The liquid height should be measured with the the float blows on a level surface using a suitable ruler. I used a metal machinists ruler which works well. I expect measuring the wet length of a toothpick placed vertically in the float bowl would work. Good luck!
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Old 03-26-2013, 01:25 PM   #7
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I used to remove the bowls quickly and target for a fuel level of 22mm (32mm carbs) until I calculated that there was enough fuel in the lines to raise the level by 3mm. That wouldn't matter so much if it drained equally into both float bowls but it doesn't. So you really need to clamp the lines or remove them to get an accurate result.

I think an equal level both sides is more important than the exact level being accurate to 1 mm. I suspect that unless the lines are removed or clamped it's more likely that the parallel float method will give equal results both sides.
The float needle shuts off the fuel right at the float bowl. What is in the fuel lines makes no difference.

I wouldn't sweat 1mm. In operation, the fuel is bouncing and sloshing all over. it doesn't have an absolute depth like when you are looking at a bowl full of fuel on the bench.

The float has to exert enough force on the needle to shut off the gas with the tank full (maximum fuel pressure). So always do this check with a full tank.

What you are after is a constant, known fuel pressure at the jets. this depends on the level of the fuel in the bowl. Deeper=more pressure. The over flow tube also has to be clear so atmospheric pressure can enter the bowl.

You can tweak your mixture slightly by changing the fuel depth in the bowl. Poor mans micro-jetting. By the same token, a lot of precision side to side is not needed. You take up the difference with your carb balancing. But it helps to get it close.

Bending the tab on the float means extremely tiny bends. You might not even see it move.

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Old 03-26-2013, 01:29 PM   #8
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And if you measure, be sure to measure below the button recess at the bottom of the bowl. I use a metal ruler with a notch cut out of it so it will fit below the recess.
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Old 03-26-2013, 01:44 PM   #9
sigpe57 OP
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Quote:
Originally Posted by disston View Post
Fuel level in carburetors has a bearing on the mixture. Too high a fuel level will make a rich mixture and too low will make a lean mixture. This is a general rule for all carburetors.
I would think fuel bowl level does not affect the mixture. The fuel flow controls the fuel in the bowl. More air draws in by the carburetor, more fuel will be sucked up, but the fuel mix ratio should always stays constant.

BTW, what does "Constant Velocity" referring to?

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Old 03-26-2013, 01:56 PM   #10
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I would think fuel bowl level does not affect the mixture. The fuel flow just want to make sure there is fuel for the carburetor. More air draws in by the carburetor, more fuel will be sucked up, but the fuel mix ratio should always stays constant.

BTW, what does "Constant Velocity" referring to?

SIG
The pressure at the bottom of a column of fluid depends on the height of the column.

The depth of the fuel in the bowl determines it's pressure and thus the amount of fuel delivered to the jets.

As the level of fuel in the tank changes the pressure delivered to the carbs changes. The idea of the carb bowl is to have a constand fuel depth and thus a constant pressure. So when testing flow (are you getting enough flow), you want the tank on reserve where the pressure is lowest.

The pressure at the bottom of a column of fluid depends on the height of the column.

Constant velocity (CV) means the velocity (speed really) of the air flowing through the carb is held constant. This is done by raising the slide. the pressure of the air is a function of it's velocity so by holding the velocity constant you hold the air pressure constant. The faster the air moves the lower the pressure. In the CV carb some of this low pressure is ducted to the diaphhram, raising it and the slide which in turn increases the size of the carb (the venturi area) and allows more air flow and lowers the velocity. When less air is flowing the pressure is lower and the slide descends to raise the pressure.

The reason your jacket wants to puff out when riding (open bike) is the high velocity air flowing over you creates a low presure area. the higher pressure (normal pressure really) air inside your jacket then pushes the jacket outwards. Same effect blows off roofs in hurricane winds. High velocity air flowing over the roof makes low pressure, normal pressure air in the house then pushes up on the roof and it pops off. Opening the windows allows the pressure to equalize and saves roofs (and trashes the inside of your house of course.). Same effect generates the lift on aircraft wings, etc.
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Old 03-26-2013, 02:31 PM   #11
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Quote:
Originally Posted by sigpe57 View Post
I would think fuel bowl level does not affect the mixture. The fuel flow controls the fuel in the bowl. More air draws in by the carburetor, more fuel will be sucked up, but the fuel mix ratio should always stays constant.

BTW, what does "Constant Velocity" referring to?

SIG
But it does. On many carburetors this is a much more critical measurement. We want the levels even from side to side, as 190e has said, this seems to be more important than actual level. But level in both carbs should be a little over 1/2 to 2/3. Some use a measurement as Jenna said.

I think the gas from the lines does get into the first bowl you remove. There will be a little in the second bowl but more goes into the first because of the cross over. I have removed the gas lines at the carb fitting when messing with this and so thought I got a better reading.

I remove both bowls and set them next to each other. I can see how even they are with out any measuring.

I had troubles with float levels for several years. I would set the level so low that I ran out of gas in high speed runs and so high that the carbs wetted my boots. I had bad float seats and my new set of carbs give me far fewer problems. It was important to find the right problem and fix that instead of constantly adjusting float levels. So set the level. Mess with it some but if you can not get it right then something is wrong. Plugged petcocks, filters, needle seats. floats too soaked (heavy). I once had problems because of damaged float pins (the axle the float is on caused the float to not want to move.)
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Old 03-26-2013, 02:33 PM   #12
Plaka
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But it does. On many carburetors this is a much more critical measurement. We want the levels even from side to side, as 190e has said, this seems to be more important than actual level. But level in both carbs should be a little over 1/2 to 2/3. Some use a measurement as Jenna said.

I think the gas from the lines does get into the first bowl you remove. There will be a little in the second bowl but more goes into the first because of the cross over. I have removed the gas lines at the carb fitting when messing with this and so thought I got a better reading.

I remove both bowls and set them next to each other. I can see how even they are with out any measuring.

I had troubles with float levels for several years. I would set the level so low that I ran out of gas in high speed runs and so high that the carbs wetted my boots. I had bad float seats and my new set of carbs give me far fewer problems. It was important to find the right problem and fix that instead of constantly adjusting float levels. So set the level. Mess with it some but if you can not get it right then something is wrong. Plugged petcocks, filters, needle seats. floats too soaked (heavy). I once had problems because of damaged float pins (the axle the float is on caused the float to not want to move.)
You can fix bad seats with a float needle you never want to use again, some powdered pumice/water paste and a valve lapping motion.
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Old 03-26-2013, 02:41 PM   #13
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You can fix bad seats with a float needle you never want to use again, some powdered pumice/water paste and a valve lapping motion.
I'll fix them someday. I believe in having spare parts so the extra set I got working last Summer is fine for now. I had the extra carbs on some years ago and they didn't work very well. Turned out the jet needles were set unevenly so when I fixed that I got a pair of carbs with good float needle seats. And I figured this out because I had no more float level problems.

I have almost a complete extra bike for spares.
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Old 03-26-2013, 02:48 PM   #14
Plaka
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I'll fix them someday. I believe in having spare parts so the extra set I got working last Summer is fine for now. I had the extra carbs on some years ago and they didn't work very well. Turned out the jet needles were set unevenly so when I fixed that I got a pair of carbs with good float needle seats. And I figured this out because I had no more float level problems.

I have almost a complete extra bike for spares.
Nothing like a ton of spares. I've parted some bikes out to myself. Always saved a ton of money.

I wonder what would happen if you threw a spare needle jet in the float bowl and just left it there. Chew up anything?
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Old 03-26-2013, 03:02 PM   #15
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Man. My BS meter has been red lining lately. Float bowl height is not about hydraulic pressure. It's about the height of the fuel in the jet tube and how far the low pressure above the fuel has to pick it up. Closer (higher) is easier (richer).

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