Bing Flow Timing

Discussion in 'Airheads' started by sigpe57, Mar 26, 2013.

  1. sigpe57

    sigpe57 Been here awhile

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    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?

    SIG

    [​IMG]
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  2. supershaft

    supershaft because I can

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    The manual makes sense if you understand it?
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  3. disston

    disston ShadeTreeExpert

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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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  4. 190e

    190e Long timer

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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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  5. Plaka

    Plaka Brevis illi vita est

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    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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  6. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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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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  7. Plaka

    Plaka Brevis illi vita est

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    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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  8. hardwaregrrl

    hardwaregrrl ignore list

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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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  9. sigpe57

    sigpe57 Been here awhile

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    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
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  10. Plaka

    Plaka Brevis illi vita est

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    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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  11. disston

    disston ShadeTreeExpert

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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.)
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  12. Plaka

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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.
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  13. disston

    disston ShadeTreeExpert

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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. :lol3
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  14. Plaka

    Plaka Brevis illi vita est

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    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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  15. supershaft

    supershaft because I can

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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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  16. disston

    disston ShadeTreeExpert

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    That seems a better explanation Physics wise but the actual function is not really important. Both explanations want an even sufficient float level. Doesn't mater how you understand it if you end up with the same result.

    But I think your right.
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  17. supershaft

    supershaft because I can

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    In the long run it does matter if you understand what you are doing. It helps you start thinking for yourself. In my experience, height effects the idle circuit quit a bit.
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  18. Plaka

    Plaka Brevis illi vita est

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    When you are evaluating rich or lean you also have to consider throttle opening so you know what circuit you are running on.

    Nice diagram on this in the bing manual.
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  19. disston

    disston ShadeTreeExpert

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    Is this what you are calling a nozzle needle? #9

    [​IMG]

    I call that one the jet needle. It may go by some other names but I think this is the most common name. What ever you call it it is a wear part. By that I mean of course that it suffers wear from use and should be replaced. Bing recommends replacing these after 25,000 miles. I think you may get more than that out of them but should maybe replace them by 30 or 35K.

    As the jet needle is raised by the slide it opens the flow of the needle jet, also #9 in the next drawing;

    [​IMG]

    This too is a wear part and should be replaced with the jet needle.

    Not all of the parts in these carbs are usually replaced. I don't know what you mean when you say you replaced "everything". That's almost impossible. I think you mean the O-rings and gaskets. Maybe floats and float needles and maybe diaphragms? All of that is commonly replaced. What we have been hearing from other riders is that they didn't realize the jet needle and the needle jet were wear parts. They rub against each other when the needle moves up and down.

    All the jets in the carbs are brass and a good cleaning is all most of them need, other than these two. One exception is the idle mixture screw. It has the small spring on it and it is the screw you turn when adjusting the mixture. This part is made of steel and it can be rusty. If so replace it.

    In operation the Bing carbs need a certain minimum amount of fuel in the bowl and too much will cause problems too. The effect of raising and lowering the fuel level will be very slight with in these confines. The action of the mixture screw will be more noticeable.
    #19
  20. Plaka

    Plaka Brevis illi vita est

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    Perfection is for the Gods. We are but mortals. If you seek perfection you seek to become one of Them, and They will not appreciate it, and exact a considerable price. I am neither particularly religious, nor kidding. But if there is one thing I try to get pious about, it is avoiding perfection. BMW owners are well known for certain neuroticisms, obsessive compulsiveness leading the pack. The more you can get over it, the more you can truly live and the more fun you will have. This isn't intuitive because the neurotic process serves a function. Abandoning that function is scary and one imagines missing the satisfaction it brings. Think again. Get help if you need to.

    You replaced a whole bunch of stuff that rarely needs replacing and definitely should be evaluated for wear before spending a dime. That was gas, oil and tire money you spent. It was a day off from work you could have been riding. I know, some people have tons of cash and could care less. But then why not just buy new carbs and not have to screw with them?

    Diaphragms last a long time. I've gotten over 100k on them. They don't get replaced unless they have a hole. I carry a spare set on really long trips. I have fitted stainless carb top screws and I keep them in good shape so it's a snap to replace a diaphragm on the road. There is a little flat wedge shaped piece of metal in with the diaphragms that captures the cable settings exactly so I can put them back where they were.

    Butterflies never come out unless they are leaking at the shafts. They get new screws properly staked if the screws come out. Some re-use the screws if they are in good enough shape..

    I've owned several vacuum carb balancers: mercury sticks, a Walus differential gauge. Gah. The old power balancing routine works better than all of them for getting a balanced and very smooth engine. I use some fancy clip on 4 plug balancers in the shop and have a very small set of the original style (spoke) balancers in the tool kit I can use anywhere.

    The principle of vacuum balancing is wrong. If you balance the vacuum drawn by each carb you have to assume this translates into the power put out by each side. Turns out it isn't a great correlation. If you just balance the power put out by each side you are working directly on the thing that matters. Let the vacuum do what it wants. Many others prefer looking at vacuum gauges rather than their tachs (and listening). I've gotten enough comments about how smoothly my engines run to know I'm doing something right.

    Light soot on the plug is normal. Happens every time you start it up, especially on the chokes. Do a chop after a run at speed and see what you get.

    Drop some of the money you spent on carb parts on a Streamlight Reach.

    http://www.walmart.com/ip/17011842?...1=g&wl2=&wl3=21486607510&wl4=&wl5=pla&veh=sem

    Then pull the plugs and with the piston at TDC look in through the spark plug hole with the Reach and check out the piston crown. Turn the motor over so you can see the lips of the valves as well.

    There is an arrow and some letters on the piston crown. If you can see them, good. If the buildup is so thick they don't show, run some Seafoam, Techron or similar for a half dozen tanks and look again. More buildup on one side vs. the other throws off your compression and in turn the balance. In general, get wary when one side takes a lot of adjustment to match it to the other side. The left side generally has more carbon. A very careful compression check will reveal this also.

    Good luck and sorry (somewhat) for getting on your case so much. BMW owners really are notorious though and it's sort of a sad affliction.
    #20