Fix a leaky Bing

Bing carburetors are notorious for leaking gas. If you're the kind of person who likes to keep their fuel where it belongs (as in, not on the ground), read on for some suggestions to help fix your leaky Bing. Start with Part 1 and go through the list. Ask yourself the question in bold. If the answer is "yes," check the next step listed under "affirmative." If the answer is no, proceed with the instructions under "negative."

Part 1

Take your carb apart. If you don't know how to do this, please watch these videos. Examine your float needle. Is it clean?
Is the rubber tip in good condition?


Replace your float needle and try again. See the shop links for places to buy a new float needle.


Continue to Part 2.

Part 2

Now, look at your carb. You see that hole where your float needle goes? Is it clean? Are there no scrapes or scores on the inside of the hole?


Clean it thoroughly and re-try. If it's scraped or scored, you might have to buy a new carb body. You can buy a new float needle seat, from Bing USA. You can take one off another carb. Heat gun and a small drift. Get the seat cold, use some heat and pack seat with cotton. Use a small drill bit and a rubber hammer, if needed. Very light love taps.

Note: Sometimes you can take a very fine piece of wire -- say, a piece of wire plucked from a wire brush -- and GENTLY scrape it around in that hole to make sure you've gotten rid of any sediment or other lurking crusty bits.

"Note2: Sometimes you can chuck a q-tip in a cordless drill and with a little polishing compound you can clean up a damaged seat... then clean any residual abrasive.


Clean it anyway. Now try again. If that still doesn't work, go on to Part 3.

Part 3

Take off your float and take it for a little swim -- specifically, a swim in some gas (or premix). Does the float actually float when placed in a container of gas/premix?


Replace your float.


Continue to Part 4.

Part 4

The brass clippy thing (Click to enlarge image)

Your carb is all clean, but it's still leaking. Now what? Now try adjusting your float.

  1. Take off the float bowl and look at where the float needle attaches to the float. See that brass clippy thing? From here onwards, that will be referred to as the "brass clippy thing." (On newer or reproduction floats, this part might not be brass colored.)
  2. While everything is still together, take note of which way is "up" for each part -- that is, closer to the top of the carburetor.
  3. Take off the float.
  4. On the brass clippy thing, there are two flanges that the needle slides into/between. Bend the flanges "up" just a hair.
  5. Reattach the needle and float with the pin. Examine the float gap with the carb body, this gap needs to be even or slightly tipped away from the hinged side. Install the float bowl and fuel line.
  6. Turn the fuel on and turn it off, Remove the fuel bowl and pour fuel on a white paper towel. Examine for dirt and rust particles. These particles stick to your float needle's rubber tip, causing your leak.

Does your carburetor still leak?

Note: Unless you fall into one or more of the following three categories, it may be best to replace your float needle before experimenting with this step (part 4):

  1. You have no money. Not even the $7-8 that a float needle costs. You're too broke for RAMEN.
  2. You want this to work NOW. Not in the 3 days that it takes to get this shipped to you. You are master of impatience, and you want it NOW.
  3. Your carb has always leaked, and it has always leaked fairly consistently. If it just started leaking, or it just started leaking worse, and you haven't done anything to it recently, chances are more likely that your float needle decided to go bad rather than that your float settings somehow got messed up.


Success! Give yourself a pat on the back and go reward yourself with your beverage of choice. Well, first you might want to start and run the moped to make sure you didn't bend the brass clippy thing too far, because if you did that, you won't be getting enough fuel, but a reward for yourself is still in order.


Continue to Part 5.

Part 5

You have a few choices now.

  • Option 1: Repeat Part 3, bending the brass clippy thing up bit by bit, checking after each adjustment to see if there is improvement.
  • Option 2: Check and replace (if necessary) the float bowl gasket. (Best choice for small residual leakage.)
  • Option 3: Replace the float needle. Yes, you already checked it, and yes it "looked" good. However, sometimes they can look good and still be bad.
  • Option 4: Make a sacrifice to the moped gods and ask for more suggestions on the forums. Please be sure to mention the methods you have already attempted to solve this problem.
  • Option 5: Give up on the Bing and replace it with a Dellorto. (Hopefully someone will add instructions for this).

Why this (theoretically) works

Here's some basic carb theory to help explain why these steps work. However, like people, each carb is special and slightly different from other carburetors. These steps should (theoretically) work for all Bing carbs, but just because something works in theory doesn't mean it works in reality.

Why Part 1 (theoretically) works

Fuel flows from the petcock through the fuel line through the fuel filter (you do have a fuel filter, right?) into the banjo and then into your carb. The float needle plugs up the hole right under the banjo when your float bowl is full of fuel. If the needle is not in good condition, it can't seal that hole, and your carb will overfill and leak.

Why Part 2 (theoretically) works

This is basically an extension of Part 1. If there is some little bit of crud stuck in that hole or the hole is scored, the needle can't make a good seal with the hole, and your carb will overfill and leak.

Why Part 3 (theoretically) works

Let's return to the path of fuel flow. Once the fuel enters the carb through the banjo, it enters the float bowl. As the float bowl fills up with fuel, the float ... well... it floats. Since the float needle is connected to the float via the brass clippy thing, the float needle also rises (moves "up"). If the float doesn't float, the needle doesn't rise, and fuel flow doesn't get cut off, resulting in leakage.

Why Part 4 (theoretically) works

On a properly adjusted carb, the float needle will fully seal the hole when the bowl is full (but not overflowing) with fuel. On many leaky Bings, the float needle does not seal the hole completely even when the bowl is completely full of fuel. When you bend the brass clippy thing "up," you are essentially moving the float needle "up" into the path of fuel flow sooner. Thus, the fuel level will not need to be as high before the float pushes the float needle into the hole to block fuel flow. This will (hopefully) stop the fuel flow before the float bowl overfills and leaks.

Notes on Part 5

  • Option one is explained above. If continued bending does not improve the situation, you should stop and seek further help.
  • Option two, contrary to popular belief, is not best initial solution to fixing a leaky Bing. If your float and float needle are adjusted properly, the bowl gasket's main purpose should only be to prevent fuel from sloshing out of the bowl when you go over bumps and make tight corners and such. Replacing the bowl gasket might keep your carb from leaking, but it doesn't solve the underlying problem that your float bowl is overfilling. If your carb is leaking just a little AFTER you've already tried everything else, THEN it might be a good idea to replace the gasket.
  • Option three ... can't hurt, right?
  • Option four: A lot of people think Bings are worthless and finicky. If you agree with those people, there is no shame in giving up on the Bing and replacing it with something different (the Dellorto SHA is a popular choice).

Square Bing Modifications

Do parts of these instructions not apply to the square Bings? If so, add instruction modification here!

To access the rubber tipped float needle on a Square Bing: The float has a small metal plate embedded in it. The plate has a slot where the float needle clips in place, and then wraps around a pin that serves as a hinge. First remove the pin by pushing it out sideways, you should now be able to pull the float and pin out. Do not leave the pin in place and try to force the float needle to unclip from the plate, the head is fragile and can snap off EASILY.

  • Another possibility if you have a square Bing is that the float bowl has a poor seal. To check, remove the float bowl and look at the sealing surface on the carburetor body. If the gasket is missing in places it could cause a leak, repair this with some gasket maker. If this isn't sufficient check to see if the float bowl is distorted from being screwed to an uneven surface. Place it on a flat surface and see if it rocks back and forth. If it does this could be a source of leaks, in this case the float bowl mating surface must be made flat. I sanded mine smooth, this process is identical to the process of milling a head with sand paper except that you can complete the sanding using only 150 grit sand paper.
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Other Important Info

  • On page 58, this manual has instructions on how to properly set the float level. You should read these instructions.