Bing Carburetor Troubleshooting

by Alan Buehner

Originally printed in the 2005 issue #28 of Still….Keeping Track

The following article was written from information provided in "Let's Do It Right by John Cobb" of the March 1973 Keeping Track.

The Bing carburetors have a lot of parts inside them and they can be intimidating if you are having problems with it. The secret to getting the carburetor to perform properly is to isolate where the problem is occurring in relationship to the throttle, what the jetting is, and if the parts are installed correctly.

The information in this article is relevant to all Bing carburetors used on the Penton motorcycles. The following are examples of the different size Bings that were used:

  • 100cc & 125cc - 27mm Bing
  • 175cc - 30mm Bing
  • 250cc - 36mm Bing
  • 400cc - 38mm Bing

Before we start talking about "How to Jet Carburetors", lets see if this is where the fault lies.

  1. Make sure the bike has good compression. If not, it will load up quite easily. If compression is low, check the rings for wear and clearance.
  2. Make sure the timing is set right. 100cc & 125cc should be 2.6 to 3.2 before top dead center. 175cc should be 3mm before top dead center. 250cc & 400cc should be 2.55mm before top dead center*
    NOTE: check the spec. sheets of the repair manual for KTM motors for the before top dead center ignition timing for 1976+ GS & MC motors.
  3. Your ignition coil may be faulty and when the bike is warm, it could cause loading up, making you think it's carburetion, when in fact its a bad coil. If its a Motoplat, have the coil and stator checked out. If it's a points system, check out your condenser.

Think of your carburetion as a chart; this will make it easier. Now if your bike is loading up from 0 to ¼ throttle, your problem probably lies in the idle, pilot jet.

If the bike loads up in the mid-range or ¼ to ¾ throttle, your problem probably lies in the following: needle, needle jet, slide.

If it loads up from ¾ to full throttle, the problem probably lies in the main jet. Now we have figured out where it's too rich.

But if it's too lean, you may get the same effect. Be careful about jetting.

CAUTION: With a lean condition, the engine is running out of fuel. Usually it will just quit. But if you go down the road and it's lean in the main jet range, problems will occur, such as piston and ring seizure, or rod failure, to name a couple. Check the color of your spark plug electrode porcelain. If it is white or light tan the carb is lean. If it is black the carb is rich. If it is light brown or dark brown your carb is "right on the money."

Here are a few settings and what we have available for 100cc and 125cc, 24- 26-27mm Bing Carburetors. These jets are listed from leanest to richest.

  • Idle jets: #30, #35, #40, #45
  • Needle jets: #2.70, #2.73, #2.76
  • Needles: #5, #4
  • Main jets: 85 to 155 in graduations of 5.

We have found the best jetting on the 100 & 125 bikes is:

  • #4 Needle
  • #2.70 Needle Jet
  • #45 Idle Jet
  • #140 Main Jet

On some earlier bikes or a bike you have problems with, this is the jetting we use.

  • #5 Needles
  • #2.73 Needle Jetting
  • #35 Idle Jetting
  • #140 Main Jetting

If you are using this jetting or use it as a base, you shouldn't have any problem. Also when doing any jetting or carburetor work, you should remove the carburetor and completely clean it inside and out. Then set the float level. The float level for the Bing carburetors are as follows:

Turn the carburetor upside down. Hold the floats up and let them down slowly. As the brass tab on the float touches the ball bearing on the end of the float needle, the floats should be parallel. Here are a few settings and what we have available for the 175cc and 250cc engines:

  • 30mm Bing 36mm Bing
  • 3rd notch needle position 2nd notch
  • #2.73 needlejet #2.83
  • 30 idle jet #30
  • 150 mainjet #165

When reassembling the carb, make sure that the vaporizer is positioned correctly. It has a half-moon cut out and the open part must be facing towards the piston. If it is aimed towards the air box, the bike will start and idle but as soon a you put the bike in gear and put the engine under load, it will die.

CAUTION: Unless you have owned the bike since new, be suspicious of main jets and needle jets. Back yard mechanics have been known to drill out the jets to make them larger. They have also been known to reassemble things wrong (without the help of a manual) by mis-adjusting the floats. On the 27mm Bings with the side chokes, make sure that the parts to the choke are installed. Check the bottom of the starter slide to see if the wax like sealer is still there. If the wax like material is missing, the choke will always be activated resulting in the engine continuously running rich.

27mm Bing showing location of vaporizer.
Side view of vaporizer showing half moon cut out.