by John Cobb

Reprint from March 1973 issue no.4 of "Keeping Track" newsletter (PDF available here.)


We have three different types of carburetors on our motorcycles. 100cc & 125cc - 27mm Bings 175cc - 30mm Amals & 30mm Bings.

Before we start talking about "How to Jet Carburetors", Jet's 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.

Your ignition coil may also be faulty and when the bike is warm, it could cause loading up, making you think it's carburetion, when in fact it's a bad coil. If it's a Motoplat, have your dealer check the coil and stator and if it's a points system, check out your condenser.

Now let's get into Carburetion.

Think of your carburetion as a chart; this will make it easier. Now if your bike is loading up from 0 to 1/4 throttle, your problem probably lies in the idel, pilot jet.

If the bike loads up in the midrange or 1/4 to 3/4 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 to lean, you may get the same effect. Be careful about jetting.

What a lean condition is, 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, rod failure, to name a couple.

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 is:
New Bikes:

#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 needle
#2.73 needle jet
#140 main jet
#35 idle jet

If you use 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 on 100, 125 and 175cc engines using 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.

On the 175cc engines, we have two different carburetors. 1. Amal, 2. Bing.

First, we'll go over the Amal. This is the jetting we use and have found it to be the best.

#105 needle jet, large opening to top
#2 long needle
#240 or 260 main jet
#4 slide

The float is set .160" from the top of the float bowl to top of the float bowl seat. Note: To set float level, your dealer should have a float level plug.

If you have a float level plug, remove gas inlet and strainer, insert it in the float needle seat. Turn the float bow I upside down, making sure the plug stays in place. Now heat the outside of the float bowl around the float needle seat. With a small punch or round dowel, push down on the bottom of the float needle seat. Push until the float needle seat bottoms against the leveling plug or until you reach .160" without the plug from the top of the float bowl to top of float needle seat. Note: If your carburetor has a plastic float bowl, then the float level is already set. Now when you reassemble the carburetor, you may find that when you start the bike, you· cannot tickle it. This is because the tickler bottom does not lift the float, preventing the gas to flood the carburetor as it should. If this is the case, your dealer should have in stock or can order a different style tickler bottom. This tickler has a longer reach, plus it is built different to keep mud and water from getting into the carburetor. The part number for the tickler assembly is No. 622-172. We also have a brass float needle with a neoprene tip. This controls the flow of gas better than the nylon one. It doesn't bounce around as much. The part number for the brass float needle is No. 622-197.

The other carburetor on the 175cc is the 30mm Bing. We have found the best jetting for the Bings is:

needle position - 3rd notch
needle jet - 2.73
idle jet - #30
main jet - #150

If the engine is pinging in the mid range, or any range, it's lean and should be richer:

needle position - 2nd notch
needle jet - 2.76
idle jet - #35
main jet - #155

For the best results on any of the Penton sportcycles a premium fuel that is leaded should be used.