Inside A Motorcycle Carburetor - Needle-Height Tuning Adjustment

We dissect the intricacies of a motorcycle carburetor’s needle height in this installment of MC Garage

Do you know what part in your carburetor controls the majority of the fueling? What does raising or lowering the needle mean? Today on MC Garage we talk about the carburetor needle.

This is it guys—the last installment of our how to tune a carburetor series. We've got four other videos that preface this video; if you haven't watched them yet, make sure you jump back and check those out. Each builds on the knowledge of the one before. We've covered how to set your float height, how to tune the idle circuit, and how to jet for rich and lean on the main jet. Now we get into the piece that transitions the fueling from the pilot to the main jet: the needle jet and needle.

The needle jet—or nozzle as it’s sometimes called—is located in between the main jet and the carburetors venturi. Fuel comes through the main jet and into the needle jet. So the main jet does affect the needle, especially as the throttle opening increases.

The needle is attached to the slide and slides up and down within the needle jet. The needle is tapered, like a needle, and has a smaller diameter at the bottom than at the top. As the needle’s position changes within the jet up or down, the opening increases or decreases because of the diameter of the taper changes within the fixed diameter of the nozzle. This is how the fuel is metered.

So if the slide is all the way up at wide open, the smallest diameter is inside the needle jet making for the largest opening. Vice versa when the slide is all of the way down. Some needles have adjustable height positions while sitting in the slide. An E-clip sets the position, and that position is often referred to as “the clip.” Moving the clip up will lower the needle, making it leaner. When there are no clip positions on the needle, you can move the needle up by placing small washers (a move called shimming) in between the needle and slide. To lower the needle? You will need a longer needle.

Needle-Height Tuning Adjustment
Needle-Height Tuning AdjustmentMotorcyclist

When someone says drop or lower the needle, that means a leaner setting. Raise the needle for a richer mixture. This is fine-tuning your carburetion in the areas you ride the most often. Honestly, how often are you wide open? Just a fraction of the time, but how often are you at 70 percent throttle? Often.

As always, your manual will have the recommended clip position, but a good rule of thumb is to start in the middle position. Then just as you did with the main jet, feel the response. If it’s sluggish and sputtery, it’s rich. A bog means lean. Also hold the throttle at the midpoint; if the revs climb or run away and will not settle into an rpm, it’s lean. When the rpms dip or drop, it’s rich. Adjust the clip position accordingly.

For even further fine-tuning, try optional needles and nozzles with different tapers and diameters. These will help to get that last little bit of perfection into your jetting and tuning, but in reality you should be able to get 95 percent there with the needle you have. That last 5 percent is usually for the mechanics who are tuning for expert racers.

That’s the gist of the needle. Get the idle and wide-open-throttle jetting set first, then pay attention to the mid. Take it step by step; think about the effects each change will make before you do it. Test and repeat. In a short time you will have a crisp, properly jetted motorcycle.