Bicycle Motor NT Carburetor Tuning
Changing the main jet size on the NT carburetor is the usual means of changing the ‘tune’ of a motor. Often, the following items are overlooked.
The NT carburetors series are about as simple as they come. The design has been used on small engines since the early 1940s, and probably even earlier. Despite its simplicity, when set up correctly, the NT carburetor is still hard to beat for performance and reliability.
NT Carburetor Tune Up Kits
NT carburetor Tune Up Kits are offered by many bicycle motor vendors. The kits provide a variety of fuel jets to assist with optimizing the carburetor performance.
Tune up kits are not essential, but provide useful assistance. Kits provide a range of fuel jet sizes that can be used to vary the fuel : air mixture. In most cases, the standard 0.65 mm jet (on 80cc bicycle motors) is usually the best option.
These notes will help you get the best performance from your NT carburetor, however if the motor is not in good running order, no amount of ‘tuning’ will make much of a difference.
Checking that the motor runs properly before attempting to tune your NT carburetor will save time and heaps of frustration.
Check the following and correct any faults:
- Does the motor run normally and produce reasonable power?
- Are there any signs of combustion gas, oil or fuel leaks? (Two strokes simply won’t run correctly with any form of leak).
- Is the ignition wiring in good condition and the spark plug clean?
- Check the spark plug gap is set to 0.03″ (0.75mm).
1. Adjusting the float height
The first item to set correctly in any carburetor is the float position. Float height on NT carburetors has a greater effect than on most carb types.
The float level effects idle as well as the mixture through the whole speed range. Use this setting to get a smooth, regular idle.
Changing the float position affects the idle mixture and may change initial acceleration. Raising the float level produces a richer mixture setting and lowering the float lever does the opposite.
The standard float position is about 1/16″ (2mm) above the carburetor body. Generally, most motors work better with the level set to 1/32″ (1mm).
2. Adjusting the float Level
To raise the float level, carefully bend the float lever until its highest point is about 1/32″ (1mm) above the body of the NT carburetor (see image).
Check the lever moves freely and the ‘fingers’ of the float lever are not touching or the body of the carburetor.
Verify that the ‘tickler’ plunger, and the float, sit level on the lever. (See image)
3. Re-fit the Carburetor Float
Re-fit the NT carburetor float, check it doesn’t touch the tickler.
If the float touches, gently bend the float lever fingers, to clear the tickler. (see image)
4. Test the Carburetor
Reinstall the NT carburetor on the bicycle engine and take the bike for a test run.
Adjust the idle speed screw to obtain required idle speed (between 1200 & 1500 RPM).
5. Check Performance Variance
Check how the motor runs throughout the whole speed range. Does it have better pickup from low speed? Does it rev out well?
6. Set the Idle Speed
Set the Idle speed by turning the idle speed screw either clockwise to increase speed or anti-clockwise to reduce it.
For good low speed acceleration it is important to have a steady consistent idle. Most engine tuners forget this point! (see image for the location of the idle speed screw).
About Fuel and Oil
Most bicycle motors perform best on UP ‘E10’ (10% alcohol) fuel with Valvoline wo stroke racing oil mixed at a ratio of 30:1.
It may seem to be overkill to use racing 2-stroke oil, however, it is usually a very similar price to cheaper grade oils.
We have found the lubricating properties of Valvoline to be the best option for all the bicycle engines we have worked on. In addition, it produces less smoke than most other brands, which means that your piston and cylinder head will take longer to coke up.
If you are not using this blend, then you may not be getting the best performance from your motor, and isn’t that what this tuning process is all about?
Some tuned bicycle motors work best on PULP ’98 octane’ fuel. The notes that came with the motor should tell you what fuel and oil blend is best for your motor.
If ‘on tune’ your motor should start easily and require choke when cold. Once warmed up it should idle smoothly, accelerate with a crisp note and pull evenly up to high speed, where it will break into a ‘fluttering’ sound. This is known as ‘four stroking’ and means the motor is running slightly ‘rich’. The ‘angry bee’ sound of a two stroke ‘on tune’ is what you want for best performance (and it sounds great!), however at the top end of the speed range, at full throttle, the slightly richer ‘four stroking’ note will ensure the motor will last longer and prevent overheating which causes loss of power, and eventually, engine failure. So our aim is to get a nice crisp two-stroke note, with good acceleration, but a slight ‘four stroking’ rumble at the top end of the speed range. If this happens at a lower speed, you will know because there will be a noticeable drop in the rate of acceleration. (At the top end of the speed range it makes no difference to the performance of the motor because it has run out of breath anyway). If, after doing all the above, your motor is not quite as you would like it to be, but close, do the following:
7. Change Metering Needle Position
Changing the metering needle position on your NT carburetor affects the whole speed range of the carburetor at various throttle & load settings.
The metering needle has only a limited effect on the overall performance of the NT carburetor and so is best used as a fine-tuning adjustment after optimizing the float level.
The needle mainly affects idle, initial low speed pick up, and part throttle mid-range speed operation. The needle setting is changed by changing the position of the ‘e clip’ (often called a circlip) and re-positioning it in a different groove of the metering needle. This changing the needle’s position when re-installed in the throttle slide.
Lowering the needle causes the carburetor to run leaner and raising the needle does the opposite. The image here shows the clip in the third groove (always count from the bottom i.e. from the pointed end of the needle).
8. Fine tune carburetor adjustment
Remove the throttle cable and slide assembly from the carburetor by unscrewing the ‘Retaining Cap’. Remove the slide and needle assembly from the throttle cable by pushing the slide against the spring until the end of the cable clears the groove in the slide. Ease the cable out of the groove and release spring tension (See images).
Remove the metering needle on an NT carburetor
Remove the metering needle and the lock washer from the slide.
Take note of the groove position the clip is located in. Move the circlip (see images) from its current position and place it in an adjacent groove (above or below depending on whether you want to go richer or leaner).
Replace the metering needle and the lock washer back in the slide. Only change the setting by one groove at a time. Any greater change could give you confusing results and it will end up costing you heaps of time and frustration.
5. Place the lock washer over the needle and circlip assembly and compress the spring to expose the cable (see image above). Slide the cable into the slot in the slide and allow the spring to seat against the lock washer. Check that the needle is firmly located on the lock washer and the spring is fully seated. This is not an easy procedure, especially the first time it is carried out. Be patient and you may find the long nosed pliers a help to compress the spring and position the cable. If you attempt to push the metering needle out of the slide and there is spring tension resisting this, then all is well. If the metering needle moves freely or is locked solid, then something is wrong so it is best to dismantle the assembly and re-assemble it again.
9. Changing the Carburetor Main Jet
After doing all the above, if you still feel you can get more out of the motor, it is time to try a jet change.
The reason that this is the last item on the list is that a jet change causes major changes to the operation of the carburetor and motor.
As most bicycle engine kits have already selected the best jet size for your motor, it is rare that changing it will make an improvement beyond the previous changes.
Occasionally, variables such as manufacturing tolerances, wear, atmospheric conditions (particularly altitude) and seasonal fuel changes (yes, the petrol companies change the make up of the fuel on a seasonal basis), there could be a gain to be made by changing the main jet of your NT carburetor.
Resellers often sell jet kits with three sizes of jet. The 0.60 is marked with (-), a 0.65 Standard (no mark) and a 0.70 marked with (|). The higher the number, the larger the jet. If you think your motor is running too lean, then fit a higher number jet.
For a rich running motor, do the opposite. The earlier image indicates the location of the jet.
When a jet has been changed, you will need to go back to the beginning of these notes and start over.
Getting the idle right first, etc. A jet change may mean you need to lower the float level to get a sweet idle.
Different types of NT carburetor
Engine kits have differnt NT carburetors. The newer carburetor behaves slightly differently to the older unit.
Both models have the same float, metering needle and main jet settings, the newer carburetor supplies slightly more fuel throughout the speed/power range of the motor.
Above are images of the two NT carburetors showing identifying features.
If you are swapping from one NT carburetor to the other, there will likely be a noticeable difference in engine operation. Tuning adjustments may be required.
A popular alternative to the NT carburetor is the Rongtong sport carb.
The Rongtong carb has fewer adjustments, however, it is a more sophisticated carburetor which can improve starting and benefit torque through the rev range.