NSR250 Carb Tuning

NSR250 MC21 Carburettors

MC16's, MC18's and MC21's come fitted with Keihin TA series carburettors, and the MC28's use the TB variant. Although perfect for the stock application, once the NSR is tuned, upgrading of the carburettor internals is essential not only for performance, but for reliability too!

This is where it becomes necessary to fit the much talked about jet kit! Several kits are available; HRC, Jha, Factory, and Daytona, to name a few of the more common suppliers.

These kits generally consist of:-

 

The following table shows jet kit part numbers applicable to each model.

 

Note: Main jet part numbers follow this pattern: 99101-393-xxx0 (where xxx is the jet size required). For example, a 185 main jet has a part number of 99101-393-1850. This formula can be used for EVERY 250 NSR, MC16 through to MC28.

Note: Check the size of the PJ in each carb, as they are often different.
Standard jetting figures can be found in the Technical Resources section.

Honda recommended lubricants for racing are Castrol A747 and Elf HTX 975, 976, or 909, @ 30:1.

These oils are suited only to track use because they should be mixed and used immediately as they are susceptible to separation if left standing.


Note: THESE OILS MUST NOT BE INJECTED, THEY ARE TOO HEAVY FOR THE PUMP.

For injecting use a high quality fully synthetic oil such as Rockoil Synthesis 2, Castrol TTS, or Silkolene PRO2. NSR-WORLD.COM recommends Rockoil products.

Adjustable needles.

When using the adjustable needles, the 'kit' emulsion tubes (HRC# 16165-NH3-911) must be used.

There are 3 different grades of needle, A, B, and C. The B needle has a profile similar to the standard non-adjustable needle, found in the majority of stock models.


Note: The kit needle holder (HRC# 16186-NH3-911) for the adjustable needles is different from the standard holder and must be used to prevent needle float.

Depending on the kit installed, and the extent of the modifications to be carried out, virtually the final job is to replace the air solenoid jets (seen below).


Note: The red cross piece is only available as part of a kit now. The kit X-piece, if you can source it is HRC part# 16181-NKD-600.

If you run open carb's, an HRC airbox (MC21 part# 17210-NH3-760 MC28 part# 17210-NKD-970), or JHA's equivalent (part# 3544) should be used. NSR-WORLD.COM Carbon Airbox can also now supply carbon fibre HRC replica airboxes in the Marketplace. The later MC28 HRC airbox also has a scoop that runs from above the radiator to the carb's providing a flow of cooler air.

More advisable than running open carb's on the street, is to fit a pair of foam filters, or modify the standard airbox.

Remember to alter your jetting from the recommended settings to compensate for the use of filters of any kind.

Finally, to make the most of these modifications, a couple of adjustments can be made to the inlet tract. 

The first of these is to fit a pair of HRC intake rectifiers.

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HRC

Stock

 

Next a set of HRC fibre or carbon reeds (Jha part# 3525) can be fitted. This will not necessarily produce a horsepower gain, but can make the throttle response crisper. (Another benefit of fibre reeds is that if they break, they generally disintegrate and pass harmlessly through the system and out of the exhaust). The reed stopper plates can be opened 1mm wider too, but a better solution is to fit some Jha spacers (part# 3529), these are  fitted between the reeds and stopper plates and cause less stress on the reeds than bending the stoppers. Use a very close fitting screwdriver to unscrew the stopper plates as the screws are very small and can break easily. Giving the heads of the screws a sharp tap with a small hammer first can often help with loosening them. While a good quality Pozi™ screwdriver will fit these screws, it should be noted the srews used by Honda are JIS type. If the original Honda toolkit is still available, use the screwdriver in it to loosen screws on the carburettors.

Shown below are 2 reed cages; on the right is a stock configuration, on the left is a Boysen twin stage assembly.

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To finish the job, if all you want is pure top end gain, remove the induction (boost) bottle. This opaque chamber, attached to the inlet manifold in front of the carb's, is designed to make the inlet tract appear longer at low rpm. Remove the bottle and plug the holes in the inlet stubs.


Note: The induction chambers are part of the airbox assembly on the MC28.

NSR-WORLD.COM ACCEPTS NO RESPONSIBILITY FOR ANY MECHANICAL OR PERSONAL CONSEQUENCE ARISING FROM THE INTERPRETATION OF THIS DOCUMENT OR THE MODIFICATION OF ANY COMPONENT REFERRED TO WITHIN IT, AND RECOMMENDS THAT ANY MODIFICATIONs BE CARRIED OUT BY A SUITABLY QUALIFIED MOTORCYCLE MECHANIC.

Note: Please note that it may be illegal to modify certain components in some countries
and may require amendments to insurance policies in others.

Special thanks to Chuck Actor for the images of the HRC intake rectifiers and reed cages.

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