Introduction.

The NSR 250 leaves the Honda factory with only a stifled 40-45hp (depending on model), but was always destined by it's designers to be so much more, indicated by the flat slide Keihin carburettors (MC18 on), optional dry clutch, and port timing!

So, although any NSR model is a terrific motorcycle in its own right, it can be vastly improved by modifying a number of its components. As listed in the index, all of the major systems can be upgraded, each adding to a final package that can produce a highly enjoyable and satisfying days ride!

Throughout this guide, where possible, HRC and/or equivalent part numbers are given and also a recommended base setting is outlined. Base settings are usually provided by each manufacturer but are a guideline only. This is because the part is often designed to be used in conjunction with other modifications, thus requiring slightly different setting up depending on your particular conditions and/or state of tune.

Genuine HRC parts are often referred to as 'kit parts'; parts from tuning companies like TYGA, Jha, Ethos, and M-Max etc., are referred to as 'aftermarket parts'.

Each modification will have positive effect on performance, and generally it doesn't matter in which order they are carried out, but the first job without a doubt is to fit a speed derestriction device.

Speed derestriction: MC18 (1989 onwards) & MC21.

Speed restriction became law in Japan in 1989.

Now, allegedly, MC18's were not speed restricted, but the R5K & R6K bikes are  factory limited.

The MC18R2J/R4J's have a very simple restriction that is easily bypassed! Not only is derestriction on these bikes free, but once completed, there will be a horse-power gain too! What more could you want?!

The restriction takes the form of an electronic limitation of the RC Valve servo, preventing the motor from opening the Valves fully, and consequently reducing the power output to around 45hp, which in turn is approximately the HP needed to get an MC18 to around 115mp! Simple but effective!

To delimit the MC18R2J/R4J harness, remove the seat unit and locate the red connector at the very rear of the subframe (near the tail-light) as shown in the image below.

Look for the black/blue wire with the "bullet" type connection. Pull the connector apart and tape up the male end to prevent it shorting on the seat subframe and you're done! Like we said - simple!! Now, instead of of the motor falling flat at 10,000rpm (despite how carefully you have set the RC Valves up!), it will pull right through to the red in every gear! A little careful jetting and maybe a change of final drive ratio should see top speed increased to a little over 120mph and power up around the 60hp mark!!

The MC18R5K/R6K restriction is similar to that of the MC21. Locate the orange/blue wire coming out of the PGM-II unit, and splice it into the black/white wire as per the instructions later on this page.

The above dynographs show an MC18R5K in various states of tune. The Green trace (45.2hp) illustrates a completely stock bike as a base-line to assess modifications against. The purple trace (61.3) shows what can be attained with nothing more than the wiring modification and some careful jetting - some 15 horsepower! The blue trace (61.7hp) sees the bike equipped with race cans, a modified airbox, and some more jetting changes to account for the increased gas-flow. The final run is shown by the red trace (63.2hp) and shows the difference between race pipes and stock pipes. Although the stock MC18's expansion chambers are very good (Honda saw fit to only make them quiet, and not restrict them very much) the race pipes add 1.5hp at the top-end and also 500rpm of healthy over-rev. This is great on the track or down the pub, but the stock pipes really score heavily over the race pipes in the mid-range, producing around an extra 9hp more than the race pipes at 8000rpm!

There are many makes of speed derestrictor on the market, the most common is undeniably manufactured by M-Max. The NSR is restricted in 4th, 5th and 6th gear by a program in the PGM (ECU or electronic control unit) activated by a GPS (gear position sensor). This detects the top three gears and when the road speed reaches 180kph (112mph), limits the ignition advance, causing the power to tail off. The M-Max box disables the sensor and the PGM releases the advance, basically acting like a simple relay. Hey presto, instant gain in speed!

HRC sell a 'kit' part (MC21 part# 30490-NKD-840) that also advances  the ignition further than standard, although you'll probably need to secure your house against purchasing it! In 1991 the part sold for ¥13000, approximately £50 or US$75, prices quoted recently have been as high as $300 - work it out for yourself! The kit part will only plug into the HRC wiring harness.

The best alternative is the KISS Racing delimiter. It is recommended by Ethos Design, an official HRC supplier in Japan. This item performs almost as the HRC box, but still only costs ¥13000 (spring '99) The M-max will suffice if all you want is speed, If you intend to modify your bike to the extreme, go for the HRC/KISS type. 

NSR-WORLD.COM has extensively used both HRC and KISS systems with complete reliability.

Speed derestriction: MC28 "Swipe Card".

The MC28 was unique in it's day as a 250, being the first to sport a digital display, and the first and only 250 2-stroke road bike ever to sport a single sided swingarm! It suffered a similar problem to the TZR250 V-twin though, in that it was a lot more difficult to derestrict than its predecessors, but due to the NSR's popularity, it was never quite the 'lemon' the Yamaha was!

The "Swipe Card" system was initially promoted by Honda as a leap forward in tuning potential, with the availability of different cards for different tracks and riding conditions. What they had really done was cleverly disguised an ignition restriction that was virtually impossible to bypass. Then, for added security, coded full power cards to disable road equipment, and made them only available to purchasers of the HRC wiring loom, for which, just to make things even more difficult, required a race licence to buy!!

A popular misconception regarding delimiting the MC28 is that you can simply buy an HRC card for the ignition. Unfortunately this 100% urban legend!!

Although 3 different cards were originally available (1 stock, and 2 HRC), the 2 HRC cards could only be used effectively in conjunction with the HRC wiring loom (part# 32100-NKD-970). The HRC loom has no provision for road riding equipment though; the lights, indicators (turn signals), brake light, oil, side stand, and neutral warning lights are all removed. The speedo is also disabled and the display's used as a temperature gauge. 

The 2 original HRC cards have now been replaced with an "all-purpose" version.

The 4 cards are designated as follows:

Any new card (HRC or stock) needs to be coded to the particular PGM unit that it is to be used with for security reasons.

The difference between the HRC 010 and 020 cards is a small variation in power delivery, managed by the ignition advance. The 020 card has a less aggressive power delivery to promote traction in wet or slippery conditions.

The HRC cards can now be bought separately from the HRC loom, and although their effectiveness in conjunction with an otherwise stock bike is questionable, they will allow a slight increase in power once expansion chambers are fitted and the carb's are re-jetted.

In 1994 HRC introduced a new flywheel and stator (part# 31100-NKD-970). The benefit of this was a more compact design, and hence, lighter weight. Less weight meant less resistance to inertia and a "snappier" throttle response, but at the expense of a little (although already virtually non-existent!!) torque. Bought separately, the cost of this item is rather unjustifiable at ¥200,000 or £1000 ($1500) for all but the professionally rich!!!

An alternative to the HRC flywheel on the MC28 is the fitting of an MC21 item. Of the MC18, 21, and 28 flywheels, the stock MC21's gives the greatest advance (the MC28's gives the least). Although not as light as the HRC item (it weighs the same as the MC28's), it will have an all important performance advantage over the stock unit. 

Lightened flywheels for race use only will be available from NSR-WORLD.COM in the near future!!

Note:	It is not recommended to use the MC21 flywheel on the MC28
	with an HRC card, only use it with the stock "PGM-4" card.

Spark Plugs.

HRC specify short reach plugs for a race set-up for both the MC21 and MC28. They will also work for the MC18. These plugs are RS250 items and come in either 10 or 10.5 heat range, and the part numbers for these are 31930-ND5-003 and 31940-ND5-003 respectively. Special caps need to be used with these plugs (part# 30700-ND5-751). After removing the old caps trim 5mm from the HT leads before fitting the new ones to ensure a good reliable connection.

Wire Splices.

Both the MC21 and MC28 ignitions can be delimited by altering the wiring harnesses. Although an extremely effective and cheap alternative to the HRC solution for both models, it is NOT as effective as utilising genuine parts on the MC21, but it does give you the benefit of using all the road-going equipment, and most of it on the MC28!

MC21 Wire Splice.

To gain full OEM power on the MC21, 2 wire splices need to be performed. the first, and more important of these disables the ignition retardation in 4th, 5th, and 6th gear. Although a stock NSR will rev in these higher gears, the retarded ignition prevents it from making any useable power and therefore acts as an electronic speed limiter. The offending wire in this restriction is the 'legendry' Orange and Blue wire! In the OEM configuration it is routed to earth (ground), but to delimit the ignition it needs to be powered by 12V. 

Follow these 10 simple steps to unleash free hp!!

 

• Switch the ignition off, and disconnect the battery.
• Unplug the black and white connectors from the PGM-III and remove the insulation from the black connector to expose as much wire as possible.
• Separate the Orange/Blue and Black/White wires from the rest.

• Snip the Orange/Blue wire approximately 70mm from the black connector, and on the connector side, remove 5mm of insulation to expose the copper core inside.
• At the same distance from the connector, remove 8mm of insulation from the Black/White wire.

Do not cut the Black/White wire, only remove its insulation.

• Solder the Orange/Blue wire to the Black/White wire. Use plenty of soldering flux to ensure a perfect connection.
• Insulate the soldered connection, and also the bare end of the snipped Orange/Blue wire with insulation tape or better still, heatshrink.

• Wrap all the wire back up with insulation tape, and no-one will ever know the difference.
• Plug the black and white connectors back into the PGM unit
• Reconnect the battery, and the job is done.

The second wire splice for the MC21 is performed to the gear position sensor (G.P.S.). This splice "tricks" the PGM-III into thinking it never gets out of 3rd gear, and as such uses 3rd gears more aggressive ignition curve from 4th to 6th gear.

• Identify the G.P.S. wire harness coming from the gearbox, ending with the light grey connectors under the seat on the left subframe rail, and disconnect the multi-plug.
• Snip the 4th, 5th, and 6th gear wires. (Light Green/Yellow, Light Green/Black, and Light Green/White respectively.)
• Identify the 3rd gear wire (Light Green/Green) and as with the previous wire splice, remove 8-10mm of insulation from it.

Do not cut the Light Green/Green wire, only remove its insulation.

• Solder 4th, 5th, and 6th gear wires from the G.P.S. to the 3rd gear wire, use plenty of flux to ensure a perfect connection.

• Insulate the soldered connection and the bare ends of the 4th, 5th, and 6th gear wires to the PGM-III to complete the splice.

G.P.S. splices have already been performed to 1st gear to utilise the PGM-III's most aggressive ignition curve, but NSR-WORLD.COM strongly advises against this. The advanced ignition in the top gears is seriously likely to cause detonation, the quickest killer of 2-stroke engines!!

 

Above is a custom G.P.S. wiring harness made to plug in-line between the gearbox and PGM-III. You can see how the 4th, 5th, and 6th gear wires are spliced together and plugged into the 3rd gear wire. This harness uses "bullet" connectors so the 3 wires can be routed to 1st, 2nd, or 3rd for research purposes!

MC28 Wire Splice.

Jha will modify a standard loom to accept an HRC card, but unfortunately this service is currently only available to customers who can deliver their bike to Jha's workshop. This can be more than a little inconvenient for the majority of current owners, we're sure you'll agree, so the following section details how to modify your stock harness to accept an HRC card. 

The advantage of this modification over the HRC wiring harness is that most of the stock electrical systems will still function. (i.e. lights, indicators, horn, etc.)

• Unplug the black and white connectors from the PGM-IV and remove the insulation from the black connector to expose as much wire as possible.

• Now separate the following wires from the rest: Green (earth), Light Green/Red (neutral switch), Green/Pink (oil level indicator), Green/White (side stand switch).

• Snip the LtGr/R, Gr/P & Gr/W wires approximately 60mm from the black connector, and remove 5mm of insulation to expose the copper wires inside.
• At the same distance from the connector, remove 5~10mm of insulation from the Green wire. 

Do not cut the Green wire, only remove its insulation.

• Solder the three snipped wires to the green (earth) wire. Use plenty of soldering flux to ensure a perfect connection.

• Insulate the soldered connection, and also the bare ends of the snipped wires with insulation tape or better still, heatshrink.

• Wrap all the wire back up with insulation tape, and no-one will ever know the difference.
• Plug everything back together, and the job is done.

Special thanks to TYGA Performance for the detailed overview and pictures for the MC28 wire splice.