NSR250 Jetting | NSR-WORLD.COM

NSR250 HRC Jet Kit

Re-jetting needs to be performed after ANY modification, no matter how small. The best case scenario of not re-jetting is missing out on a possible performance gain, the worst case scenario is seizure!

 

Note: Although a fairly straightforward procedure, jetting involves very fine adjustments, the replacing of very small components, and a large amount of patience! So if you are in any way in doubt whether to adjust the factory settings - DON'T!

 

NSR-WORLD.COM Jetting Terminology.

Jet Kits.

HRC, Jha, and Factory are the three most common names covering jet kits for the NSR series. The Jha kit is the most comprehensive, with fitting and set-up instructions available in English.The Factory kit is much less common, and rarely found outside Japan.

A base setting.

A base setting is usually supplied by an after market parts supplier. The problem here is that this setting is often based on a standard machine fitted with their item only. For example, when Ethos supply an exhaust system, they also supply 2 MJ's to suit. This may be fine if you are replacing your stock chambers only.

Undoubtedly, a basic understanding of what's going on inside your combustion chamber, and of how each component interacts with the next is very important!

Where to start?

Start by noting down your current settings! If everything "goes to pot" you can always revert to your initial setting and start again!! As mentioned in the carburation section, NSR's came from the factory with a variety of configurations, probably due to both model evolution and climatic conditions. (Areas of Japan are very mountainous and because of this, bikes sold at higher altitudes could've come fitted with adjustable needles etc.)

The three main areas to cover when jetting are the idle and slow running, the progression, and top end, and each is as important as the next. Without all three areas being optimised, you will never begin to appreciate the full "fun" potential of the NSR!

The slow running is metered by the SJ and AS, the progression by the needle, and the top end is governed by the MJ (and PWJ if used).

The following diagram shows which system is in operation at each throttle position.

The slide governs the primary airflow through the carb into the crankcase, and is directly proportional to the throttle grip position.

Main Jets.

The MJ is usually the first item set.

If you estimate an MJ setting and it's even only vaguely close, you could ride around town all day without the engine going bang! In fact, as mentioned right at the beginning of this guide, you will only encounter problems at this stage if you get carried away thinking all is well, and blast off into the distance on your favourite stretch of road without extensive refining!

Don't let this detract from the overall importance of the MJ though, or the first straight you come to, with the throttle wide open in any gear, the engine will seize quicker than you can say, "new pistons, please"!

Two things to understand about the MJ are first, fuel is 'sucked' through it, and second, bigger is not always better!

Air is drawn through the carb by the vacuum in the crankcase created by the upward motion of the piston. As this air passes over the top of the emulsion tube, fuel is "sucked" from the float chamber up through the MJ, into the emulsion tube where it starts to atomise with the airflow, this 'mixture' is then drawn into the crankcase.

 

Note: Jha's and HRC's settings and parts are intended solely for racing purposes. Both companies naturally assume that along with the removal of the oil pump, all other race modifications have been carried out!

Both HRC and JHA recommend the plugging of the PWJ's, preferring only one variable, the MJ, for full throttle fuelling.

 

Slow Jets.

The slow running and progression are a lot more complicated than you may think, with a number of variables to account for.

As the throttle is opened, the slide is lifted, and as the slide raises, more air is allowed to flow, 'sucking' on the SJ. (The straight section of the needle effectively blocks off the MJ at minimal throttle.)

Along with the raising slide, small pre-drilled passageways within the carb body, called galleries, assist the SJ in metering fuel from idle to 1/2 throttle.

Air Screws.

Air screws are the final element of the idle and slow running circuits, they are often referred to as idle mixture adjusters.

Due to the limited sizes available for the SJ's, some form of finer adjustment is needed, and this is where the AS's come into play. The AS's are a tapered, well, er, screw! As shown in the above diagram, an auxiliary air gallery is cast into each carb body, and the taper on the screw, depending on its position, allows extra air to bypass the opening created by the slide, weakening or richening the mixture.

Needles.

The needles, along with the emulsion tubes, sometimes referred to as needle jet/main jet holders, take care of the progression phase. The progression phase is the transition from the slow running circuit and 1/4 throttle, to the MJ (and PWJ if used) and full throttle.

Needles are available in different grades, usually denoted as A, B, and C type, with profiles suitable for both leaded and  unleaded fuels. In the NSR's case, the A type is the richest, i.e., the slimmest profile, allowing the greatest flow of fuel, and the C type is the leanest. As mentioned before, B needles have a profile extremely similar to the non-adjustable needles found in the vast majority of stock carburettors, and are usually suitable for most applications.

Fuels.

Often overlooked, the type of fuel used can regularly effect the jetting process, particularly on a 2-stroke engine.

Japanese sports bikes have been designed to run on unleaded fuel since the mid '80's, and a stock NSR will happily run all day long on it without any adjustment.

JHA and HRC advise the use of 100+ octane race fuel with their kits, which accounts for, in part, for their choice of base setting of the MJ's, SJ's, and pre-mix ratio. Even higher (than regular) octane fuel such as Super Unleaded can adversely effect the jetting on a modified motor, and will often necessitate the richening of the mixture.

You will not necessarily notice a difference in feel when using the higher octane fuels, especially on a stock motor, and consequently wonder if it's worth the extra expense over regular leaded fuel, but a higher octane will allow you to run both more compression and more ignition advance over stock without as much fear of detonation. This is a point worth noting when an HRC or equivalent ignition delimiter is used on an MC21, or the HRC harness, card, and flywheel conversion, is used on the MC28. 

NSR-WORLD.COM recommends the use of 97+RON Super Unleaded
by virtue of it's increased detonation prevention characteristics,
as a general precaution against premature piston and cylinder failure.

 

Note: NOTE: DETONATION CAUSED BY HIGH COMPRESSION/HIGH IGNITION ADVANCE & LOW OCTANE FUEL HAS TO BE RATED AS ONE OF THE QUICKEST, IF NOT THE  QUICKEST KILLERS OF A 2-STROKE ENGINE... BE WARNED!!!

 

Note: Using the higher octane fuels will make reading a "plug chop" harder, as the fuel burns more fiercely and less residue is left on the electrode.

 

Finally, also consider the fact that as Super Plus unleaded is more highly refined than regular unleaded, it is therefore cleaner. This can only be a bonus as you are less likely to encounter blocked jets, especially the small PJ's, another 2-stroke killer. Remember though, that most fuel nearly always comes from the same refinery, so it doesn't matter where you buy it from!

Jetting Guide - Quick Reference.

© Aktive Reeds : http://www.aktive.com/

Quick Jetting Guide: © Aktive Reeds

1) IDLE. Set idle speed to proper r.p.m. by adjusting the IDLE SPEED SCREW. Turn the IDLE MIXTURE SCREW or AIR SCREW to achieve highest speed and best response. After adjustment has been made reset IDLE SPEED SCREW to proper r.p.m.

2) OFF IDLE to 1/4 THROTTLE. The SLOW JET and SLOW AIR JET are most effective in this range. When you want a richer mixture in this range use a larger SLOW JET or a smaller SLOW AIR JET. The opposite holds true for a leaner mixture.

3) 1/4 TO 3/4 THROTTLE. The JET NEEDLE is the most effective component in this range. Raising the needle by lowering the clip position at the top of the needle will richen the mixture. Lowering the needle will lean the mixture

4) WIDE OPEN THROTTLE. Changing the MAIN JET effects this range. Select the size which offers the best W.O.T. performance, then install one size larger MAIN JET for ideal engine durability.

 

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, AND RECOMMENDS THAT ANY MODIFICATION ONLY BE CARRIED OUT BY A QUALIFIED MOTORCYCLE MECHANIC.

Carbuettor Strip-down and Jet Kit Installation.Carburation Index.Engine Tuning.