FAQ Answers

Motor Brand & Model

Why isn't Stock Moto open to other motors like the YZ125?

Different brands of motors, even different years of the same motors have differences in power output.  Originally when Marty Casey of SKUSA was compiling the rules there was an opportunity from Yamaha to use their Karting Kit version of the YZ125, and they were even considering offering some sponsorship to the class but an executive decision overruled the negotiations, and resulted in discontinuing the YZ125 kart package altogether.  The YZ125 is a superior motor to the CR125.  It comes with a domed piston, O-Ringed head, straight manifold, an Exhaust Valve that needs no replacement, and a Digital Ignition Curve that blows the socks off the CR125 Ignition Curve.  There were about 100 of these motors motors imported however as they have been discontinued they are not a viable choice.  Typically a YZ125 out of the box has about 2HP advantage on the CR125 in fully tuned condition.

The Stock Moto Standards have some provisions for Local Options to allow the YYZ125.  It is always up to the promoter to determine whether they will support Local Options.

What is the CR125 Kit Motor?

In 1999 JR Clasen worked with Honda to assure a stable supply of CR125 kart friendly packages and came up with a Bill of Materials to provide a CR125 Kit package.  This was based on the the 6 gearsets from the 1997 model (Honda went to 5 speeds in 1998), the 1999 Ignition, and the 1999 Cylinder & Head.  These kits are shipped un-assembled and have some flexibility in configuration;  For example you can order a CR125 Kit motor without the CR125 Manifold.  If effect you can add or delete any parts you choose.

What makes the "99" Spec so special for Stock Moto?

The 99 Spec is the most common package of shifter power in America.  Over 2000 99 Spec motors have been sold, roughly twice as many as all brands of ICC motors combined.  Given this volume of sales Honda will continue to provide a discounted CR125 Kit package as it maintains a flow of manufacturing and production orders which allow the discounting to continue.  It also assures that stock locations will maintain a decent supply of parts.  Due to those volumes, a new 99 Cylinder costs about 75% of what the 2000 and later cylinders cost.

Just as important is that a single year of major performance impacting components like the cylinder, head, and ignition make the job of enforcing tech much easier.  One of the key features of the 98-99 cylinder (they are the same) is in the design of the Exhaust Ports.  The roof of the ports is machined rather then cast assuring a consistent port height.  In fact all the dimensions provided in the Stock Moto Standards are only repeatable in manufacture on the 98-99 cylinders.  Earlier and later years of cylinders have exhaust port heights controlled by casting only with deviations that encourage modifications that are hard to spot.

Different years of ignitions have different ignition curves as well as different connectors.  Just changing a connector will not allow non-matching models of components to work together as they have different electrical requirements as well.  In the past there has been great suspicion over "OEM" Ignitions as more then one Custom Ignitions have been potted into a stock appearing module.  Testing a CDI Module is a costly task.  The most practical solution is to enable Tech to perform a Grid Swap of Ignition Boxes at their discretion which is the most likely way to assure "honesty" with ignition modules.  That requires all competitors to have the same module, hence the 99 was chosen as it is the most common.

5 speed or 6-Speed?

Although the complete 1999 Honda CR125 motor is actually a 5 speed the CR125 Kit includes the 6 speed gearsets.  The first 5 gears are the same ratios but since most Motocross riders never use 6th gear, Honda decided to eliminate it in 1998.  Most organizations allow either 5 or 6 gears, just be aware that 6 gears will be a better choice.  If you have a 5 speed motor it will cost about $200 to add the gearsets and 6-speed shift drum to upgrade to 6 gears.

How long will Honda supply the parts?

Honda has one of the best Supply Chains in all of industry in fact they are considered by many including GM, Ford and Mercedes Benz to be a model to follow.  Most of their motorcycle and ATV parts are available for products that are 15 years old and more.  Obviously if a part has some demand they will sell them.  There have been over 20,000 1998 & 99 CR125 motorcycles sold in the US and most are still running.  There have been over 2000 CR125 Kit motors sold, and they are currently selling 400 a year, several times more then all the ICC's combined.  CR125 parts will be available for at least another 10 years.

What are Motor Builders doing to "Blueprint" Stock Motos?

Fortunately there isn't much that a motor builder can do, and what they do is within the scope of most karters.  As the motor is a Kit, it comes un-assembled.  There are 343 parts in the kit and they are packaged in sealed bags - even the bolts & washers.  Take the person who is familiar with teardown/assembly of 2 strokes like the KT100 and hand them a CR125 Kit Motor and the assembly instructions in the Building & Tuning the CR125 Guide, and they will take about 12-16 hours to assemble the motor.  On the other hand, let someone with experience do it and it takes 4-5 hours - over an hour of that is just ripping the bags open!

Blueprinting is relatively simple, and the following gives a list of what's done: 

The most important part is shaping the aftermarket Exhaust Valve Plugs.  It takes about 10 minutes and requires a Dremel Drum Sander and a Magic Marker.  As the Exhaust Valve bores are the machined surface that intersect the cast passages, no two plugs will be the same.  Mark the backside where the plug matches the cast passage and taper the front edge towards the cylinder to a knife-edge and thats done.  10 minutes tops.

Install a set of Medium Carbontech Reeds.  As Reeds are difficult to tech to OEM, and they are replacement parts, they are open.  Reed Cages MUST be OEM.  Medium Carbontech Reeds provide the best performance.  Not a big improvement, but every little bit helps.

Be sure the Reed Stops allow the reeds to open as far as possible.  The OEM Reed Stops are required but it's not really feasible to put a spec on the exact shape - and it will vary a little from one to the next anyway.  Slide the assembled Reed Cage in the Reed Plenum and wiggle it up & down.  It should fit snug with no wiggle.  Remover the Reed Stops and put the screw side in a vise and bend them out just a tad.  Be sure the upper and lower Reed Stops are bent to equal amounts and slide in snugly.  Again a small gain in power, but worthwhile.

Advance the static ignition timing to about full advance.  This of course will depend a little bit on which pipe and what fuel you use.  Also some organizations like SKUSA allow "slotting" the timing plate holes to allow more advance.  It's really not necessary but the argument is that some ignitions may be a bit off.  This is where a dyno is handy - as setting the timing to the pipe and fuel will give a small advantage.  Then again, if a driver is at the level where that small of an advantage is worthwhile, they'll be adjusting static ignition timing at the track anyway.  The difference in lap time may be measured in hundredths of a second at best.

Careful assembly of the Top End to meet specifications is a best practice assembly method.  In most cases the Combustion Camber Volume (CCV) will range from 10.8 - 11.0 CCV depending on the method of tech chosen.  Give yourself some room on this as base gaskets tend to compress a .001-.003" after running them a while.  The closer you can get to the maximum compression the better, but again it won't make much difference.  What's important is staying on the legal side with CCV and base gasket thickness.  (While big HP gains are available from stout compression, generally that means you'll be very illegal before you'll see a noticeable gain).

Installing the exhaust Header Flange may seem like a no brainer, but the better you can match it to the port, the better it will work with the port.

Pistons are generally not going to need any special attention, but rings do.  Most new OEM rings have about a gap range of .010 - .020 and the optimum is more like .007-.009".  Wiseco buys their rings from the same OEM Honda does and they are an exact fit except Wiseco specifies a tighter gap tolerance.  Most Wiseco rings come at .004-.006" gap so you'll need to file then to fit (007-.009").  Rings always should be lapped a bit with a ring lapper on 400 grit - this helps hold an oil film both to lubricate and seal better in the ring land.  Again small improvements but worthwhile.

Finally at the center is the Crank & Main Bearings.  Honda has a .0002" press fit for Crank to Mains making assembly and disassembly a bit more difficult.  You can sand the crank journals down by .0002" either by hand (about an hour) or by putting the crank in a lathe and sanding (about 10 minutes).  In either case the motor won't go any faster, but it makes the assembly and disassembly go much easier.

Honda seems to have nailed crank alignment over the years although it it can't hurt to check.  A crank thats out by .002" does begin to show noticeable losses on a Dyno.  Most motor builders will check and align a crank for about $20, as it's about 10 minutes of work.

Potentially this blueprinting amounts to 2-3 HP on the average, and maybe 4 HP on a worst case example.  There simply isn't that much that can be done on the Stock Moto like there is on an Open Moto, ICC, or even a KT100.  Most motor builders are going to charge around $300 for the blueprinting and another $300 for dyno tuning.  For some people thats worth while, for others it's not.  The bottom line is the steps outlined above are about the only ways to find any power and most people can do themselves.

What about converting modified Legacy Motors to Stock Moto?

With 2000 Motos sitting around you would think a few of them would like to join in with the Stock Moto class.  It could be as simple as putting the stock Reedcage, 99 Ignition, 99 Cylinder & Head on, and go racing.  A lot of folks have their old 99 OEM ignition, and their Reed cage sitting around somewhere, so for under $400 they can convert to stock with a fresh top end, at least in theory.  Most of the Mod Motos have had bottom end mods that make them illegal to the Stock Moto Standards.  Removing the Kick Start Boss is legal as it's not a performance enhancer. 

Removing the ramps on the floor of the Reed Plenum can be considered a performance modification as it does provide some gains at the top of the RPM band.  This mod generally has a positive effect from about 11,500 RPM and up, but also has a detrimental affect below about 10,000 RPM.  It's probably more of a hindrance then a gain on a Stock Moto and we don't want to encourage people to make these mods.  As a result this is covered in the Local Option where adding 10 Lbs will serve as a deterrent, as well letting people know that they will likely need to buy new cases in future years also serves as a deterrent.

Matching the Port Plenums in the crankcase to a ported cylinder actually reduces performance with a stock cylinder.  The flow is interrupted at a critical entry into the port passage.  This is treated in the same regard as the Reed Plenum ramps referring to Local Options.

2 Carburetors and Intake Boots

If it's supposed to be Stock why is the RS125 Manifold allowed?

The CR125 carb boot was designed to both make the best power yet allow for easy access to the airfilter.  Great on a bike, but a packaging issue on a kart.  Its not too big of a problem for Medium seats but any larger and the seat struts just won't fit in a usable manner.  Install an XXL seat and you have to move the seat over so far that is also takes shifting the rear axle off center by about an inch to get the counterweights equal.  It may stop straight but it handles different on left & right turns.  Thats a problem.

Just because an RS boot comes from the RS and is straighter then the CR boot doesn't mean it makes more power.  On the contrary, when used with the CR Reed Cage it actually gives a little power up.  Why?  Reeds are opened by pressure which is affected by the velocity and changes to the velocity. 

An RS Reed cage has 4 petals - 2 on top, 2 on the bottom.  The highest point of pressure is in the center of the reeds while along the outer edges near the rubber stuffer "ears" the pressure is lower.  Thats OK because the pressure at the center still has enough pressure to open the reed all the way across to the edge.  Now put that same stuffer in a 6 petal CR reedcage and guess what? The center reeds open fully, but the outer four reeds just don't open all the way. 

The CR intake manifold has huge stuffers and not because Honda wanted to restrict the flow.  They increase the flow on a 6 petal system because they increase the pressure on the outer reeds.

Just how much difference?  We did some dyno testing and found about about 1/2 HP drop with the RS Boot.  We cut off the little stuffers for the RS Boot and that dropped it almost 1 HP under the CR boot.  While 1 HP sounds like a lot, consider this;  if a competitor with an XL seat is off on turn speed by only .3 MPH it's going to take 2 HP to make up the difference on a 600' straight.  So it's a reasonable trade off.

Keep in mind that most of the first folks that embraced Stock Moto were average sized adults most between the ages of 30 and 40 years old.  They don't mind a little disadvantage to a smaller or younger driver, so it's best for everyone to compromise on this.  The committee votes were 10 for, and 2 against.

Is a Pumparound allowed in Stock Moto and do I need one?

Yes the Pumparound is allowed.  There are a couple of vendors who will convert a carburetor  to  a Pumparound for about $150, or you can do it yourself in about an hour with the instructions found in the Tuning & Building the CR125 manual.

The most common carb found on a Moto is the Keihn.  Keihn and Mikuni carbs were designed for gravity flow which is quite a bit less pressure then the pumps produce, so the pumps can overwhelm the float needle even with a return line that bleeds off the pressure.  Mikuni offers a a range of float needle seats which can take care of the over pressure problem.  The Keihn has a fixed seat and sometimes it works, sometimes it doesn't.

Another problem with the Keihn is that some have "sticky" floats.  Basically the float may jamb open or closed on high G-force turns.  It usually doesn't cause the motor to stick, it just loses power momentarily.

So if you find that the carb works fine on turns and doesn't seem to be losing fuel through the vent hoses, the you don't need a pumparound.  But if the carb is an incurable hesitation on a high G turn at the same place or you're losing a lot of fuel out of the vent hoses, you need a pumparound.

3 Pipes & Silencers

What are the best Pipes for a Stock Moto?

From our testing the RCE T3 works the best, and it's probably no coincidence that the top 2 Moto builders Paul Leary and Darcy DeCoste also use the RCE T3.  Whats motre interesting is that the RCE T3 is practically a clone of the 1995 RS125 B-Kit, which is still the hottest pipe used on the RS racing bikes.  When SKUSA adopted the rules they chose from 3 pipes; The RCE T3, the RLV 6800, and a pipe that Paul Leary was having custom made until he returned to the RCE T3.  We'll cover each of the pipes we tested in the order in which they work.

RCE T3

This is the best pipe going and fits the ignition curve like a glove.  It begins to come on as low as 9000 RPM and about 9800 it surges to the peak torque at 10,200 and makes about 35 HP peak at 10,500.  It has a shift point of about 11,800 and overrevs to about 12,200 with a "brick wall" effect on revs.

RLV R4

Kind of like the RCE T3 except move the whole RPM band up about 300 RPM with a little less peak torque.  This pipe also makes 35 HP but at 10,900 RPM. Shift point is about 12,100.  Its actually a little easier to drive then the RCE T3.  It would probably work better then the T3 on a Road Course since it doesn't brick wall at the overrev point.  The powerband is slightly compromised as it's designed to operate at RPM's beyond what the 99 ignition curve is optimized for.

RLV B4

The RLV B4 is out of production but there are a lot of them around.  Its a good pipe but it's not really matched to the ignition and needs higher compression or mildly oxygenated fuel to work effectively with the 99 ignition curve. 34 HP peak at 11,100 RPM.   It has a very wide power band usable power from 9000 to 12,200 and overrevs to 12,600.   34HP at 11,000 RPM.

MacMillian

This is an old technology pipe that was great in it's day but not compatible with the OEM ignition.  It has a wide power band, at the expense of torque.  It also needs more compression or highly oxygenated fuel to work effectively with the 99 ignition curve.  34 HP peak at 11,300 RPM.

RLV 6800

The RLV 6800 was chosen because it was rated as a "soft" pipe.  Its an ancient design and lacks the power of the others.  Originally it was offered to detune the motor, something not very popular with racers.  33 HP peak at 11,100 RPM with usable power from 9200 to 12,000.

What are the best Silencers for a Stock Moto?

Silencers are sort of a fine tuning instrument.  At the bottom of the powerband a free flowing silencer allows the pipe to come on early yet at the upper end of the powerband the silencer should build backpressure, just enough to stretch the efficient operating zone of the pipe a couple hundred RPM.  RLV, RCE and M4 all make good silencers but knowing which one is best depends on the track.

Generally using the silencer recommended by the pipe supplier is the best one to run.  There are a few silencers the "Moran Spec" that are 14" long with the end pipe turned down that have been shown to work very well, at a reduced noise level.

4 Variations in Organizations & Rules

Why do some Stock Moto rules allow multiple years of Cylinders and Ignitions?

As the class was first beginning to grow in Southern California a few new motors began to show up with 2000 and later cylinders as they were never ruled out.  As Marty Casey of SKUSA was evaluating a rationalized version of the Stock Moto there was an initiative to grandfather the existing motors running in Stock Moto that wouldn't conform to the 99 Spec CR125.  That would have included the 2000 and later CR125's as well as the YZ125's.  No one came forth to register these motors.  Marty left just as the new rules were to be implemented for 2005 and as a result the multi year components were left in the rules.

For 2006 the only regional series that allows a multi-year mix is ProKart and SoCal karters, both in southern California.

Why do some Stock Moto rules limit the choices of Pipes?

As various regions were working with developing the Stock Moto Standards the one thing that seemed to cause the most concern was from pipe selection.  Some wanted brand X, some wanted brand Y, others wanted brand Z.  There was an overall concern that pipes would turn into pipe war if a certain pipe & silencer wasn't specified.  No one seemed to understand that pipe technology had progressed much faster then it did.  Further if a specific pipe was to be specified it needed to be measurable and repeatable in production.  Moreover it shouldn't be single sourced.  In effect there really weren't any perfect solutions.  So what we decided to do was propose a circumference limit to prevent anyone from exploiting that dimension and leave the selection of a pipe to each of the regional promoters.

North California Karters wanted to build on a singular spec pipe and chose the RLV 6800 with a spec silencer.  The main reason they chose the RLV 6800 was with the intent of getting a longer life out of the motor.  However the side effect was to divide regions up.  For 2006 the Southwest Racing Association followed suite effectively outlawing 95% of the existing Stock Moto's from running their series.

All other series are either allowing a choice to 3-4 pipes or just leaving the pipes open.

Will the Stock Moto rules ever be all the same?

Eventually they will come closer together but until some of the promoters get over the "not invented here" syndrome that won't happen.  The funny part is the same promoters who wanted no part of the Stock Moto Standards committee are the same ones that have to create their own rules.  We've been down this road before with TAG and look at it today.  And yes, we had the same problems with the same maverick promoters a few years ago with TAG.  If they repeat with Stock Moto what they did with TAG they will fall in line with the Standards.

We're pretty optimistic that the majority of the country will be on the same page in 2007.  Moreover, we now have some very good low cost tech tools available for the 99 tope ends that have been proven at the Florida World Tour this year - even used as the basis for a DQ.

How stable are the Stock Moto rules?

The Stock Moto Standards are very stable.  Given the quality Control of the Honda parts and the specifications are a mirror of what's been in production for several years now there is very little that will need changing.  Probably as stable if not more so then Rotax classes are.  What you can expect is that those regions who do not conform to Stock Moto Standards this year will do so next year.

What are the Stock Moto Local Options for?

There are a handful of stock YZ125's and CR125's with various years of parts that would benefit their fellow racers by being a part of the class.  Each local area always has their exceptions.  This includes legacy motors who would like to backdate their motors to run Stock Moto.

How do the Local Option Shifter Groups apply to Stock Moto?

The Shifter Group concept is an idea borrowed from John Bishop of IMSA from years ago with prototype GT racers.  As the sports car racing world went through times of feast to famine Bishop knew the only way to put on a good race was to combine several different class formulas into a couple of classes - a rationalization of classes.  The result was GTP, GTO and GTU.  As the 9 major classes of shifter karts often resulted with 5 entries at the average local club we created a formula and implemented this formula a few years ago at the Denton track for club racing.  Group 1 is for ICC's and Mod Motos with a 25 lb handicap on the ICC's.  group 2 is for Mod Motos at 420 lbs, Stock Motos at 385 lbs, and 80 Shifters at 350 lbs.

Why did Kartweb get involved with Stock Moto Standards?

Most Org rulebooks look like they've had so many insertions and deletions it's difficult to understand them.  Further, very few karters have the time to really get involved and say what they really want.  Most rules are written by people that either don't know how to build the motors they are specifying, or when they do, they are writing rules that they benefit from somehow.  In essence most karting rulebooks stink.

We did our first Standards thing with TAG and it worked.  That wasn't because we are good at Standards, it succeeded because it was what the karting customers wanted.  The karting retail industry was never known for brilliant marketing moves.  Think about it;  How many kart retailers do you see in the advertising at Kartweb?

Stock Moto was the right thing, but left to the marketing geniuses of the retail industry, and the rocket scientists who write the rules for the orgs, Stock Moto wasn't going anywhere.  A year after the Stock Moto Standards and look at it now.