Home Forums Shifter Karts Building a Stock Moto CR125

This topic contains 8 replies, has 5 voices, and was last updated by  Ben Lewis 10 months ago.

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  • #53958

    Benjamin Lucas
    Participant

    In the finest tradition of biting off more than I can chew, I have made the decision to build my own CR125 motor. The main goals are to save money, learn stuff, and hopefully wind up with an adequate club-level motor at a low price. I’m looking for any help that anyone is willing to provide.

    I’m starting out with a ’99 bottom end (eBay item# 391238280978). I figure I’ll use a new crank, and the piston looks like it may have seized, so I’ll replace that as well. I’m keeping my eyes open for a ’90-’97 6spd at a low price.

    Does anybody think it crucial to have the factory crank trued for a local club motor?

    Also, with kart-specific power valve plugs going for ~$100, it seems to me that getting a cylinder from a kart supplier justifies the expense over ordering off the shelf from Honda, not that it’d be hard to shape and install the plugs, but it doesn’t seem like the cost savings would be worthwhile. I don’t think I’d trust a used cylinder anyway without getting it refreshed, so factor that cost into any used cylinder and the cylinder is not looking like the right place to save a few bucks.

    It does seem to me though that a cheap radiator is a place where I could reasonably skimp (bearing in mind that this is a low-budget endeavor here).

    So I’m curious. What other areas are prone to kill your budget when trying to set up a low-budget Stock Moto engine?

    Anyway, this is my first post here. Just to introduce myself, I’m an older guy, nearing 40. I have some autocross experience, and rental karts, and motorcycling. I will be starting 2016 out with a LO206 package, but I’m just sort of piecing together this engine, slowly, for shits and grins as sort of a winter project, just spending time scrounging for parts and seeing what I can come up with.

    Anyway, your help, guidance, insight, etc will be greatly appreciated.  Cheers!

    #53966

    Patrick Roth
    Participant

    Bejamin,

    Welcome to the forum!  I am by no means an expert but will chime in with my two cents…  If you were starting with a motor with a known karting history I would say the repair/rebuild would be relatively easy to deal with yourself; however, I recently watched a friend of mine try to save money by buying a bottom end off of ebay only to have it grenade and cost him a lot more in the end (costly bottom end rebuild in addition to a second top end).  I’m not saying to not go the ebay route, but my recommendation would be to pay for a complete rebuild by someone who does this for a living so they can inspect all the components.  It’s a lot of $$$ and takes away the fun of learning and rebuilding, but IMHO it’s better than the risk of learning the hard way and costing a lost more $$$.

    With that being said, I believe everyone here would use a “slip fit” crank since a stock crank would require somewhat specialized assembly of the main bearings.  I’m sure each engine builder has their own black magic but Honda is a world class manufacturer and I doubt they would put their name on anything that wasn’t durable/reliable/consistent (not to mention they have used and perfected their motors for many years).

    There are so many nuances about getting the setup that has the cost, speed, and reliability to match your needs I would highly recommend talking to an engine builder.  I have been extremely satisfied with my dealings with Musgrave Racing (MRC) and highly recommend a call and conversation with Willy Musgrave.

     

    #54022

    jack lachut
    Participant

    Benjamin, do a web search and look under Honda cr125 rebuild guide or

    http://www.metal -matrix.com/cr125rebuild

    good info on disassembly and reassembly with pictures.

    #54025

    Benjamin Lucas
    Participant

    Patrick, you raise a good point. If I do wind up having issues with the build or come across things I’m not comfortable doing, I wouldn’t hesitate to bring in the box of parts and have it rebuilt by a pro. And I may even farm out a few of the jobs and if I do that, it’ll be a good time to pick somebody’s brain about it.

    And Jack, I did see the metal-matrix site. Great info, nice pics. I am also thinking about maybe ponying up the bucks for a kartweb membership. I’ve read here that they haven’t really don’t much updating in the past 5 years or so, but it sounds like they still have a lot of solid information for a new builder.

    I’m also thinking of buying a full 90-97 motor for the trans. If I did this, the plan would be to swap trannies and then build up the 90-97 for use as a practice motor. It’d be nice to have a spare that I could throw on the kart for practice and fun without having to use my legal motor. If I went this route, I’d be able to use things like sleeved pistons and the like to keep the costs way down.

    #54027

    Patrick Roth
    Participant

    Now that I have the disclaimer out of the way, I’ll see if I can’t be more helpful… 😉 I’m unable to view the ebay item so I’ll make some assumptions.

    1) A key decision is to go with either a 99 or a 01 top end.  Based on price alone, the 99 is cheaper and there is arguably little, if any, performance gain by going the 01 route (but the 01 is the choice of just about every SKUSA racer out there).  I’m going to assume you’ll be going with a 99 top end.

    2) A complete rebuild kit (both the top and bottom, less the crank) can be had for about $500 from Fastech.  You could also identify the parts and try to save a few bucks by going to a large distributor such as Partzilla.

    3) If you go with a new crank make sure you get the slip fit available from a karting specific supplier.

    4) For the top end you’ll need a 99 cylinder and head with a set of power valves installed.  The nice thing about the 99 is the power valves can easily be removed and installed on another cylinder.  Depending on how complete your bottom end/engine is you may or may not need some additional items (cylinder studs and nuts, water hose joint, etc).  DO NOT HONE THE CYLINDER (it has plating)!

    5) Hopefully your motor will come with an intake boot and reed cage.  If so take a real close look at the reeds and replace if there is any sign of cracking or fraying.  The safe bet would be to replace the reeds to put you at ease.  If it doesn’t come with the intake boot and reed cage, you’ll need to buy them.

    6) The “hot” carb is the Keihin PWM carb.

    7) As long as your track allows, the cheap route is to go with an air filter intake rather than an airbox.  The air filter is the preferred route.  I recommend a support of the filter/airbox to keep the intake boot from ripping.

    8) On the exhaust side you’ll need a flange coming off the cylinder.  In my opinion, they all stink so pick your poison.  The o’rings constantly need replacing or the OEM metal band also will crack and fall apart over time (I don’t like this one because it seems like an opportunity for small metal parts to get into the cylinder).  The 01 has a nice manifold with a wide rubber seal but this won’t work on the 99.

    9) SKUSA runs a SK1 exhaust pipe; however, check your club rules to see if the RLV pipes are allowed.  If the RLV pipes are allowed you may want to put a wanted to buy ad on this site as there is likely a SKUSA racer that has a pipe laying around from before the switch to the SK1.

    10) For a 99, a 14″ silencer is what I ran and is what I’ve seen many other people run.

    11)  Mounting brackets – I highly recommend the MRC brackets for the coil and CDI as it moves everything to the back of the seat and takes away the vibration which seems to be the cause of several failures.  MRC also makes/sells nice exhaust pipe and silencer brackets at good prices.

    12)  I recommend going with the MRC fuel pump and mounting bracket.  This is a single pump and simplifies everything.  I haven’t had a single issue with this setup.

    13)  Electronics – I hope you got the CDI and coil with your bottom end; otherwise, you’ll need these items ($$$).  I’m assuming the stator is coming with the motor (if not this is another big $$$ item).

    14)  Cooling – Closely inspect your water pump impeller and bearing (not sure if this is included in a bottom end kit) when you have the bottom end in pieces.  Replace if needed.  As far as the radiator goes there are a lot of options.  I recommend calling RPM Racekarts as they sell low cost radiators with a nice mounting bracket.  I’m sure there are others out there, this is just what I have found.

    Hopefully this helps!  If the $$$ adds up to more than you are willing and decided to go the used kart motor route, I just notice a DDR 01 package go up for sale for $4k from Roy Montgomery.  Included in the package are two cylinders so you could buy the package and sell the other cylinder.  I know Roy and he is meticulous about his stuff and since he is a karter his package should fit the bill.

     

     

    #54028

    Patrick Roth
    Participant

    I just noticed where Roy’s ad includes a motor mount which is another ~$200 item…

    #54841

    Todd Kageals
    Participant

    Ben,

    I did exactly what you are doing, even got the lower end on eBay!  I spent about $1000.00 rebuilding my motor (new cylinder, new crank, new head, new piston, all new bearings, all new seals and gaskets).  Get your parts from a place like discounthondaparts.com .  There are other places.  You can buy a new cylinder for what it would cost to buy a sleeve and have it installed.  You can easily do this if you have some general rebuild experience (I had only done car motors before).  One tip on the transmission.  I THINK I remember reading that only the 96 97 is a direct swap.  The earlier transmissions have one shaft that is shorter or something like that (IIRC).  Also, you can easily slip fit the bearings/crank.  If you are doing it by hand, it will take a while but easily doable (I did it on a lathe with some sand paper).  Good luck.  Feel free to email me if I can help (it’s been three years though).

    Todd

    #54842

    Todd Kageals
    Participant

    Ben,

    I did exactly what you are doing, even got the lower end on eBay!  I spent about $1000.00 rebuilding my motor (new cylinder, new crank, new head, new piston, all new bearings, all new seals and gaskets).  I can’t remember if that included the electrics or not.  The CDI is not that expensive but the stator setup is $$$.  Get your parts from a place like discounthondaparts.com .  There are other places.  You can buy a new cylinder for what it would cost to buy a sleeve and have it installed.  You can easily do this if you have some general rebuild experience (I had only done car motors before).  One tip on the transmission.  I THINK I remember reading that only the 96 97 is a direct swap.  The earlier transmissions have one shaft that is shorter or something like that (IIRC).  Also, you can easily slip fit the bearings/crank.  If you are doing it by hand, it will take a while but easily doable (I did it on a lathe with some sand paper).  Stick with OEM Honda parts (especially if you are going to run in Stock Honda).  Good luck.  Feel free to email me if I can help (it’s been three years though).

    Todd

    #61104

    Ben Lewis
    Participant

    Think I’ll chime in

    Go for it!

    I did this too and it was a blast. Keep in mind you need to add +20% to your budget. Even if you are mechanically inclined doing this for the first time you will mistakes. But, that’s part of the fun. If you plan on doing this for a while then build it first yourself. Rebuilds again and again once you’ve got the bug will save you way more than your first build will. Plus you can put a sticker on it with your name and people will say “hey I never heard of Lucastech”. It seems to me that Stock moto should be about the club guy who can build his own stuff that’s one of the main reasons I chose it…plus there’s some real speed there too.

    1)Don’t skimp on the radiator you don’t have to go out and spend big $ to get a quality one. Don’t be tempted to try and mount a rotax banana style or 1 dirt bike radiator. The cr is more demanding with that big hot pipe in front and heat = death. Do not run without at least a trailtech temp sensor.

    2) You will also need the kick start plug. I run mine without the idler gear IDK if others do. I’ve done 3 tops and a bottom and its been fine. 1oz weight savings! Hoorray

    3) Absolutely do the slip fit on the crank it makes rebuilds a breeze. If you are putting it on a lathe make sure the crank is secured from swinging out. Duct tape will not do the job. Safety wire will just don’t put it through the bearing surfaces.

    4) the 96-97 is a direct swap. I’ve done it but they are hard to find. It took me 6 months to find a cheap one that wasn’t abused.

    5) The 99 CDI is going to be $$ too. You’ll have to go to a kart shop for this

    6) You didn’t say if you were racing or not. If you aren’t the Wiseco kit is cheaper and has predrilled holes for the exhaust rib. And it’s more durable imho.

    7) Don’t try and fit this to the L206 chassis. You need a 32mm shifter chassis. I used to race l206 and then some jerk let me test drive his S2. Now I’m hooked for life. It’s a big curve make sure you drive one first. I’ve let several friends try out my shifter and they were like nope ok the TAG is good for me.

    8) There is much more to this setup than just the motor. This package has a lot of ancillaries. J arm, Shift shaft, Kart Clutch cable, Motor mount, engine stop, Air box (don’t do filter many tracks are banning them), air box mount, as many seat struts as will fit 4-5 is ideal, chain guard, pipe mount, silencer mount…

    Let us know how it turns out! It is very do-able and you will be sweating like heck but smiling inside that helmet. Mine came in around $1500 with a full rebuild included.

    Top tip: start doing pushups now

     

     

     

     

     

    " Turd Ferguson is real and he will pass your a** "

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