Home Forums Shifter Karts What goes into a mod moto build?

This topic contains 3 replies, has 4 voices, and was last updated by  Tim Pappas 5 years, 3 months ago.

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  • #28290

    John Reikes

    I blew up a top end at my last race and I have some downtime before I’ll be racing again. I had a cir-clip fail, so it scored the cylinder and put a thousand little pits into the head. I already have a fresh top end for stock moto, but I was wondering if I can put the damaged parts to good use and build a modified cylinder for open class.

    I’m guessing that I would need to:
    – Overbore the cylinder to get rid of the scoring
    – Do something (don’t really know what) to clean up all the pitting in the head
    – Mill the cylinder and/or head a little for extra compression
    – Get a high compression piston (overbored to match the cylinder)
    – Do some sort of porting to the cylinder (is this something I can do at home with a Dremel?)
    – Advance the heck out of the ignition?

    Is that the basic idea? If so, it could be worthwhile for some big HP fun once in a while.

    Do I have the basic idea or is there a lot more to it than this? If it’s much more complicated and/or expensive than this, then obviously it won’t be worth bothering.

  • #28338

    James McMahon

    You can get real deep if you want, there is a lot more involved especially when it comes to grinding ports. Its as much as art as science.
    Avoid upping the compression too much, kills entry speed, harder on motor internals and is less forgiving with jetting.
    Simply advancing the ignition statically will probably end in tears, there are mod ignition boxes with custom curves, thats probably a better route.

    If you wanted a quick pick me up, I think leave the ports as they are, reduce the cylinder head vol a smidge, get a “mod” ignition CDI that has a curve to match your pipe and cylinder. Of course, having done that you will need to re-explore your jetting setup and moving to a higher octane fuel is probably not a bad idea either.

  • #28401

    Chris Reinhardt

    Building a good Mod Honda is a science as McMahon has said…  It’s lots of things and they all have to work together…

    You can’t “bore” a nikasil cylinder without replating it, and it’s not legal in any kind of karting….

    Case prep was a big part of it, porting the cases and cutting and welding the kick starter hump for a straight intake would probably make the biggest change, along with a good pipe and a stock 99 ignition advanced all the way, run a TM 125 piston, shim the cylinder up, and tighten the head up.  That’s the way Banke had us run them until he came out with the programmable ignitions, albeit with RS125 pistons…

    All that and the stock Honda guys will still be faster!!!!  :)


    Chris Reinhardt

    CR2 Motorsports


    XV Racing Products



  • #28442

    Tim Pappas

    Port the cylinder, port the cases, reconfigure the head, cut the kickstarter boss for a straight intake, build a pipe to match your porting, cut the choke section out of a 39pwk weld the snout back on and bore to 42mm, dry sump your transmission, cut your clutch down to 3 disks, develop exhaust water injection system, and you will be close.  Or fix your stock honda and have fun.

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