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XR100 - Top End time

Discussion in 'Mini Racing' started by pscook, Nov 23, 2015.

  1. pscook

    pscook Well-Known Member

    I noticed that my XR100 was blowing ore smoke than usual this weekend. It's been ridden hard, so not surprising. I'm thinking either rings or valve stem seals, since either are cheap I might as well do both. However, upon reflection, it might need valves and guides, as well as a piston. And if I get a piston, I should probably bump up a size and give it a fresh bore/piston clearance.

    Then one thought lead to another, and I'm looking down the barrel of this: http://tboltusa.com/store/tbparts-120cc-big-bore-kit-br-for-honda-xr100-crf100-p-1625.html

    Give me your thoughts on a big bore vs replacement piston (probably going up one size), and what else needs to be done. One thing to (maybe) consider: I have two of these lil' fellas, and I would rather keep them "even"-ish in performance. The motto is "as goes one, so goes the other." Right now they have identical everything except pilot jet (which doesn't seem to affect WOT at the little flat track I race), so a big bore in one means double the cost as I have to then through the big piston into the other.

    And who has the Honda/Mini parts hookup? OEM or aftermarket.
     
  2. ACDNate

    ACDNate Well-Known Member

    BB kit is definitely easier and cheaper than sending a cylinder to a machine shop. I went with the BBR kit of the same size. It started me down a path to throwing stupid amounts of money at a XR100. You've been warned :)
     
    Photo likes this.
  3. TurboBlew

    TurboBlew Registers Abusers

    you can get valves pretty cheap from a couple internet OEM sellers. Think I paid about $80 for intake & exhaust with new cotters for each.
    If you go big bore like the kit pictured... get a complimenting cam to really reap the benefits. There are a couple of companies that sell valve spring kits. Seals are cheap too.
    You can do your own "lapping" of the valves if you have the patience & a piece of hose/drill and a valve springs compressor. Real easy to check if they are sealing. Then the next step is going bigger on the intake/carb. If youre keeping the stock airbox...run an old ATC200 carb. They fit almost perfect in the stock location & can be bought for $34 all day. Lastly... put some HD clutch springs in. All told there you are looking at $450 or less in parts and almost a 50% increase in power.
     
  4. sharky nrk

    sharky nrk Rubber Side Up

    I have the TB kit and its pretty good. If you don't have to stay in 100cc displacement category I see no reason not to go 120cc. BBR cam is same as stock and waste of money but there are some good cams out there. Everything about the bike is better with the 20% displacement increase. I have the 200 carb as well but have not installed it. You need a CR80 throttle cable/setup with the bigger carb. Only reason I have not installed the bigger carb is laziness lol. Are you sure you need the valves, seats, guides? If I was going through that much effort I would probably pick up one of the $200-250 big valve head from ShiftUP on fleabay.
     
    jschuster2 likes this.
  5. TurboBlew

    TurboBlew Registers Abusers

    except that head will run like ass on any of the flat 120 piston kits (TB, BBR, etc). There is a special high compression piston to use that can be ordered off J parts. And you'll need an adjustable cam sprocket as well. :D
     
  6. sharky nrk

    sharky nrk Rubber Side Up

    ^^ oh really? Thanks for the heads up. I was reading through a thread on Thumpertalk or Planetmini or somewhere about that head specifically and talk was that is was fine on a TB/BBR kit. But if not, definitely want to know as I was looking for a budget head option.
     
  7. sharky nrk

    sharky nrk Rubber Side Up

  8. TurboBlew

    TurboBlew Registers Abusers

    yeah I bought one last year for $175 shipped off web bike. Its a few ccs larger than the stock head. Once together the compression is ~10.5 to 1ish with the flat 120 pistons. The hi comp piston pushes it to around 13 to 1. I was warned about bolting it to a stock crank without proper balancing! :D
     
  9. pscook

    pscook Well-Known Member

    I'm not looking to spend stupid money, so I'll probably pull it apart first to see how bad it is. If I can get away with a new piston, rings, and stem seals, I'm going that route. I know how to lap valves (I used to race an FZR600), so that's a gimme when the head comes apart. I'll check the valves in the guides, but probably just leave those and do the easy stuff. If I do a big bore, well, I will end up with a $3k investment on a $400 bike. I really don't like that ratio for something I ride 10 hours a year, even if I do ride it really hard.

    We have a winter flat track series with a 150cc limit. If I decide to drop serious coin, I'll just get a CRF150 and be done. But it's a 9 race series, and I never make all of the races. Of course, with a more reliable bike, I could do the complimenting arena cross series as well...

    Thanks for the inputs so far, I'll see where I end up after this weekend when I crack open the top end.
     

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