RS125 CR125 swap

Discussion in 'Mini Racing' started by Racer156, Dec 4, 2019.

  1. Racer156

    Racer156 Member

    I’m putting together a RS125 Honda based mini road racer and the rules allow either a water cooled 85 or an aircooled 125 two stroke. So a 76-78 CR125 could work (chain switched to the wrong side for 79), but a yz125, kx125 and rm125 from the late 70s could also work. Anyone ever tried this? Wondering what would fit with the least amount of effort. Any insight would be appreciated, especially pitfalls to avoid.

  2. Spang308

    Spang308 Well-Known Member

    I gotta think a modern water cooled 85 motor makes more power than an old air cooled 125 and would drop right in. No personal experience, but a good guess.
  3. Racer156

    Racer156 Member

    Yea, most people go with the 80/85 watercooled engines but the short gocart tracks have a lot of stop and starts so I think the extra torque would provide an advantage. There’s a 25 hp limit on the class that an air cooled 125 can easily meet and the old air cooled motors are waaaay cheaper than a newer 85.
  4. d_alexand

    d_alexand Well-Known Member

    Try a 1980 rm125, its left drive. The rm's are pretty popular in AHRMA mx so they must be pretty good for that era. I'm not sure about mount it, the vintage 125 engines are small but it'll probably still need the kickstarter cut/removed like the 85's.

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