Yamaha MX/DT/RT100 builds?

Discussion in 'Mini Racing' started by britx303, Dec 28, 2019.

  1. britx303

    britx303 Plagued with FZRs...........

    Anybody do anything with any of these? My daughter and I have an RT that we've had for 5 years and recently got into really waking the thing up after blowing up my TTR225 engine at last months hare scramble. We took all the suspension from the TTR and fit it to the RT,so now it rides like a full size bike. We opened up the stock exhaust and gutted out the inner baffling and reshaped it into a proper chamber and rewelded back together and bored out the stock muffler. Flows a hellova lot better. Experimenting with NO head gasket for now and so far no pinging or anything odd. The RT is without a doubt running 100% better than when it was stock. Actually lofts the front wheel believe it or not with little effort..........definately no longer the tame little bike it started as,and its been loads of fun building it with my daughter. So now onto the questions..........will a V-force reed cage improve anything,or boyerson reeds? Ive read one is good for more mid-range(v-force) while the other is good for top-end(boyerson). Because I will be racing it in a hare scramble,im thinking mid-range will be better to gain a bit more of. Also, anyone familiar with doing an MX100 cylinder swap? I will try to put up some pics of the little,yet growing mongrel:D For an RT playbike,its a beast:crackup:
     
  2. racer_11

    racer_11 turn one fng

    I have run an mx100a in vmx for about eight years now. I can tell you about what not to do to an mx by showing you my box full of holed pistons, thrown rods, destroyed case halves and broken frames, if that counts for anything!! by nature, the mx engine consumes itself rather quickly during race days, so running the bike as reliable as possible turned out the better option for me. what works for me is keeping in mind it's a 100 and therefore no midrange, I focused on all top end. the rt flywheel will help lug if you aren't spot on with shifting. yes a mx cylinder will bolt on an is a better cylinder than the rt, but make sure to use the mx head. I run in a true vintage class, so run a 74 mx100 frame which allows easy adaptation to race mods. the rt frame might limit you some e.g, chassis, suspension, airbox, pipe). also the rt clutch is prone to fading easily and transmission is spaced odd, compared to the mx (but definitely can still get the job done). I've tried many combos of reeds and ended up using the oem steel reeds (again focusing on reliability first and then top end). I've lost more points and races focusing on a fast bike first, but at the same time have pulled my share of holeys!
     
    britx303 likes this.
  3. fastfreddie

    fastfreddie Midnight Oil Garage

    Where do you get plastic fenders for a DT? Preston Petty is gone...
    While we're on gone vintage products, where's Bassani?
    I have a '73 CT3 I'd like to resto-mod for pure dirt...
    IMG_0749.JPG
     
    britx303 likes this.
  4. racer_11

    racer_11 turn one fng

    I think preston petty stuff is still available. I've seen a booth of theirs at some race if I recall. I don't see many new bassani pipe much anymore! that ct is sweet!
     
  5. britx303

    britx303 Plagued with FZRs...........

    Gotcha! Reliability is definitely key too. So far with the exhaust mods,air box mod and rejetting I’ve been able to find some hidden power easily with that alone. I ordered power reeds to try it out and supposed to come today. Race is on Sunday so may stick with the stock reeds if it throws all I’ve gained off too far. What’s your opinion on running without the head gasket? So far I’ve had zero issues without it, and that definitely helped perk it up too. But as mentioned reliability through the race is key
     
  6. fastfreddie

    fastfreddie Midnight Oil Garage

    Saved it from a Harley crowd that wanted to violate it with a sledge, one dollar/three whacks. I convinced them they could make more money auctioning it off. Surprisingly, it was a bitter battle. I took it for $300. :beer:
    It had been street registered, has low miles and runs strong but appears to have been neglected.

    Older brother had one when I got into riding. He bought it new for $695, broke it in with 695 miles, then stripped it of all street parts...milled head (.010″), matched ports, Bassani, Petty fenders, forkbrace, MX pegs, gearing...
    I road the shit outta that thang...ran like a 250! :D Then Kawasaki had to come out with the KX in '74...what a beast! Scary fast, but not ludicrous like Yamaha's MX series SC500. :eek:
    My 'tween and teen years were some of my best. :)
     
  7. britx303

    britx303 Plagued with FZRs...........

    Does the head have 2 plugs? Pic is a little fuzzy.
     
  8. fastfreddie

    fastfreddie Midnight Oil Garage

    Yup. My brother used one of the plug holes to install a compression release valve, activated by a finger lever on the clutch side of the bars...gave the bike some engine braking.
     
  9. britx303

    britx303 Plagued with FZRs...........

    Welp, managed to work past a few bottlenecks at some of the mud bogs etc. and rode in first place until the last mile and a half. At some point my air filter came off and sucked a lot of mud/dirt into the carb and shut me off for good, dropping me way down and I had to watch the guys in my class catch up and go past me on the sideline in the woods.....oh well that’s racing. Need a better filter setup for next time. The RT has a surprising amount of ability In the woods with the suspension swaps and little bit of tuning and exhaust work only. All in all, damn fun bike that you can pin it while giggling under your helmet.
     
    racer_11 likes this.
  10. britx303

    britx303 Plagued with FZRs...........

    Pulled the top end off to inspect it............I effectively sand blasted the piston and cylinder when I took in all the mud:D I guess it’s time for a new piston and bore:D A local dealer has a few great deals on yz450f’s that are damned tempting but this little engine was so fun I’m going to commit to a full season hare scrambling it after I get the rebuild done. I’m thinking I need another rt100 to turn into a mini roadracer.............because......2STROKE!!!!!:p
     
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