Why do you have to rebuild 2-strokes ALL the time???

Discussion in '2-Stroke Machines' started by SpeedyE, Oct 3, 2019.

  1. grendels_arm

    grendels_arm Sam Green Eggs and Ham

    4-strokes get rebuilt all the time by race teams as well. MotoGP teams get like 6 motors or so a season I think for this reason. That's like 3 races a motor they have to figure. I heard a race or so ago that Rossi and a few other guys had one motor to go.
     
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  2. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    Yeah, but the OP is comparing experiences from street diesels vs GP strokers; as was noted, a street smoker like an RZ has a MUCH longer maintenance interval vs a TZ, with a similarly low expectation of replacing pistons or cranks. With the TZ, pistons are considered a consumable, much like tires and EZ-Up canopies. Replacing an RZ piston means something went wrong.

    At the loony end of 4-stroke maintenance, Top Fuel dragsters are torn down after each PASS, never mind each weekend! o_O
     
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  3. Kurlon

    Kurlon Well-Known Member

    And they don't even make 1000 complete revolutions in that pass, including burnout IIRC.
     
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  4. pscook

    pscook Well-Known Member

    MotoGP allows seven motors a season, no rebuilds allowed. You start the season with seven and they to last the season (or you start from pitlane with your new engine). No further development or rebuilds area allowed.
     
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  5. Senna

    Senna Well-Known Member

    Seems like a good thread to ask this question.

    I am considering purchasing a Honda RS125R sometime late next year when funds allow for it. I'm attracted to the two-stroke GP bikes mainly as they are incredibly narrow, which helps me a bunch due to some hip issues I have that cannot be remedied outside of surgery. I also like that they are purpose-built, no BS street stuff to deal with.

    I've only owned four-stroke motorcycles - no two-stroke owning or riding experience. I'm not the most mechanically inclined - I send off my bikes for any engine work, including valve adjustments. However, I'm not opposed to learning if the maintenance and rebuilds are relatively easy to complete for 125GP bikes. I have a local independent mechanic who is quite comfortable working on them, too.

    Am I setting myself up for endless frustration and disappointment if I were to go the 125GP route? Are parts for these bikes still available, or is it becoming tougher and tougher to source critical items?
     
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  6. Kurlon

    Kurlon Well-Known Member

    I've helped a friend rebuild his RS125, disturbingly easy motors to work on. Really, the 'complex part' is the transmission, but that's the same on any engine. I'd say they're a good choice to learn to wrench on, but you'll be spoiled by a purpose built machine the first time you offer to help your friend check the valves on their GSXBRR-R mess. :D

    No idea how parts availability looks these days, RSCycles says cranks are no longer available but rod kits are to rebuild existing ones?
     
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  7. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    It takes less time to do a piston replacement than it does to change the jets on a diesel. I'm prejudiced in favour of the TZs, obviously, but the RSs are good machines too.
     
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  8. pscook

    pscook Well-Known Member

    If you want to be competitive, then tuning a two stroke is an art to get it running on the knife's edge. However, it's very rewarding when it all comes together. The flip side of the coin is that it's an art to get it running properly at times, as you can easily tune it to a standstill and then be exceptionally frustrated that it won't do what it just did not two hours ago due to weather changes.

    If you are willing to ride it at 80-85% performance, then the bike can be tuned to an almost "set and forget" the jetting. and you can get a feel for gearing as you progress. But the guys who have been doing this for a while will help you a lot, but on the track they will disappear in a light fog of sweet blue smoke.

    I would hazard against a 125 as your first two stroke experience; rather, look for an Aprilia RS250 Cup bike as they suffer fools a little more happily than an RS or TZ. This one says sale pending, but maybe reach out in case the deal falls through.
     
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  9. dave3593

    dave3593 What I know about opera I learned from Bugs Bunny

    There are a lot less parts in the top end of a two stroke but they are amazing in their use of gas dynamics.

    The service manual for my TZ is excellent on the engine. I suspect the other brands manuals are also. It shows clearances to be checked including two crankshaft checks that can be done by only taking the cylinders off. The pistons I am now using need set up differently than the originals. Maybe due to a metallurgical difference. There are people here that can guide you and some knowledgeable parts suppliers like Rick at Accu Products.

    Real GP engines have "cassette" gearboxes. They can be removed with the engine in the frame.
     
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  10. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    You know the OEM intends for the owner to work on a bike when there IS no 'owner's manual', but the service manual and parts catalogue are considered integral parts of the package ...
     
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  11. mattology

    mattology Well-Known Member

    The aprilia RS250 also has a cassette gearbox that can easily be removed with the engine in the frame as well... you can have the gearbox out in about 10-15 minutes.

    [​IMG]
     
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  12. DonTZ125

    DonTZ125 Purveyor of Neat Toys

    Is that the GP bike, or the challenge cup bike (street-bike based)?
     
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  13. dave3593

    dave3593 What I know about opera I learned from Bugs Bunny


    A thing of beauty!
     
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  14. motion

    motion RockyMountainMotos.com

    Cup bike. Both Cup and street use the Suzuki VJ21 motor.
     
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  15. mattology

    mattology Well-Known Member

    I think it's actually closer to the VJ22 variant, 3 piece power valves, carburetors, and crankshaft taper...
     
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  16. zrx12man

    zrx12man Captain Amazing

    The Honda RS125 service manual is excellent, and should be your bible. If the bike doesn't come with it, it is available online for download. Keep it with you at all times and follow it meticulously. We built and raced RS125's for 4 years, and I tuned the bike for max power exactly once. In the interim, we won dozens of races and many club championships. Tuning to 85-90% saves parts, time and aggravation, not to mention running a bit rich everywhere makes it easier to ride, allowing you to work on learning how to corner it rather than worrying it will seize and highside you if you close the throttle a bit too abruptly after a long straight.
    These bikes will teach cornering technique and bike setup better than any other available machine. Don't be intimidated by the service intervals, but do take them seriously and do everything the manual says without fail. If you do that, it will be a fantastic, reliable machine.
    Also no mechanical DNF's for our team in 4 years. I'm proud of that, but I learned from people who knew their business like Tryce Welch (RIP) and Brian Billings.
     
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  17. Senna

    Senna Well-Known Member

    Thanks for the input, guys. Seems like it's still a viable route to go. Where do you RS125 guys source your parts?
     
  18. Sullivan

    Sullivan Active Member

    Rs cycles, TSO (technical sports one), other bike owners. Just to name a few.
     
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  19. zrx12man

    zrx12man Captain Amazing

    2 additional notes:

    1: The only time I allowed someone else to wrench on one of our 125's while I was there, that guy had his own idea about rear axle nut torque after a gearing change, and felt the 51 ft/lbs in the factory service manual was excessive. Our rider threw his chain on the warmup lap and missed out on his only appearance in a USGPRU 125GP race as a result. Lesson learned, follow the manual without fail, it was written by people who know.
    2: We set 2 125GP lap records on softly tuned machines ridden by a competent, experienced rider at The Ridge in Shelton (1:50.1) and Pacific Raceways in Kent (1:34.1). The records were set in 2012, and as far as I'm aware they still stand. Stock machines, no "A-kit" or carbon stuff and slightly rich tuning. We did use a quickshifter and dataloggers, but that's it.
     
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