How to DE-tune a YZ250!

Discussion in 'Bike Mods, Performance, Maintenance & Repair' started by bashn, Aug 6, 2001.

  1. bashn

    bashn Rookie DRN Member

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    What is the most effective way to take some of the the hit out of a 2-stroke while getting re-aquainted with this type of power? After riding 4-S YZ's for the last 4 years, I want to try a '01/'02 YZ250. I've spent some time time on one this weekend and have determined it will take a few weeks to get back to full speed. The only thing I can think of is to add some flywheel. I simply want to make the power more controllable, not reduce it.

    Mark


    :
  2. Derryl Leverett

    Derryl Leverett Rookie DRN Member

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    Smoothing YZ 250

    I have a 2000 YZ 250 that I needed to smooth out for woods riding and hare scrambles. I have added an 11 oz. flyheel weight, FMF Gnarly pipe and Boy. dual stage reeds, and most importantly sent my cylinder to Eric Gore. The bike is awesome. It has plenty of power , but just pulls now, no big hit , just smooth strong power. Give Eric a call, he can advise you on what you need. I might also add, that W.E.R revalved and resprung fork and shock ,and they are Magic.
  3. KMG

    KMG Rookie DRN Member

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    I have a 99 YZ 250 and fitted a 12 oz flywheel weight last week after getting advice from this forum.
    It works great and smoothed the hit somewhat. Also the bike cuts out less over tight rocky sections or roots etc and is more manageable. Did'nt notice a loss of power either. Pulls off much better and smoother as well.
    Much more pleasurable to ride .
    Do it.:)
  4. bashn

    bashn Rookie DRN Member

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    Thanks for the posts. My buddies '01 is almost managable as is, but the '02 has a serious mid-range punch compared to the '01. I've ridden flywheel equipped bikes on the track before and I liked the way they hooked up but stayed "snappy", at least compared to a four stroke. I'm going to buy a new YZ250 and keep my YZ250F, at least for now. The YZ250F made me a better rider, but I just can't stand to loose a bike length on every straight. I'll ride the YZ250 for a week before I decide what to do. I'll probably start with the flywheel. It's cheap and can easily be reversed. I'm sure Eric Gorr can take the engine where I want it to go if the flywheel doesn't do it. BTW, Eric did a great job on the 2mm overbore kit for my YZ250F. I know I have the fastest YZ250F around, but it's never going to be as fast as a YZ250, even with me riding it!:)

    Thanks Again!

    Mark
  5. MARK IT

    MARK IT Subscriber

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    I added the metal clutch plates and Eric did a 1mm over on my cylinder
    and worked the head (dont know what he did to the head) but the bike
    is totaly different, very good in the woods!
  6. EricGorr

    EricGorr Super Power AssClown

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    Depending on the year model of your YZ, a solution of aftermarket accesories or simple machining changes can accomplish virtually any tuning or detuning of the powerband.
    What is the year of your bike?
    Eric
  7. Yamamoto

    Yamamoto Uhhh...

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    Gearing ?

    Maybe if ya gear it taller you like it better. Everyone gears it lower for the Extra HIT.
  8. bashn

    bashn Rookie DRN Member

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    Hi Eric,

    I plan to buy a left-over 2001 YZ250.

    I just want to re-iterate that I plan to use this bike for MX only. I'm a 44 year old senior novice who just wants to "take the edge off" the mid-range hit these bikes have. I'm currently riding a modified YZ250F. I've ridden a lot of enduro bikes and love that type of power. In particular, a friend has a '92 KTM 250 EX/C with extra flywheel wieght that is just magic. It pulls hard, it's responsive, but wont kill you if you happen to grab a hand-full of throttle at a bad time.

    Mark
    Last edited: Aug 8, 2001
  9. EricGorr

    EricGorr Super Power AssClown

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    That's great! You'll love that bike. Recently I worked on a similar project with a 60 something experienced rider and football athelete named Bob from the north shore of Chicago, on a KX250. Bob's kickstart leg had suffered from overuse and injuries in sports so he wanted a bike easy to start as well as one with a smooth electric powerband.

    Bob has all the TSR software and had experimented with different porting and head maps on his computer prior to contacting me about machining his engine's parts. We modified the cylinder for less port-time-area, reshaped the transfer port flow pattern for better scavenging at low rpm, and the compression ratio and bowl shape of the head was reduced by enlarging. The changes to the cylinder are non-permanent and can be returned to stock but the head mods are permanent. However if Bob ever decides to go riding in the Rocky mountains someday, we can just machine a new head and interchange it.

    Regarding examples of bolt-on accessories; Boyesen Dual-Stage reeds, a Steahly flywheel weight, and a spark arrestor tail-pipe will help and work with any internal engine mods you may choose in the future. Check out Boyesen's web page for the reeds and MX South for the tail-pipe and flywheel weight.
    Good luck
  10. IBWFO

    IBWFO Rookie DRN Member

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    Put an FMF spark arrestor on it and keep the stock pipe too. That will really change things, will be relatively inexpensive and when you get to where you want a bit more you can take it off. Another inexpensive thing would be to alter the power-valve, and again when you get to where you want more you can change it back, or you could go with a thicker head gasket?
    All quick inxpensive fixes.
    Regards



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