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Honda CB250 K4 -72 racer mods

This is a discussion on Honda CB250 K4 -72 racer mods within the Vintage Motorcycle Racing forums, part of the Caferacer.net Forums category; Back from race. The Honda as usual reliable. Minor problem was that a piece of steel holding left silencer broke and had to be welded. ...

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Thread: Honda CB250 K4 -72 racer mods

  1. #71
    Senior Member Mike 40M's Avatar
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    Back from race. The Honda as usual reliable. Minor problem was that a piece of steel holding left silencer broke and had to be welded. First race 2nd place as one usually faster 2-stroke broke down. Second race, another 2-stroke seized so it was my son's first victory. As one wise old racer said: "To win you have to finish"
    TrialsRider likes this.

  2. #72
    Senior Member crazypj's Avatar
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    Cool stuff, glad he got a good weekend
    I dabble in rocket science, when I\'m not picking my nose
    The other night I gave my girlfriend an orgasm, but, she spat it back at me

    'you can take my word for it or argue until you find out I'm right'

  3. #73
    Senior Member Mike 40M's Avatar
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    My son didn't have to push hard, just enough to keep the next rider 1/8 mile behind.
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  5. #74
    Senior Member Mike 40M's Avatar
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    Next meeting done. My son got a flu and could not ride. So was alone there with both the CB, the RG (fixed after last race crash) and the Manx. After that the Norton tank had started to leak, I switched bike and rode the Honda. As I haven't ridden it for more than a year (son raced it), it was a pleasant surprise how nice and easy it is to ride. Handles well and brakes good. The modified steering head, reinforced swing arm mount and the lengthened swing arm makes it handle really well. Both the Norton and the Honda is completely useless under 5000 rpm, but as the Manx stops at 7800 and the CB at 11500, it is much easier to ride the Honda. Though it should go at least over 8500 rpm. But on the sunday warm up it started badly, powerless and misfiring. Time to replace the electronic ignition system. As a thunderstorm with heavy rain came in, I didn't took out the RG as it have slicks. Not enough time to replace with rain tyres. Called it a day. Anyhow had a lot of fun.

  6. #75
    Senior Member crazypj's Avatar
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    Cool, nice to have an update now and again
    I dabble in rocket science, when I\'m not picking my nose
    The other night I gave my girlfriend an orgasm, but, she spat it back at me

    'you can take my word for it or argue until you find out I'm right'

  7. #76
    Senior Member Mike 40M's Avatar
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    Last practice this year. My son rode the Honda today as I played with the Manx. The ignition trouble I had turned out to be dirt on the optical sensor. So now the Honda was performing as usual. I have fitted an lambda sensor to the exhaust, and was interested in the AFR readings. Son reported air/fuel ratios between 12.5 and 13, which means carbs are fairly well set up. Though this was at air temperature around 40F. Will be interesting to see readings at a more decent temperature, have to wait until late April for next test. Only some routine maintenance to be done.
    So with very little work to be done to the Honda, the Manx and the RG, I got another race bike to prepare, a Yamaha RD-LC.

  8. #77
    Senior Member crazypj's Avatar
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    F/A ratio's are good at 12.5~13 :1, should make bike a lot easier to ride.
    Are you doing a 250 or 350 LC (Elsie ) I don't know how much information is available for them today but someone has probably posted tuning info from the 1980's? I found the reed blocks need a lot of work, you can make them flow much more than a Boysen set and still have 'street' like throttle response. Bigger carbs work, 34mm makes bike VERY peaky but 30 or 32 gives wider spread of power. (I think they have 26mm?) Oh, don't bother to work any of the intake covered by the reed blocks. Cut out piston bridge and match cutout to port width. Decent set of expansion chambers make a world of difference, even TZ350 ones will bolt on but they are bit noisy
    I dabble in rocket science, when I\'m not picking my nose
    The other night I gave my girlfriend an orgasm, but, she spat it back at me

    'you can take my word for it or argue until you find out I'm right'

  9. #78
    Senior Member Teazer's Avatar
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    Quote Originally Posted by crazypj View Post
    F/A ratio's are good at 12.5~13 :1, should make bike a lot easier to ride.
    Are you doing a 250 or 350 LC (Elsie ) I don't know how much information is available for them today but someone has probably posted tuning info from the 1980's? I found the reed blocks need a lot of work, you can make them flow much more than a Boysen set and still have 'street' like throttle response. Bigger carbs work, 34mm makes bike VERY peaky but 30 or 32 gives wider spread of power. (I think they have 26mm?) Oh, don't bother to work any of the intake covered by the reed blocks. Cut out piston bridge and match cutout to port width. Decent set of expansion chambers make a world of difference, even TZ350 ones will bolt on but they are bit noisy
    Lots of ways to make an LC faster - depending on the rules. Forget about old TZ750 reeds and think about YZ85 reeds with single petals each side as an easy upgrade. Porting is also fairly easy to do but pay attention to squish and head shape. Programmable ignition will add low rpm advance and more punch out of corners. 26mm carbs are fine up to about 60 or so HP but pipes are the key to making the package work. 34mm are fine on a 250 or 350 TZ but the revs have to stay up and that will require a different set of gears.

    A stroker crank and extra ports would sure help but I'm guessing that would be outside the rules.

    Lomas makes some great pipes and so does Dave Swarbrick. There are probably others available that work.

  10. #79
    Senior Member TrialsRider's Avatar
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    imho the rules suck whenever they hold back the advancement of a technology and ultimately participation in the sport.

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