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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; Originally Posted by Mike 40M Thanks for advice on the RG. I must have been lucky as I've not seen detonation marks like that before. ...

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Thread: Honda CB250 K4 -72 racer mods

  1. #61
    Senior Member Teazer's Avatar
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    Quote Originally Posted by Mike 40M View Post
    Thanks for advice on the RG. I must have been lucky as I've not seen detonation marks like that before. Sadly I can't check because the engine is already in pieces because one big end bearing had seized. Doublechecking looks like a very good idea.
    When we build engines, we try our best to do it right and fail to check our own work. With a 2 stroke it's important to check compression and leakdown after the motor goes back together and before it's fitted in the frame. It's amazing how often there's a ding or scratch that we overlook that's just enough to cause a tiny air leak and that's often enough to melt it down.

    While the carbs are off, check and clean every jet and drilling to make sure they are clean and flow the same. Use a can of WD40 to spray through one drilling on one carb and repeat on the other carb and compare spray.

  2. #62
    Senior Member Cyorg's Avatar
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    Quote Originally Posted by Mike 40M View Post
    Thanks for advice on the RG. I must have been lucky as I've not seen detonation marks like that before. Sadly I can't check because the engine is already in pieces because one big end bearing had seized. Doublechecking looks like a very good idea.
    Does the seized big end belong to the same cylinder that has the blasted piston and head?
    "Non urinat in ventum"

  3. #63
    Senior Member Mike 40M's Avatar
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    No, on the other side.

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  5. #64
    Senior Member Mike 40M's Avatar
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    Back to the Honda. As I mentioned before, a part in the clutch acutating mechanism has mysteriously disappeared. Searching the internet gave "Sorry, out of stock" except for a single one. E-mailed the vendor. No answer. So it will be a homemade one.
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  6. #65
    Senior Member Mike 40M's Avatar
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    Small update. Arrived in my mailbox today. Not on the net, but in the real world. New old stock.
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  7. #66
    Senior Member Cyorg's Avatar
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    You know the original one will magically reappear now....
    Mike 40M likes this.
    "Non urinat in ventum"

  8. #67
    Senior Member crazypj's Avatar
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    Quote Originally Posted by XB33BSA View Post
    MY THEORY AND IT MAY HOLD WATER
    for whater reason a small chunk of piston alloy,that fractured loose fom the area outer to the dcirclip groove
    maybe the wrist pin reached a back and for hammering that stress cracked off a small chunk
    of course up and down goes the piston and at the change of direction the recip,the minor bit of metal gets wedged and rolled up to go the other direction
    after couple hundred 100,000 recip motions the balled up metal is eroding iron liner material
    for sure there was never anything close to melting temps
    I've seen it before a few times, The rings start fluttering due to high rpm and piston ring groove wear, ( because of the constant high rpm) bit of oil gets in combustion chamber, usually only had a single hiccup, detonation, etc.
    Piston crown gets hammered, ring grove closes on top ring cracking it in pieces Rings break up and start cutting cylinder , various bits of cast iron get into pin bore.
    what red-line are you using? You need to lighten pistons if your going over 12K.
    It would be a real good idea to get crank fully checked. Oh, check crankcase bearing bores, you will see fret marks but need to make sure bearing bores are still round. Any signs of piston touching head (pretty common if you have 'correct' clearance) If you don't have any marks on piston crowns or head squish band you probably have to much clearance and con-rod thinks it's a sine wave
    Just went back and read all the posts. Pretty sure Cyorg was right about ring groove wear, possibly ring butting (not enough end gap) and resonance. Ring groove wear is something almost always overlooked on 'budget' re-builds where pistons are not being changed as they 'look OK' I've been told I'm OCD about measuring things but really I just don't like 'wasting' good stuff (change it cos your in there) or having engine blow up.
    Last edited by crazypj; 04-16-2019 at 10:39 PM.
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  9. #68
    Senior Member Mike 40M's Avatar
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    First race for the year. A 700 miles trip to Norway. The Honda was going without any problems. Simply check air pressure and oil level. Recharge battery ( total loss system ), and add some petrol. First race 4th, second race 3rd. Best lap two seconds better than it was 2015. Probably because my son is a bit faster now.
    More problems with the Suzuki RG250. First practice it didn't go well. After float level adjusted on right carb, second practice performed ok. As we has changed it to Yamaha wheels and front fork, we are happy that it handles very good. Reason for change was that slicks are unobtainable in 18". It competes in a 80-87 class, 600cc 4-stroke or 400cc 2-stroke. First race as expected, went well in the corners but lacked speed on the straights. On warm up second day, it shattered the clutch. Hopefully, I can find another clutch in the barn.
    Next race 6th July.
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  10. #69
    Senior Member Teazer's Avatar
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    PJ raised a very valid point. There is a finite limit above which rings will flutter. On our old CB77 motors (54mm stroke) that limit for stock rings was just under 10,000. With a shorter stroke yours should be a little higher. But for sustained high RPM you need pistons that have thinner rings if you rev it hard. On the CB77 it was initially built with super light valve train and big valves and it revved to 12,000 but didn't last. We ended up with stock valves and higher compression and created a true squish band and in that configuration it did not rev as high but came out of corners much harder and that made a huge difference.

    With a 56mm stroke and heavy, tall pistons, that's not so easy to replicate, but take a look next off-season to see what you can do to improve squish with a flat(ish) combustion chamber.

  11. #70
    Senior Member Mike 40M's Avatar
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    Ooops. Next race on the 29th June. Nothing to do with the trusty Honda. So we fixed the RG clutch, and a faulty fairing fastener and loaded the bikes and all other things in the racevan. Next problem will be to transport the Manx to the race. Managed to wreck the pickup yesterday, never experienced an airbag before. So now borrowing a horsebox.

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