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Honda CB250 K4 -72 racer mods

13801 Views 80 Replies 10 Participants Last post by  crazypj
I bought the Honda three years ago and have raced it for two seasons now. In 2015 me and my son didn't change it much, only changed silencers in order not to get black flag, and changed shift pattern in normal race practice. Only minor problems, had to weld a broken ignition coil bracket and weld an alloy tank leak. Improved rubbers under tank. Season ended with a problem with exhaust too low. Result nothing more than new foot peg, brake pedal and hand brake lever. Plus some frame welding. Winter spent with changing exhaust and making a GRP diaper (bellypan). Season 2016 it run well except that we got oil on our right knee, could not find out why. This winter bike will get an overhaul and some modifications. Chassi behaves well, previous owners had lengthened swingarm, reinforced swingarm mount. Modified steering stem and usual frame welding. Front disc brake works well and the rear Suzuki wheel no problem. The 30mm Mikuni VM pair perform well.
Thought it could be wise to check the engine so it was overhauled. Fairly good condition, only one broken piston ring and a broken valve spring. The improved camchain and tensioner was in good condition. The (not the fanciest) racecam was in mint condition. Another year I might polish the rockers. Right engine cover changed to a standard one because the oil lines to an external filter didn't fit with the new exhaust. Took some time matching head and carbs to inlets and exhausts.
We decided to change from half to full fairing, so yesterday we made new fairing brackets and fitted the fairing. As nothing ever is as easy that you expect a new tacho mount was needed. New ignition coil mounting parts had to be made.
Except for paintwork the bike is almost raceready.
Problem left to solve is a smart quick detachable upper T-bracket between screen sides and centre steering nut.
Any ideas?
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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.
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Cool stuff, glad he got a good weekend
Cool, nice to have an update now and again
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 :eek:) 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
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Nice to have an update, thanks. Glad you found the broken up roller before it trashed the head. After 3 years, I've forgotten all you did. About 6~7 months ago I did oil transfer piece modification for a CB350. Turns out Honda were aware of the problem and made modifications enlarging the space between transfer piece and guide tube.
I did a write up and took some pictures, it's a very worthwhile mod for all the CBCD two valve twins (160 to 500)
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