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Honda CB250 K4 -72 racer mods

13803 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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that is a beautiful 250 mike
hope you dont mind me posting this link here but it is my friends in sweden
they race the cb's and are building the cyb
i sold them some engine parts nos crankshaft ,piston,super radical cam,rockers etc all very speacial parts
i am the kenny guy they mention on the "motor" page Honda CYB350 RSC Projekt |
Got a X-mas package. Engine back from tuner. No big problems, he changed pistons and cylinder, rest of the engine seemed OK. Never seen piston and barrel worn this way. Left cylinder has 4 deep indentions at TDC and BDC, each side. Too deep for overboring so he got another cylinder from eBay. Top piston ring in several pieces, second ring tight fit but in one piece. Oil ring quite OK. Piston pins OK. Piston crown has some signs of excessive heat. Looks as piston has melted over and under the piston pin, both sides. Right hand piston had both rings in pieces. Any suggestions why this has happened?
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we need pictures that we can actually see some detail
the cylinder picture is shit,you are awesome you are not shit just the picture
but did i mention,the pictures you gave us suck ?
please make good pics for us
it is very interesting damage you have there
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
A chunk of piston alloy wouldn't erode the cylinder wall to that degree. How would a piece of cast aluminum small enough to come from that eroded area and small enough to travel through the inner diameter of the pin cause 4 divits that size? If there was a chunk of piston pin missing, I would assume he would have mentioned it.
of course you are right and after i wrote that i then seen your images from the bmw
and of course that image explained the whole thing
i have seen wear on wrist pin ends from clips
there can be also some strange harmonics at high rpm especially a 180 crank 4t twin
it has a rocking couple as well as a uneven firing
wrist pin clips should be installed with the tails vertical
i think for a race engine telfon buttons are the way to go
a clp cannot escape if its not there
here is the harmonics i speak of they kick in about 9500rpm on my 350
same engine as the 250 just smaller bore in he 250
about 38 seconds in there is the harmonics
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