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Honda CB250 K4 -72 racer mods

13820 Views 80 Replies 10 Participants Last post by  crazypj, 
Mike 40M
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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Mike 40M
Sorry, some trouble finding any pics at all. As it was on Arctic Raceway in -15.
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Credit picman.co.uk
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Mike 40M
How it looks today, fairing had to wait until epoxy has cured.
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Not weather to take it out for a proper background
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Mike 40M
With new fairing, only numbers left and possibly some paint.
But as an old quite successful racer said: Paint ain't fast.
So now I'll start setting up a 350 Norton racer. That's another story....
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Mike 40M
Finally got the bike out of the workshop. Son didn't approve of my no-paint approach. So he did a bit of spraying. Could not do as much with the other bikes. Painting stinks.
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Mike 40M
Sadly IKEA started in a part of sweden where people are very economical. Compared to them, Scots are big spenders. No chance of sponsorship.
First start of engine this year. Checked and a minor adjustment of ignition timing after engine rebuild. Only problem was a silencer bolt that disappeared. Next weekend first practise.
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Mike 40M
Did a barnfind yesterday. Found a half-forgotten CB500/4 racer. Spent half an afternoon to get it running and raceready. Now we have two bikes ready to race.
Nice to have a barn full of might-be-useful-things.
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Mike 40M
The CB250 seemed to be slightly faster than last year. Now some minor problems. New fairing, clipon and clutch lever needed. I think I'll borrow the foam filters from the Vincent. If son will go out in the gravel again, some sand might find its way into the engine.
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Mike 40M
Tried to explain to the enduro son that roadracing shall be on the track, not beside it.
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Mike 40M
Just an ordinary lowsider in a lefthander. Front wheel slipped. He said he was going a bit too fast. Low track temperature ( about 5 centigrade ).
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Mike 40M
A short update on the Honda 250. Bought two fairings for the Honda and one for the Norton. Cheaper shipping from UK that way. One is on the Honda, painted about the same as the previous one. I did one race but had some front brake problem. Solved it and did another race. Made a big mistake so it stopped at the start line. Eventually got it started. Was one third lap after the other. Managed to get past some others rider anyhow. Must have been a bit angry, because best lap speed was 4 seconds better than previous year. Then my son did a fairly good race on it while I was away to watch the isle ofMan classic TT. As the Honda was going well, it was simply stored away in a barn. Me busy with other bikes and my truckdriving son earning his living transporting stuff like army tanks, really big loaders, diggers and windmill gearboxes. You won't believe it but those gearboxes wheights 60 tons. He bought a Suzuki RG classic race bike which we has prepared. He tried it on a track day, seized on second lap. If any interest, I might make a thread on it.
Back to the Honda. We took off the dust sheet, filled air in tyres and petrol in the tank. Charged the battery and put it on the newly bought roller starter. Ok, so it went in the race van. The Norton went in soon after.
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Mike 40M
A quick check in the entrance list tells that only #22 Johan will be there this weekend.
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Mike 40M
I think it should be possible quite easily as US riders do compete at different levels in different motorsports. The following link gives a pair of race invitations in english.
Tävlingsinbjudningar 2018 | MCHK-Racing
There is a couple of email addresses in them to people who should be able to give correct answers.
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Mike 40M
Back from race. Third after a Suzuki T20 and a Greeves. The Honda came in heavily breathing blue smoke from the catch tank. Last race for the season, so plenty of time stripping the engine to see what has happened. Best fourstroke anyhow.
Better luck than than for a guy in a supermono class. Engine exploded at the finish line making a big oil spill. He fell off after the finish line and thus got his second place.
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Mike 40M
Took out the engine. Since I have some other bikes to fix, it is now attended by a fellow racer, who has a lot of experience of tuning and preparing CB twins.
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Mike 40M
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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Mike 40M
Sorry, was not able to take better pictures. One of the circlips is intact. I was not told about the other one. The wristpin has not damaged the cylinder wall. When I restored a 1930 Harley 45" back in -63, a wrist pin had made a long straight cut in a barrel. In this case the dents in the cylinder is about 4mm wide and a about 1mm deep. The four dents differ from 2mm to 5mm in length. No damage worth mentioning between endpoints of stroke. If the wristpin had come loose it would have made two long cuts for all of the stroke, eventually progressing to a single big cut. The wristpin is not damaged in any way, except some discoulouring in the middle, indicating excessive temperature.
You can see the intact circlip on the photo.
If a circlip was to blame it should first make damage on one side and then went through the wristpin and made its work on the other side. And only at both end of stroke.
I think it must have been the engine gremlins.
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Mike 40M
On the piston, wrist pin is tight fit, but I can push it in with my thumb. No more marks found. A fractured circlip, one part stayed, one part slipped through the wristpin might be what could have happened.
I'm still curious about the four distinctive pits in the cylinder.
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Mike 40M
Agree, that barrel pic is shit. I'm not good at taking pics. But since you asked, I made another try
Upper left side
Wood Wood stain Varnish Caramel color

Upper right side
Caramel color

Lower left side
Wood Wood stain Table Caramel color Varnish

Lower right side
Wood Wood stain Caramel color

Sorry that the lower marks are upside down.
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Mike 40M
Cyorg said:
So all 4 pits line up with the pin boss area of the piston at top/bottom of stroke... yes?
Click to expand...
Yes.
Finally got a talk with the guy who fixed it. ALL 4 circlips was intact when he dimantled it. The one seen in the photo can't be removed, alloy melted it in place.
So the circlip theory out and no signs of piston ring parts in the wrong place.
Anyhow the bike is assembled again. Ready to start except one part has disappeared, the clutch retainig ball. Will see if I can find it or find one to buy.
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