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Honda CB250 K4 -72 racer mods

17K views 83 replies 10 participants last post by  Mike 40M 
#1 ·
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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#3 ·
Construct your fairing stay bracket with a center post that mounts to the steering head tube and extends to the front of the fairing. Next attach fairing ear stays to the center support.
You can get fancy by making the fairing stay be a quick disconnect with a pin. Do the same for the side stays. Pull three pins and remove the fairing.

Using the top nut gets weird because it is another thing that has to be disconnected to remove the fairing.

matt
Tannermatic
 
#8 ·
a very swedish paintjob.

you need an Ikea sponsorship now
 
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#9 ·
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.
 
#11 ·
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.
 
#15 ·
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.
 
#18 ·
Mike,
How easy is it for a foreign rider to classic race in your series? My wife's parents live in Bergen, Norway and when we visit, it might be fun to go racing. I have US racing licenses from a variety of sanctioning organizations.
 
#21 ·
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.
 
#23 ·
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?
Auto part Metal

Brass Metal
 
#31 ·
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?
View attachment 96209
View attachment 96211
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
 
#25 ·
Hard to diagnose things from photos, but there are a few reasons why a circlip comes adrift. Could have been the wrong clips were installed, wrong clip diameter, wrong wire diameter so the clip doesn't sit all the way home in the groove, clip was damaged during installation or the rod is out of whack. A bent or damaged rod, or a small end that isn't running true to the big end can cause the wrist pin to move back and forth hitting against the clip. Eventually the clip and or groove will fail. The clip will erode the piston as shown in your photo and the pin will run up against the cylinder wall.
 
#26 · (Edited)
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.
 
#27 · (Edited)
I didn't realize that was the clip still in there, I thought it was light reflecting off the end of the pin. It is odd.... the only time I have seen erosion like that on a piston, it has been from either a circlip or a piece of ring. You stated that the oil ring was ok and the second ring was intact. Pieces of the top ring wouldn't likely get in there without some sort of obvious trail past the lands and the two lower rings. Have a look at the top piston ring land and see if it shows any witness marks (above the pin area) from a end piece of the compression ring being snapped off during installation. How does the circlip groove look on the other side? Are there any odd marks on the inside of the pin compared to the pin from other cylinder? Something was bouncing around in there.
 
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