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Honda CB250 K4 -72 racer mods

13819 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 looks familiar. We used to take boxes of spare parts to every race and of course the part you need is always the one part you don't have with you. That clip on is pretty bent. Don't remember doing that to a bike. How did it happen?
That is a really unusual wear pattern. It appears that the pin has repeatedly hit the liner at BDC and TDC which are the times in a cycle where the rod is more or less stationary. That in turn suggests that the pin must have moved laterally (side to side) to impact both sides and then to not create a groove down the liner.

So maybe it is related to a harmonic effect of some sort. What sort of revs does it run to and did the rider notice any particular revs that it used to vibrate worse than others?
But.....if you look closely, the pits appear cratered from being repeatedly struck with something small, the pits are not rounded in a way that you would expect if it was the pin and Mike said there weren't any witness marks on the pin. Granted the pin is harder than the wall and would wear less. The pits also get deeper as they get closer to the end of the stroke, so an undamaged pin running at right angles to the bore couldn't make pits that look tapered outward like that.
Agree that it's not actually the pin moving side to side and impacting the walls. That was before my first cup of coffee.

The damage to the piston looks like a clip came loose, but it's still there. A typical clip failure produces a groove from top to bottom as the pin broaches the liner. Inb this case it's something that's happening when the piston reverses direction which is what I was exploring earlier.

What does happen at direction reversal is that inertia becomes an issue as loads are reversed and that raises a question of where the holes are in relation to pin and ring positions. If it's at a ring height in the bore, that would suggest that a ring is causing the damage. We normally think that rings rotate and would not consider the rings as a possible cause of the issue, but if the oil ring was pinched or had excessive clearance, a ring issue might be postulated. It's easy to speculate that once teh damages started and the ring was unable to rotate that the process would continue to deteriorate, but what started things going south?

Perhaps a ring was slightly damaged and was unable to rotate. That is most likely a handling/fitting issue. And why only on one side and not the other? That suggests that it was less of a harmonics issue and more of a mechanical damage issue.

It's possible that the ring groove was excessive on one piston and that allowed a ring to rock at TDC and BDC which in turn allowed the end of the ring to ding out a tiny amount of metal each time it reversed. Another possibility is that the expander ring was either overlapped or incorrectly fitted.

Mike: Do the divots align with the oil control ring in terms of position from top of the liner?
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If the two rings in the oil ring had been set with ends 180 degrees apart, that could explain both sides, but with them apparently close to each other, that would create wear at top and bottom on the same side as the ring gap and the other side would be clear of damage. If Mike was the liners or old barrel, he should be able to measure the top of the top divot and bottom of the lower ones and determine which part of the rings that correspond to at TDC and BDC.

That might, and the emphasis is on "might", shed more light on the cause and mechanism involved.
From what I'm seeing in the photos, the pin boss is eroded downward (below the pin), I'm pretty sure that when measured, the lower extent of that erosion on the piston will lineup with the lower edge of the pit in the wall at BDC and vice versa at the top. Unless I'm missing something, all that can tell us is that something was rattling around in there. Based on the depth of the pits, if either the oil ring or second ring played a part, then there would be damage evident on the rings.

Yeah, but that doesn't explain why the wear is most extreme at BDC and TDC......
The RG is easy. Detonation for sure. What's less easy is the cause. I have a client RD350 that melted down and it wasn't obvious what happened. It's tempting to tear it down but first I did compression and leakdown tests and it would not hold pressure. Using a spray of soapy water the bubbles started to appear in all sorts of places - enough to cause a problem. Then I checked the ignition timing and it was slightly advanced on the good side and even more advanced on the bad side. In addition there was difference in height between the two barrels leading to ineffective squish on one side. It's all those small things that add up.

If it's not too late, try a compression test and leakdown and see where the ignition is set - as best as you can before you start to tear it down. If it's too late for that, double check everything from jetting and fuel flow to ignition timing and squish.
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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.
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.
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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
Lots of ways to make an LC faster - depending on the rules. Forget about old TZ750 reeds and think about YZ85 reeds with single petals each side as an easy upgrade. Porting is also fairly easy to do but pay attention to squish and head shape. Programmable ignition will add low rpm advance and more punch out of corners. 26mm carbs are fine up to about 60 or so HP but pipes are the key to making the package work. 34mm are fine on a 250 or 350 TZ but the revs have to stay up and that will require a different set of gears.

A stroker crank and extra ports would sure help but I'm guessing that would be outside the rules.:cool:

Lomas makes some great pipes and so does Dave Swarbrick. There are probably others available that work.
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