This is a discussion on Honda CB250 K4 -72 racer mods within the Vintage Motorcycle Racing forums, part of the Caferacer.net Forums category; Originally Posted by Mike 40M Thanks for advice on the RG. I must have been lucky as I've not seen detonation marks like that before. ...
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.
Back to the Honda. As I mentioned before, a part in the clutch acutating mechanism has mysteriously disappeared. Searching the internet gave "Sorry, out of stock" except for a single one. E-mailed the vendor. No answer. So it will be a homemade one.
Small update. Arrived in my mailbox today. Not on the net, but in the real world. New old stock.
You know the original one will magically reappear now....
"Non urinat in ventum"
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.
Last edited by crazypj; 04-16-2019 at 10:39 PM.
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First race for the year. A 700 miles trip to Norway. The Honda was going without any problems. Simply check air pressure and oil level. Recharge battery ( total loss system ), and add some petrol. First race 4th, second race 3rd. Best lap two seconds better than it was 2015. Probably because my son is a bit faster now.
More problems with the Suzuki RG250. First practice it didn't go well. After float level adjusted on right carb, second practice performed ok. As we has changed it to Yamaha wheels and front fork, we are happy that it handles very good. Reason for change was that slicks are unobtainable in 18". It competes in a 80-87 class, 600cc 4-stroke or 400cc 2-stroke. First race as expected, went well in the corners but lacked speed on the straights. On warm up second day, it shattered the clutch. Hopefully, I can find another clutch in the barn.
Next race 6th July.
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.
Ooops. Next race on the 29th June. Nothing to do with the trusty Honda. So we fixed the RG clutch, and a faulty fairing fastener and loaded the bikes and all other things in the racevan. Next problem will be to transport the Manx to the race. Managed to wreck the pickup yesterday, never experienced an airbag before. So now borrowing a horsebox.