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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.
 

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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.
Does the seized big end belong to the same cylinder that has the blasted piston and head?
 

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Discussion Starter #64
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.
Clutch part.jpg
 

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MY THEORY AND IT MAY HOLD WATER
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
I've seen it before a few times, The rings start fluttering due to high rpm and piston ring groove wear, ( because of the constant high rpm) bit of oil gets in combustion chamber, usually only had a single hiccup, detonation, etc.
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.
 

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Discussion Starter #68
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.
 

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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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Discussion Starter #70
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.
 

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Discussion Starter #71
Back from race. The Honda as usual reliable. Minor problem was that a piece of steel holding left silencer broke and had to be welded. First race 2nd place as one usually faster 2-stroke broke down. Second race, another 2-stroke seized so it was my son's first victory. As one wise old racer said: "To win you have to finish"
 

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Cool stuff, glad he got a good weekend
 

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Discussion Starter #74
Next meeting done. My son got a flu and could not ride. So was alone there with both the CB, the RG (fixed after last race crash) and the Manx. After that the Norton tank had started to leak, I switched bike and rode the Honda. As I haven't ridden it for more than a year (son raced it), it was a pleasant surprise how nice and easy it is to ride. Handles well and brakes good. The modified steering head, reinforced swing arm mount and the lengthened swing arm makes it handle really well. Both the Norton and the Honda is completely useless under 5000 rpm, but as the Manx stops at 7800 and the CB at 11500, it is much easier to ride the Honda. Though it should go at least over 8500 rpm. But on the sunday warm up it started badly, powerless and misfiring. Time to replace the electronic ignition system. As a thunderstorm with heavy rain came in, I didn't took out the RG as it have slicks. Not enough time to replace with rain tyres. Called it a day. Anyhow had a lot of fun.
 

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Cool, nice to have an update now and again
 

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Discussion Starter #76
Last practice this year. My son rode the Honda today as I played with the Manx. The ignition trouble I had turned out to be dirt on the optical sensor. So now the Honda was performing as usual. I have fitted an lambda sensor to the exhaust, and was interested in the AFR readings. Son reported air/fuel ratios between 12.5 and 13, which means carbs are fairly well set up. Though this was at air temperature around 40F. Will be interesting to see readings at a more decent temperature, have to wait until late April for next test. Only some routine maintenance to be done.
So with very little work to be done to the Honda, the Manx and the RG, I got another race bike to prepare, a Yamaha RD-LC.
 

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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
 

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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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imho the rules suck whenever they hold back the advancement of a technology and ultimately participation in the sport.
 
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