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PostPosted: Mon Sep 09, 2019 1:40 am 
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On the street

Joined: Tue Jan 09, 2018 6:59 am
Posts: 14
Country: Denmark
Bikes owned: 1968 T20 (once owned T500)
My T20 has new pistons, std. bore, running in a new (right) and a used but very fresh cylinder and by measure as new (left).

I have some top end rattle when not under load, goes away as soon as the gas comes on.

1) Wonder how much rattle there was originally and if I can and should do something about it?

2) About 120 kms into running the thing in - the noise seems to gradually decrease. The manual says 800 kms keep under 4000 rpm and up to 1600 kms keep under 5000. Did people really do that back in the day?

Cheers
Rolf


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PostPosted: Mon Sep 09, 2019 4:27 am 
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On the main road

Joined: Thu Nov 07, 2013 1:59 pm
Posts: 172
Country: United Kingdom
Bikes owned: A100 GT250 & 4t Yams!
Did you measure piston/bore clearance and ring end gap when refitting? In spec??
If yes I wouldn't be too worried if I was you...

'Yes, they all sound like that sir' :wink:


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PostPosted: Thu Sep 12, 2019 3:58 pm 
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Expert racer

Joined: Tue May 18, 2010 5:38 pm
Posts: 1459
Location: New Hampshire
Country: USA
Bikes owned: Suz, Yam, Honda, Kaw.
my 2 cents

I do agree 60s two strokes were noisy and some 4 stroke riders back then swore some of these 2 strokes were about to blow (didn't though). It may be fin rattling noise and you maybe able to place rubber dampers in between fins on head for example.

Also the rings during this error were thick. DId you use the OEM piston for this build? Can a modern Suz 250 piston fit that has thinner rings?

You have one new cylinder and one used cylinder if I understand? which you measured as new. how did you measure...what clearance did you find curious. Did you hone cylinders? Hone can take another thou or more off very fast. If larger piston to cyl gap you should hear some slapping/rattling while idling too and frequency of sound increased with rpm. Idle is light load too

Can you tell if one side or the other rattles?

light load while riding: Is it RPM dependent? I mean you are around 5K-6K RPM and then you back off throttle to closed position to cause this noise: if so could be fuel pinging/detonation/ AFR lean, timing off.

You can pull off head and look also look into Ex or Intake port for piston scuffing. If you want a real quit engine after a brief run it, tear it down and smooth piston skirt if high spots or build up etc.

It is hard to run a small 2 stroke at low RPM for break in... no torque down there. If you rev it too high it may "break" in too fast and have those material high spot/build up causing the noise. As above take a look see and tear down on smooth things out.

Can it be that you had rust stain/stripes in the con rods eyes due to original roller bearing setting still for decades in one spot and those stain (metal decay) finally fatigued as you ran it - now you hear wrist ping rattling?

If you can see cylinder wear like on intake side about piston skirt (no hone marks left, cast liner polished and wearing you may have a sloppy big end con rod play. I think you have to pull cyl to see that side of liner

I think any of these can cause a sound you mention if I can really understand that sound solely based of you writing.

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PostPosted: Fri Sep 13, 2019 8:32 am 
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On the street

Joined: Tue Jan 09, 2018 6:59 am
Posts: 14
Country: Denmark
Bikes owned: 1968 T20 (once owned T500)
Vintageman wrote:
my 2 cents

...
I think any of these can cause a sound you mention if I can really understand that sound solely based of you writing.


Thank you: Wonderful collection of ideas. I'll work my way through them.

Cheers
Rolf


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PostPosted: Mon Nov 25, 2019 2:45 am 
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On the street

Joined: Tue Jan 09, 2018 6:59 am
Posts: 14
Country: Denmark
Bikes owned: 1968 T20 (once owned T500)
daxman wrote:
Did you measure piston/bore clearance and ring end gap when refitting? In spec??


I will pull the top during the winter and re-measure the whole thing.

Been looking without luck: Where can I find specs for bore, ring gaps, etc?

Thx for any help! 8)


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PostPosted: Sat Nov 30, 2019 11:18 pm 
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To the on ramp

Joined: Sat Jan 29, 2011 10:51 pm
Posts: 220
Location: New Zealand
Back in the seventies we used to run it in to the end of the driveway then away we go.if a two stroke doesn't rattle the key is usually turned off.

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