Help needed with TYZ 250

[izzywithus]

Hi David,
I bought the bike as a non-runner and was told it needed a new piston. When I got it home I started the bike and it ran OK. I took the exhaust off and looked up the port and could see some scoring on the piston, hence removing the barrel.
<<How does the big end feel when you move the rod?>>
As far as I can tell it is smooth. There are no rough or tight spots but I have never done this before.
<<Do you think it might be rusty, or just dusty?>>
Looks clean inside.
<< Why does the piston need replacing?>>
Scoring on side of piston which is just deep enough to hook your fingernail in. There is no damage to the barrel that I can see or feel.
<< Did the motor ingest water while it was running?>>
I think so.
<< Did the rings or piston sieze to the bore? Was the motor run with no air filter? Did a circlip fall out? Has the piston crown been holed, or cracked? Are there any pieces of metal missing?>>
No to all I think.
<< Do you think the motor has seen a high RPM runaway event?>>
No idea at all. What do I look for?
<< The history of the motor is important in judging if you need to replace the conrod, and you should be able to feel for damage of the main bearings.>>
Is it worth taking the flywheel off and trying to spin the crank without it? There is a slight rumble when the crank is spun now.

Cheers
Iain

[SQUAT 405]

Hi there izzywithus!,,,,,,I have had one of these for about 15 years now the first model pink one If the piston is scored replace it with a new one dont risk damage to the bore,replace rings and crank seals as well its not that hard to split the cases and replace the main bearings !!!a little trick is to heat the cases when apart with a hot air gun only,around the bearing area, dry heat is the go! not an oxy, turn the case split side down and gently drop onto a flat wood or similar soft type surface the bearings will just about drop out ,dont dry press them as you can damage the case,pre freeze the new bearings and while the case is still hot drop the new ones in, no press required I used bearings from Bearing service centre as i remember they were not the expensive so i replaced most of the gearbox ones as well I used new yamaha crank seals,this rebuild was done about 10 years ago and my tyz still runs great!!!!small end bearing is easy to replace and should be done anyway with a new piston!iff the big end needs replacing as well its a bit of a different story,Hop this will help you in some way,,,,,,,,,, interesting tyz never heard of one like that before this one, the decompresser is trick even the decal is !!!!!

[David Lahey]

Yes Iain any rumble in the mains means it's worth doing them.
My Ty250Z was the same model as as Squats's but probably hadn't seen as much use because it was still on the original everything and no noises and good compression. I did decoke it though and it runs like a dream. Now owned by someone else in our local club, I bought it in 1998 as a second-hand import from Japan.

[izzywithus]

Hi David and Squat 405, its great to have something a bit different and interesting. Thanks for all the help and advice, I am sure I will be back for more as the bike progresses.

I have had a search around various bookshops and on-line but cannot find a service manual for the bike. Do you know where I can get one from?

Thanks
Iain