Help needed with TYZ 250

[izzywithus]

Hi all,
I am new to rials and have just bought a Yamaha TYZ 250 Scottish as project and I need some information please.
There is a little cable operated doohicky on the carb just below the top engine mount. What is it?
How much play is acceptable in the big end bearing? I bought the bike knowing that it needed a new piston and wanted to check the bearings with the head/barrell off.
What are the differences between an standard TYZ and a Scottish?

Any help greatly appreciated.

Thanks
Iain

[David Lahey]

The only cable-operated doohickys on my TY250Z were the throttle and the clutch.
Maybe someone has fitted a different carby and it has a cable operated cold-start device. Standard carby has a little red knob on the plunger.
If you post a picture of your bike it should be pretty easy to tell the differences to a standard one. My first thought was that it might have a larger fuel tank. There is not much else you could do to make it better for the SSDT.

[PA]

Maybe a choke that could be opened from the handle bars when doing the road sections as I understand that opening the choke is a common thing to do in order to save the engine on the higher speed road parts.

[izzywithus]

Thanks PA and David, I am working out how to attach photos and will reply soon.
This doohicky is mounted on the barrel above the carb and the cable runs under the frame. Will post a photo later. If it is a cold start how does it work as there is a choke as well mounted on the carb.

Cheers
Iain

[izzywithus]

With a bit of luck I have attached a general view of the bike as I bought it, including lights, horn, indicators and tin tank.

The next photo shows the location of the doohicky control and then the unit itself.

Any ideas on what it does and how it works would be great.

Cheers
Iain

[David Lahey]

The doohicky is a decompressor, When the cable end pulls the arm, it opens a valve inside that allows pressure from above the piston to escape into the crankcase.
I haven't seen one on a TY250Z before.
The road-going gear (speedo, lights, battery etc) came on some TY250Z models. The bigger fuel tank and seat are obvious differences to a normal TY250Z.
A normal TY250Z has a flat plastic tank cover and the tank is a black PE moulding of about 3 litres and is confined to the space between the frame spars. Your fuel tank extends above the frame spars and underneath the seat.
I see there is an over-centre type cable lever under the RH top frame spar. That could be the lever for the decompressor, or may be to actuate a remotely located fuel tap - I can't tell which from the photos.
The bike was probably sold/modified by Nigel Birkett (Nigel Birkett Motorsport) in the UK. You can probably phone him for detailed info about this bike.

[izzywithus]

Hi David, thanks for the reply. Yet another question. Is there any way to tell if the big end bearings are worn without stripping the engine?

Thanks
Iain

[David Lahey]

The crankshaft (main) bearings can be felt by moving the crankshaft ends up and down by hand.
Turning the motor by hand with the sparkplug out can also reveal rumbly main bearings, as can listening for rumbling with it running.
A failed con-rod (big end) bearing can make a knocking sound, or it may rumble like failed main bearings, or it might make no noise at all (audible to a naked ear) prior to failure.
For a fee you can have vibration spectrum analyisis performed by a bearing monitoring specialist, but that would probbaly cost more than replacing the con rod.
The normal test for the big end bearing requires taking the cylinder off which you will have to do to replace the piston anyway. The usual test is to feel for up and down play in the bearing. If there is any movement up and down, the bearing is close to catastrophic failure. There are also recommended limits for side to side movement but I don't know what they are for that engine.

[izzywithus]

Well I guess since I have got the barrel etc off already I may as well do the bearings. I can feel no play up or down or off the vertical but there is a very small amount of movement left right.

Thanks for the advice to date.
Cheers
Iain

[David Lahey]

I wouldn't split the cases just because you have taken the barrell off. The main and big end bearings may be fine. It takes a lot more labour to split and reassemble the bottom end than to refit the barrel and head.
What do the main bearings feel like when you turn the motor over?
How does the big end feel when you move the rod? Do you think it might be rusty, or just dusty? Why does the piston need replacing? Did the motor ingest water while it was running? 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? Do you think the motor has seen a high RPM runaway event? 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.