feetupfun

More HP? Have you ridden it yet? They are known for a few things but not for lacking power

Not me but I've seen some. Eligibility can be tricky here. OK for P65 motocross but not for P65 trials

I use NGK B6ES with 0.5mm gap with a standard ignition TY250 engine and B6ES with 1.0mm gap with an aftermarket electronic ignition TY250 engine. On both engines the spark plug gets looked at about once per year. They usually start to get rounded edges after about 5 years. I've been setting the plug gaps on my trials bikes with points ignition at 0.5mm for about 45 years now and have no recollection of where the 0.5 mm first came from. A B7ES is a conservative plug for a TY250 twinshock and was recommended by Yamaha because they wanted to minimise the risk of customers melting a hole in the piston crown when they went flying along the road on a TY250 (like commonly happened in the 1970s)

I use car manual gearbox oil in my 348 clutch and it works beautifully. I was using ATF in the clutch previously but it squealed during engagement.

Take the cable plugs off the thermostat and join them together electrically. Set a multimeter to measure resistance and measure the electrical resistance across the connections on the thermostat as the wet end of the thermostat heats up. A saucepan of water on a stove works for this. It should drop to zero ohms when the water gets hot enough - somewhere around 80-90 degrees celsius.

Sounds like the thermostatic switch has stopped working. It's a fairly common failure. They are easily available. You can test yours with a saucepan of hot water and a multimeter. People usually bridge them out while waiting to get the new one which means that the fan will run whenever the motor runs. The motor will run too cool at times but at least the coolant doesn't get blown out.

It might not be the right pipe or it might be a bit bent or the tank might be out of position or it might be the wrong tank Does the rest of the bike bike look like this?

What is so important about being pre-80?

To answer your other question, the KT carby on my KT is still doing great service. Haven't had to do replace anything in it the 20 years I've been using it. I'm also running a KT250 carby on my 250 OSSA trials bike and did have to lift the needle a groove compared with standard KT settings to get it to run right.

Looks like an O is an O for any Mikuni needle jet

There's a reference chart somewhere that gives the hole diameters for Mikuni needle jets. I'll see if I've still got it

Thanks PMK that looks great. I'm having a go at guessing which Yamaha the rear hub came from. Being brake and sprocket on the same side I'm thinking late model (post -81) IT175 or YZ125

What you are looking for is any model Alpina or Sherpa T after the time when they changed to using the rear engine mount as the middle support for the swingarm axle.

When my wife wanted to get a modern bike we got a Beta because it was the easiest 2 stroke for her to kickstart.

Machine the brake drum surface smooth and true and fit brake shoe linings that are sized to exactly match the drum diameter. This usually requires relining the shoes with oversize lining material (5mm or 6mm thick) which is a service available through automotive brake repair shops. The linings are then machined or ground back to match the drum. Choose a lining material that provides the most friction. The ones I use are forklift brake lining material and after doing what I have described, are every bit as good as the two-pot Brembo disc front on my TY250Z. I've heard there is also lining material designed to work on brakes that operate under water and that this material works well on trials bikes (that often have wet brakes) but I haven't tried it yet. If you try and do the radius matching without fitting thicker linings the angle of the actuation cam will cause sliding of the edge against the cam follower and this creates extra friction and springiness. There is a cheapskate way to get away without getting thicker linings. By packing out the pivot end of the shoes and then machining the linings back to match the drum. This can even be done with shoes that have a full circle pivot hole like a Bultaco (by rebushing the pivot hole off centre)

Jon you can fit in a much larger weight band than the one in Guy's photo. I'm working on a 360cc TY250 motor at the moment and it has a weight band that is at least twice as heavy as the one in Guy's photo. People tell me that the weight band on this 360cc motor looks like what was fitted to the 320 Majesty motor. The owner of the 360cc TY250 I'm working on told me that the huge weight band flywheel was fitted to his Yamaha factory supported TY250 in 1975 at the same time as the Yamaha fuel injection was fitted, to tame the motor down. The super-heavy flywheel was then fitted to this 1977 TY250 motor when it was converted to 360cc (in 1977). I'll post up a photo of the flywheel soon.

The cheap ones I've seen come with an M8 x 1.25 thread slave cylinder mount/adjuster which I see as an advantage compared with the arrangement on that Magura

I haven't used one myself yet but have seen them in use on some old trials bikes and was thinking of trying one on a KT250 as part of a silly game with a friend who has an unusual clutch system on his KT.

What does this mean?

For me that would spoil the look of the bike

Yes they are made in different lengths

I'm stuck in the 1970s with my welding. Oxy-acetylene and stick. I tell people that the welding technology fits perfectly with the age of the rider and the age of the bike.

They can both be made a lot lighter than standard

A standard TY250 weighs about 11 kg more than a standard TY175 and 10 kg of that difference is the weight difference of the motors

The hubs are exactly the same on 175 and 250. The 250 uses a dished sprocket and the 175 uses a flat sprocket because their front sprockets are at different distances from the centreline.