feetupfun

If I had a decompressor on my 348 I would use it to ease the piston over TDC before giving the kickstart lever the starting stroke

Don't get hung up on the front hub. The IT range had speedo drive from the front wheel and a YZ hub

https://www.yamahaty.com/english/tyusineen/majestyen.html

Slide number is usually the height of the cutaway in mm

Here's a photo of the adaptor plate I was talking about

As for your other question, yes when you use a rubber connector for the carby, you no longer need the thermal insulator piece. To seal that joint, you can use a paper gasket or machine an O ring groove or use a sealant that stays soft

You could alternatively fit a flat adaptor plate that fits the existing studs and has 2 holes in a vertical plane for a connector that has the mounting holes further apart. I've seen those adaptors for sale but would be very easy to make. Connectors with wider spaced mounting holes are easy to source. Yet another option is to use the inlet connector piece from a later model Alpina or Sherpa T, which are made to suit spigot mount carbies. That's what I did on my M138 Alpina when I replaced the AMAL flanged carby with a Mikuni, then an OKO

Yours is an early A model TY250. It will probably have a engine number prefix 434 and frame number prefix 434. There is no reed spacer on your bike. The alloy plate you can see the edge of is part of the reed cage. That carby is an aftermarket Mikuni. Easy to tell by the large spigot that the fuel hose goes onto and the idle air screw is on the wrong side. The original carby on the A model TY250 is a TK brand. The TK carby is good until it wears then not worth keeping because the wearing parts are expensive/unobtainable. Later model TY250s came with a Yamaha-Mikuni carby that was unique to the TY250. The B model TY250 Yamaha-Mikuni is different to the C,D,E models TY250 Yamaha-Mikuni. The important parts are not interchangable between the off-the-shelf VM26 Mikuni and either of the VM26 Yamaha-Mikuni carbies. Your carby may well work well. If it does work well and is 26mm, please share the jetting specs because I've never been able to get the one I bought to work well on a TY250 motor.

I'm old and unfit but can still manage to start my 348 dozens of times during a trial without using a decompressor. Having said that, I find that starting it for a section often takes more effort than riding the section. If it wasn't so hot where I ride, I'd probably leave it running more than I do. They do make a cool noise if you use one for braking. The 348 kickstart mechanism is robust apart from the return stop so there is no need to fit one to protect the mechanism.

The open class Pursang cylinder that fits that bottom end is also 325cc. The later open class Pursangs (360 and 370) have a longer stroke, as does your model 213 Alpina

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)