feetupfun

One thing I have noticed is that the standard Sherpa N has a very long horizontal rear frame loop (for a dual seat). The prototype Matador, Mk1 Matador and Mk2 Matador all look like a Sherpa S frame to me because of the steep short rear frame loop. On the Miller Sherpa T prototype, the rear frame loop and shock mount area doesn't look like any of them. It's rear loop is short and steep but the tubing looks smaller diameter and the gusseting at the shock mounts is very different. The photos I'm looking at are in Bultaco Todo un mito and Don Morley Spanish trials bikes. I've also looked at two frames. A 5 speed Campera frame from 1966 which I think is an M21 and a 1963 M3 Sherpa S frame.

It's fairly common practice on multiple brands of twinshock trials bikes to have the top mounts closer to the mid-line of the bike than the bottom mounts are. It was done to make the bike slimmer where it counts

Yes that is great value. They are a fabulous motorbike. The ignition system is not connected to the lighting system. Each has its own stator coil in the magneto. Blowing 6V bulbs was standard back in the day for TY250A. Fitting 12V bulbs was one way to avoid it, but the lights then became dimmer. They came with no voltage regulation on the lighting circuit so as the RPM rises, the voltage rises. Nowadays there are a myriad of options for the lighting circuit on motorbikes.

My memory of riding the big-wheel Beta 80 is that it was OK for me at 5'10" but I would think it might be a bit cramped for someone tall. The motor was easy to use in sections. Modern 125 two-strokes are excellent. The modern mecatechno (electric) trials bike is extremely light.

I've just done a top end decoke on a friend's (low hours) 2022 TRS 300 and was astounded at how tenacious, dry and thick the carbon deposits were that completely covered both the piston crown and the inside of the head. It ranged from 0.25 to 0.5mm thick. The tenacity and hardness of the carbon in this TRS reminded me of the carbon that formed in my 1970s air-cooled two stroke enduro bikes running on 20:1 castor oil premix. Sure enough, when I then asked the owner what premix oil they use, they told me it is castor oil based. I'm accustomed to seeing soft, wet smudges of carbon that cover about 40 % of the piston crown on my 2 stroke trials bike pistons. Is using castor oil premix in modern trials bikes common practice?

Piston position can be used to make timing marks and it's easy to measure through the sparkplug hole

You could take the exhaust header off and measure the bore diameter through the exhaust port. The Beta 125s I've seen have a different exhaust header shape to the 200/250/270

Yamaha was selling genuine spoke sets for TY250 last time I bought some. They are zinc plated. Stainless steel spoke kits for them are available from In Motion in the UK. I know you can get spokes custom made by a bloke in Brisbane. You will probably have to send him samples to copy. There are probably people all over Australia who can make spokes for you from samples. Another option that people do is to re-plate the original spokes.

If its is a standard 348 engine then it is approx 310cc displacement..

When that casing is on a 348 motor there is a screwed plug there.

There was nothing in place apart from the cable sheath to stop the wires being damaged. A common improvement for reliability was to run the wires along the top side of the mudguard.

It is a modified Yamaha TY175 or TY125 frame.

Another factor in rear tyre clearance is that the rear competition tyres in the 1970s were nowhere near as tall as modern competition rear tyres

So here's a photo of a Spanish Betor trials shock from back then. I have no way of knowing if its actually from a model 92 or not but I do have quite a few and they all have the same length shock shaft. Some have this type of seal retainer and others have a different type of seal retainer.

My memory is saying 3.25". I'll have a look to see if I have one to measure

The ratchet mechanism you can clearly see in the photo should click as the kickstart lever returns towards the stored position. Just before the lever reaches the stored position, there should be a cam or ramp that moves the ratchet teeth apart. If not for the ratchet mechanism, the kickstart shaft would be rotated by the motor when the motor starts.

If you aren't sure, do a test fit both ways in the hub holes and see if they will work OK.

I've seen a weird thing like what faussy wrote about. A bloke who had lots of road bike experience had a try on a modern trials bike in the parking area at a trial. We showed him how to do fairly open figure eights on the flat ground. He got on, did about three figure eights (while looking a bit stressed), got off and had a real good vomit. He said it was because he was exerting himself too hard. Another time a rider with hundreds of hours trail and enduro riding came along to a trial and had a go on a modern trials bike. He was fine apart from not being able to stay inside the section boundaries even on totally novice lines.

The rubber carby mount piece (between the carby and the reed cage) are known to crack and let air in

Sid to get a photo of the front end of the cable, the bike needs to be on it's side or upside down and my OSSAs are currently packed away. I'm not willing to unpack one of them until I'm convinced that you really need a photo. So here it is in words. Pedal bar end: The pedal bar arm has a receptacle for the end of the cable outer. The nipple on the cable inner fits into a holder on the frame. Wheel end: The brake plate arm attaches to the cable inner. The cable outer is held in a receptacle on the brake backing plate.

I'm assuming you are asking why the mounting holes do not perfectly align with the muffler sitting there. How close does it get to the mounting holes lining up if you lift the muffler? If that's not enough, you could loosen the exhaust mounting bolt near the airbox and loosen the bolts at the cylinder and try moving the exhaust upwards. If that's not enough you could bend the end section of the exhaust upwards.

The Leonelli has a longer red plastic bit that houses the magnet, so is a bit more easily bumped off its seat. They have opposite magnetic polarity. I haven't looked close enough to see it the magnets are held in place differently. Having the magnet pop out of the holder seems to be a fairly common problem, based on how many people have asked to borrow a lanyard after theirs lost its magnet. I didn't notice a difference in price between them.

There will probably be an aftermarket rod kit available because the TY250Z motor is based on the YZ250 motor of the same era (ie probably the same conrod)

when you say 04 TY4, do you mean an SY250R or a SY250F or SY125F or something else?

It sounds like you haven't pulled a motorbike engine apart before. The Cota 348 is far from being an easy one. To get that casing separated on many other bikes is straightforward. The primary drive components are removed, the shifting components are removed, the gearbox output sprocket is removed, the ignition flywheel is removed, the shift drum indexing plunger is removed. All the fasteners holding the halves together are then removed and the crankcase halves are separated. Cota 348s have a few unusual things that can prove to be more challenging than on most other bikes. The drive gear on the crankshaft is on a taper that can be very difficult to release. The sprocket is on a taper that can be very difficult to release. The gearbox components are positioned precisely left/right with shimming that can fall off the shafts during disassembly. The crankshaft seal holders, gasket thicknesses and shimming are what positions the crankshaft in the right spot left/right and provides correct end float.