feetupfun

Broken engine mounts are very common on twinshock TY250s. Usually one of the top rear mounts but the latest one I've repaired also had a crack in the steel cross piece between the front engine mounts. It had been used for sidecar trials though which is a tough ask of any frame. One TY250B I bought had been ridden for some time with all the engine mount bolts loose. How could I tell it had been like that a while? The holes in the engine casings were elongated and had cracks at the ends of the holes in the casings (and the bolts were loose). It had lots of other problems too but it was a good parts bike for the price. I've never seen a TY250 broken engine mount bolt but have seen the threads worn off from movement.

The thing that got me interested was a magazine test I read in 1975 when the B came out. It listed some differences between the A and the B models. Then over the years I collected motorbikes and motorbike parts and one day was able to compare the dimensions of the two conrods. I could see that the B conrod has a smaller beam section of the conrod so I finally believed the motorbike article. I haven't weighed them though. I suspect that Yamaha changed the conrod design on all their similar motors at the same time as the TY250 changed, much like the primary drive gear went from spline to key drive with the B model at the same time as the Yamaha MX and trail bikes changed. The same happened with the fork sliders design and the clutch design.

Leading axle 35mm Bultaco Betors (Sherpa T or Alpina) with parallel top tube ends are a good alternative, having the same axle offset as the 348 forks. Just be aware that there is a range of fork tube lengths for different models. I've seen a couple of 348s with Bultaco forks. Also seen a couple of 348s with 247 fork bottoms.

I may be wrong but I thought that Bultaco spoke threads are 55 degrees thread angle (Whitworth) so the nipples would be too tight if your nipples are indeed metric thread (60 degrees) I'll determine the thread form on my Bultaco spokes later on if you haven't worked out the problem by then

I reckon I know what's happened. You said that you took out the actuator camshaft. The only way to do that is if you remove the clutch pressure plate and let the pushrod move to the right far enough for the camshaft to come out. I reckon that while you had the pressure plate off, the mushroom part of the pushrod slid out and so did the ball bearing behind it, without you noticing the ball bearing. That would explain the symptoms you have now with it reassembled

I reckon your rear wheel is 1970-1973 Sherpa T or Alpina but may also be same era Pursang or Matador

I'm glad I'm not the only one who weighs wheels. I suspect that the rim type is also a reason for why the later Sherpa T rear wheels are so much lighter than the M49 and M80 rear wheels.

YZ250 piston but you need to make mods to the shape of the crown if you want to replicate what the original piston crown looks like. Here's a photo I found on the web that shows what I'm on about

1/8 inch is probably even more than standard. I've never bothered measuring it to know exactly. There is normally slack in the rotation of the camshaft before it touches the pushrod. If the clutch was working OK until the lever perch broke, why do you now think something has gone wrong in the mechanism?

There was a plastic model kit made for Ty175 and they sometimes pop up for sale

Probably need a bit of history about the bike to someone to work out what has happened here. It could be lots of things

Looks like a mechanically standard series 1 model 49 and yes the patina is excellent. Great to see. Thankyou. I'll bet the seat foam is rock hard ?

Notes added to your questions

The flat tool is what I described in a private message after you asked me about how to do the job. I don't use a tube type puller. Yes I've pulled in many TY175 cranks with a flat tool.

They did it to create the lightest wheels possible at the time. I'm not saying I like the idea of the plated brake drums either, but you did ask

I'm in Australia and have a spare I would sell but freight to the UK would be expensive. It is the type shown in Vintagenut's photo but not as nice as that one. Beware, there are two types. The early M49s saddle/bridge/sidecovers didn't have the scallop for clearance of the top shock mount (because the shock mount location was further back)

I had a test ride many years ago on either an 88 or 89 at Bennoble (Robert Marsden's bike) and absolutely loved it. After riding it, I couldn't believe that people still bought Yamaha monos after that TR34 had come out

The monoshock DT frame is made from larger section tubing and has a longer (heavier) steering head and heavier hubs and brakes than TY175. On all the DT175 twinshock and monoshock models, the swingarm is about 75mm too long for trials use and the extra length is in the gusset at the front end of it. The monoshock DT175 front end with leading axle forks has very limited turning angle due to the lack of offset in the yokes and the width of the frame at the steering head. The frame is much deeper between bottom frame rails and seat rails than than a normal trials bike so body english is limited. Even with all that wrong as far as trials competition is concerned, all you need to do to make them great fun to ride in extreme terrain is to move the footpegs back a few inches so you can more easily ride standing up.

Where I live, the only time my trials tyres see daylight is when I'm out riding and it takes about 5 years from new for Michelin X11 to go hard enough to noticeably lose performance and about 15 years to when they start falling apart

Looks like a very neat job. There's something similar over here made from a 1980s monoshock DT175 converted to twinshock rear end and the big frame tube where the shock normally goes has been used as the fuel tank on it. How does your TS185 go?

Firestone trials tyre with a distinctive vee or chevron pattern to the blocks. You can see them in photos from the period. I've kept a front one for reference.

I've got an early M49 which shares frame design with the M27 and I have found 360 mm shocks work the best for me

What is it about the bike being road registered in 1983 that convinces you that it is a 1980 model?

As a matter of interest in the vibration stakes, did you know that when Yamaha produced the second and later models TY250, they used a lighter conrod design, compared with the A model. This had a noticeable effect. I have a few bikes with TY250 engines and the one with an untouched, original A model engine in it has enough vibration at one particular RPM that the vibration transferred to my body causes loss of vision. This doesn't happen when trials riding because the RPM never stays constant long enough and I'm standing up, but when I'm trail riding in 4th or 5th gear and find that particular RPM and am sitting down, the vision thing happens. When I do the same sort of riding on my other TY250 powered bikes, I don't get the vision effect. The other motors are TY250D model with original conrod and TY250B model with original conrod.

There is usually a price premium for competition eligibility. From what I see advertised in the US, that TY at $US1500 is fairly expensive even accounting for it being eligible for "old bike" class in trials competition. The best value trials and dirt bikes are the ones that have depreciated in value due to age but not old enough to be eligible for bike age class competition. If you aren't a nostalgic type rider there is no good reason to buy a 1970s bike.