feetupfun

OK my experience with electronic ignitions for TY twinshocks is with three types: Trails and Trials UK Rex's Speed Shop Electrex World I've got two bikes with the Trails and Trials UK ignition and never had a problem and neither of them has ever kicked back. I also have a TY250A that still has the standard ignition system working perfectly and it kicks back a fair bit when I'm knackered during a hot trial but not when I'm fresh. Both the bikes with the T&T UK ignition run very well. One of them was fitted to allow the timing to run only a small advance at low RPM to tame the motor response down at low RPM yet still run well through the RPM range. The other T&T UK system was fitted because I was having problems with the original points ignition cutting out unexpectedly. I've ridden a friend's TY250A that had been recently rebored and had a Rex ignition fitted. It was similar performance at low to mid RPM compared to my TY250A that has the points ignition system but from mid-rpm to high RPM it had noticeably more power than mine. I helped a friend to fit an Electrex World system to his TY175 and was amazed to find that the mounting location for the HT coil was beside the airbox. This means that the HT cable passes over the carby and past the fuel hoses. The bike ran fine with this ignition. They may have found a more suitable HT coil by now.

Sorry but to reduce kickback while kickstarting you should run less advance not more advance.

I already said you could fit an electronic ignition with advance. This will give you less advance at kicking speed. If still using points, set the timing with a strobe light. Be patient. Lie the bike on its side. Wear a head light that shines from between your eyes.

The rim might not have the correct offset from the hub

Yes of course you can fit the diode. The rest of the TY250A ignition is the same as the later models. However the diode is there to stop the motor running backwards. It won't stop it kicking back which is caused by firing prematurely with the crankshaft turning in the forwards direction but too slowly. The tendency to kick back is made worse if the spark timing is a bit too far advanced. If you want to minimise the possibility of a kickback then use an ignition that has an advance curve.

Have you asked Richard Kent (the aussie EM agent) yet?

If it has the original setup inside the magneto cover, probably not, but you would have to remove the magneto cover to have a look to know if it is original and to know if it has a lighting coil. Another thing you could do, to help people help you, is to mention what digits the engine number starts with so the motor can be better identified.

I did answer. I made two posts. Maybe you only saw the second one. Riding weather where I live is absolutely fantastic. Having lots of fun test riding and practicing where I live. I've been concentrating on getting a 1968 Bultaco going as well as it can. We are in the middle of the trials riding season here but there is still a couple of weeks to go before trials competition starts up because of social distancing requirements. Preparations for the first competition events post-lockdown are in progress.

You need to work out the ideal length for the preload spacers for yourself to suit your springs and how you like your forks. A suitable starting point in tuning the forks would be to fit spacers that provide about 20 mm of preload with the forks fully extended.

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)