feetupfun

I'm 55 and have been competing in observed trials since 1974 and also enduro, hare and hound, MX and trail-riding, which is by a huge margin the most popular form of two-wheel off-road activity. I stopped doing all the fast stuff 15 years ago after I started having a few too many crashes caused by loss of concentration, and now confine myself almost exclusively to riding trials on Twinshock era bikes I studied Mechanical Engineering at Uni in the late 1970s and also did a Trade apprenticeship (fitting and turning), and have worked since 1980 in engineering roles for large companies that have interesting manufacturing processes (BOC Gases, Alumina Refineries, General Electric and the past 11 years at a large coal-fired power station. I love working at old plants that are nearing their end of life, because it is always interesting to find newly emerging failure modes Because I don't get to do trade work in my paid employment, I get great enjoyment from doing trade type work as a hobby, which fits perfectly with my love of Twinshock trials bikes. Tweaking the old bikes is my cup of tea. I have a brother-in-law who also did mechanical engineering and the same trade at the same time who is a wizard with racing engines and is also a specialist welder and we tend to feed off each other's enthusiasm I learned to ride on a 1950s CZ125 road bike in the early 1970s which my Dad had restored and he also restored a 16H Norton and a plunger Tiger Cub during my youth. Back at the time my Dad married my Mum, their only form of road transport was a motorcycle and sidecar. They had to buy a car so they could build a house so that was the end of motorbikes for a few years. His favourite bikes from his youth were his Matchless and his Norton International. I have two teenage kids and I write the trials column in VMX magazine

as suggested by a previous poster, on the model 10 you may find the frame number above the swingarm mount as shown in this photo

Both the 348 and the OSSA are old bikes, but the standard 348 is a quiet old bike and the standard OSSA MAR is a noisy old bike

Maybe it would help you work it out if you listened to an OSSA with a standard muffler

Mk 1 round bowl AMALs behave like that if there is a drop of water in the float bowl

It would take something pretty special to reduce the exhaust noise to that of a 348 Cota

mine has an MCS universal two stroke muffler with one side flattened a bit to clear the tyre. It is not particularly quiet either

what carby is it?

I would check the diameter of the barrel spigot on the 175 before you get too excited about using a 250 barrel. It might be OK but it would be a very thick sleeve in the 175 motor if they are the same OD By the way, there are quite a few Sherpa Ts out there with Alpina bottom ends and they go fine in sections once geared down

yes the bearing seats lose metal when people replace the bearings without heating the hub. What you have is quite common on old motorbikes with aluminium hubs. If there is no radial play, you can fit them with a retaining compound to prevent them from moving and making the fit even worse. If there is radial play, the hub can be sleeved. If there is radial play and you use a retaining compound, the brake drum surface may not run true Cota 348 trials bikes were sold in 1976, 77 and 78. There were trail versions (348T) sold later on (probably to use up the unsold 348 frames after the 349 frame was released) The piece of perforated plate may have been removed from your frame to allow easy cleaning of the underside of the motor. Another possible reason was that one brand of aftermarket bashplate made for 348s fitted between the motor and the frame tubes, so they might have taken the mesh section out to prevent debris building up above the mesh plate

Brake pedal looks like 1977 or 1978 348 Someone has removed the drilled steel bash plate that is normally welded between the frame tubes under the motor Looks like the ignition flywheel cover has been cut away, probably to allow easy cleaning around the sprocket Can't see much in the photos but from what can be seen, the rest looks like normal 348 to me Have a good look at the frame while the paint is off, looking for cracks and bends One of my 348 frames has tubular ears/loops welded to the frame tubes under the front of the engine as a protective measure, but I don't know which year it is. My other 348 frames don't have those ears

suggest you post photos of (at least) frame and motor

There are second hand magneto covers available but I wouldn't call them cheap. The alloy bashplate from the BCDE models and that engine cover are probably the most commonly cannibalised parts on TY250s wrecks

I made one from 5mm aluminium plate that was a simple curve (not fancy, no welding) using a piece of wire as a template to do the curve at the front. It cost about $40 for the piece of aluminium. It could also be made with two creases instead of a curve if that is easier for you to bend

I run TY250A twinshock forks on a TY175, and found that the standard TY250 spring rate and preload were a bit high for that bike. I reduced the spring rate and preload by replacing the standard preload spacers with springs. I have not fiddled with the damping as the standard damping seems to work fine with the reduced spring rate and preload

looks like the wheelbase would be a few inches longer than standard

yes

Even if they did fit, they would look very strange on a TY50, because they are made for bigger diameter wheels

also the slide looks quite worn so I'm thinking maybe the slide needle and needle jet might be worn. That would make it run too rich from about 1/4 to 3/4 throttle. Maybe you could take a photo showing the slide needle up close. You seem to be pretty good at close-up photos, if possible can you also photograph the hole through the needle jet?

There are still quite a few possibilities for what is causing the smoke. Did you work on the bike at all between your first ride (when it didn't smoke) and the second ride (when it did smoke)?

The best test for clutch slip is acceleration in top gear It is extremely unusual for a clutch to slip when the engine is working as a brake, because there is much less torque being transmitted by the clutch in a given gear when the engine is used as a brake, compared to when you are accelerating

Glenn are you sure it is the tyre that is moving? I had a tyre/wheel combo on one bike (KT250 I think) a few years ago that I tried different locks, and even put screws through the side of the rim to grip the tyre, but the tube stem kept leaning over during a ride no matter what. Because it still happened without the screws moving in the tyre bead, I figured it was actually just the tube creeping around inside the tyre. After that I got a lot less fanatical about rimlocks and have even been riding a twinshock for over a year now with no rimlocks or screws on the rear wheel - TY250D rim and Michelin X11 tubeless with a grooved rim bead seat.

I run the standard springs and preload spacers in the majesty modified TY250 frame and that seems to work fine to me. I also have a Godden Majesty 250, which carries a more weight on the front end than the Yamaha frame, and it is still fine with the cartridge emulators, standard springs and standard preload spacers. Yes 1/2 inch to 3/4 inch would certainly help. If it was me I would extend the swingarm on the TY250-framed bike by 25mm, which would make it the same wheelbase as the Godden Majesty

squirt things near the front sprocket with WD40 one at a time with a test ride in between until you find the culprit Yes it could be the bearing behind the sprocket, if the bearing inner is spinning on the gearbox shaft for some reason. The sprocket nut is meant to be tight. If it's not tight the shaft can turn inside the bearing inner. Could also be the chain adjusted too tight. The chain gets tighter when you get aboard. The WD40 will find it whatever it is

Both are hard to get some parts for so make sure they are complete The Aprilia is rare and may be seen as desirable by a collector Italjets are not rare but are not commonly seen in competition