feetupfun

and here is what it looked like after fitting the extension piece

The new sprocket was machined to provide a half-width recess to match a half-width recess machined on the original dished sprocket. I then drilled 6mm holes and bolted the two pieces together. An alternative is to machine the teeth off the dished sprocket and weld a new sprocket on. These photos were taken while I was preparing to fit an extension piece outboard of the sprocket to reduce the bending moment on the axle

I went and counted the rear yesterday (before I had anything to drink). It is a pretty big sprocket and yes it is slow in first but it's never troubled me. However I don't actually like the way it looks (so big) and now with this discussion am thinking of fitting a smaller rear and giving that a go

My 250 OSSA runs 10/52 gearing. I never use the clutch and I don't remember ever riding a section in anything except first gear. I remember having to make some sort of spacer for the front sprocket to provide clearance for the chain with the 10T sprocket, maybe to clear the sprocket nut. Be aware that gearing is to suit the rider's own riding style. Just because someone else has a certain gearing doesn't mean it is right for you. If you have a clutch with a light, fast and predictable action, you can usually ride in whatever gear best suits the fastest part of the section. If you have a twinshock clutch that has a soft takeup, or an unpredictable takeup, or a wide takeup, or a delayed takeup, it is usually better to ride (in the old-fashioned way) without touching the clutch. You would probably find that your "very experienced guy" with the custom TL125 has a clutch that works like a modern bike. OSSA clutches while predictable are usually a bit heavy with standard leverage ratios and have a wide take-up if you increase the leverage enough to make them light.

re main jet size for highway use: Test ride with bigger and bigger main jets one size at a time until it four strokes at full throttle, then go down one size. This can be done on the road or on a dyno. The reason why you need to do this is that because the engine produces so much more heat at sustained full or near full power than it does when ridden with a variable load, the engine temperature will go too high if the fuel/air ratio is even slightly lean.

Yes putting a bend in it takes the tip higher when it is stored. I have already posted photos showing what I did to mine (yes they have a bend) on these forums somewhere

Using the start lever "enrichment valve" is sometimes used to cool the motor when it is being hammered along at full or near full throttle. Yes the extra frictional heat generated by the rings from riding at constantly high RPM can cause problems. Two strokes are very sensitive to jetting when used at consistently high output, which is why it so tricky to get a road racing two stroke to run properly, and not seize when you back off the throttle at the end of a long straight. If you are going to ride your TY250 along a freeway at 60 MPH, correct jetting is way more important than if it is used for trials or trail riding, or low speed road use. Problems can be minimised by gearing the bike to suit the speed you want to ride at. For general road use a TY250 should have at least 16/53 gearing.

I lengthen the stand by welding in a piece of steel tubing

The front trials tyres we buy now are taller than the trials tyres from the mid 1970s so it should be expected that if you still are using the original mudguard mounts, there will be less clearance

Some models don't have an exhaust port bridge so don't benefit from having holes there. One of my Bultacos is an Alpina 138. It has no exhaust port bridge, but the only piston I could buy at the time I had it rebored (about y2000) had those holes.

OK I see the O ring you are seeking in the A model parts book showing the TK carby. Yes, if it leaked, some fuel could bypass the main jet.

The 26mm OKO from mid atlantic trials is fabulous. I've got one on a TY250B motor, replacing the original Mikuni which had seen a lot of use. I haven't tried the OKO on my A model yet.

pmk, I figure you are enjoying the process of developing your Curnutt shocks to the point of getting them working well for trials use. It is quite interesting to hear your voyage of discovery. As far as shock lengths go, the Betors that came with my OSSA are 350mm long and TY250 twinshocks originally came with 325mm shocks. Most people nowadays ride TY250s with something between 340mm and 360mm and my OSSA steers nicely with both 350mm and 360mm shocks. The reason why trials shocks in the 1970s had shorter travel and heavier damping than trials shocks do in 2017 is because the development work that has produced the amazing trials shocks we can buy now, hadn't happened yet.

yes count grooves starting at the top. When you say the "needle jet set screw", I figure you are meaning the air screw. As well as the diameter, measure the groove width and depth to size the o-ring. A leak past the air screw o-ring will not make the bike run rich. In a TY250A TK carby that has had a fair amount of use, a likely cause of the bike running rich is: Wear in the needle jet and the needle (where they rub against each other) or a passing float needle valve

I've got an A model like yours and the motor has never even had the cylinder head off, but a few years ago when I was servicing the points, I did notice a small oil witness around the magneto side crank seal. The main bearings were still quiet and with no detectable free play, so I assumed that the seal had just started leaking a bit because it was so old. The motor still ran fine and if I hadn't looked would not have suspected that there was any leakage there. The crank seals on both sides can be replaced without splitting the cases. If that was my bike I would inspect the magneto side crank seal for any evidence of oil residue, feel for any up-and-down play in the LH main bearing and have a good listen with the motor running for any rumbling noises, which are a tell-tale for failing main bearings. They are an incredibly sturdy motor so can do an amazing number of hours of regular use with little effect on the bearings, but like any two stroke, any rolling element bearings on the crankshaft assembly can be damaged fairly quickly from the bike being stored in damp conditions. If there is any up and down play or rumbling, it is not worth changing the seals unless you also replace the main bearings, because the movement of the crankshaft will probably cause even new seals to leak

Those bars are what came standard on the first (1976) and subsequent Cota 348s. They are 1" in the middle and stepped down to 7/8" for the lever mounts and grips. They have a nice shape but beware of unexpected failure. Mine looked perfect externally but broke unexpectedly in about 2014.

When you say "The big end pin won't allow the crank webs to close that side play down even more", are you saying that the big end pin is stepped?

That one looks like a Fraser frame with a Bultaco front end (Sherpa T triples, Bultaco Betor forks and Alpina/Frontera front hub)

A standard TY175 carby mounted on a TY175 should not have any problems due to riding steeply downhill if the float height is right and the float valve is sealing off. They normally work perfectly. The float valve does not last forever. I think I'm on my third float valve in the TY175 I've had for 41 years. If it is a wear problem causing the valve to pass, you should see a band on the needle surface that is not shiny (where it touches the seat) and the rest will be shiny. The needle taper should be shiny all over. If the problem has developed over time, it will probably be the float valve passing. If you don't know the history of the problem, it could also be that the float height has been set wrong.

It's fairly common to need to open the fork tube clamps a bit. If you've had the tubes re-chromed sometimes they end up slightly bigger in OD than original, but even standard tubes sometimes need it too. If it is only the second (upper) clamp that is causing problems, it might pay to check if either of the clamps is not quite straight. Another option is to fit the tubes through the bottom clamps without the top clamp, then fit the top clamp, rather than try and slide them through both clamps at the same time.

Common failure on that type of hub. It is a Honda hub and good ones are rare, not just because it is a rare model bike, but also because they break). Cota 242s also had that front hub. The earlier Cota 247/348 type front hub can be used and are far easier to find. I've also seen Yamaha drum-brake wheels fitted to 348s and 349s. Some 349s have narrow spacing between the fork tubes which limits the type of hub that can be fitted (unless you also get a set of wide triple clamps)

The rim and tyre should be in the centre between the forks. The rim can usually be moved left or right by adjusting the spokes. It's possible that it has a non-standard wheel so if you post up photos, you will get help with that.

How the air screw is set makes a big difference to the off-idle response

On the SWM in the photo there is still cable between the clutch and the device which would operate at full cable loading. Your suggestion to have an intermediate lever just above the clutch cover with a rod to the clutch arm would have less friction overall than the setup on the SWM because the whole cable run would be operating at reduced loading.

Have you tried Hell Team (GasGas importer and a wealth of GasGas knowledge) in Sydney, Australia? I think they helped my friend out who bought either rings or a piston kit for their GG70