feetupfun

Large capacity single, short kickstart lever located high up and too far forwards. I ride a 348 and find that in the course of riding a trial, kickstarting the thing at each section on the first lap tires me out more than riding it, and mine starts perfectly. I'd hate to try and ride one that was a cantankerous starter. It's too hot here to leave it idling while you walk a section, but I am certainly tempted. I think the decompressor is there to ease it over TDC before having a decent lunge at the lever, and to make rude noises with. As for the mudguards, you can buy new replica mudguards easily. I can't tell if yours has the new-fangled lopolene? plastic tank, or is a fibreglass tank, or has a fibreglass cover (shelter) over an aluminium tank. Fibreglass gelcoat can be polished.

GasGas GasGas 280

That's the reason I got one

pmk, those bikes are all in the same class in Australia. Our classes are Pre-65, Twinshock Pre-87 (air-cooled mono excluded) and Air Cooled Mono. For a while the middle class was Pre-84 (to exclude air cooled monos), but the cut-off date was later extended to allow the (1985) TLR250 in

I read it as a humorous comment. To me on the other side of the world, it meant "contrary to the increased stress generated by the Brexit process, we are happy to reduce stress levels by loosening our rules to increase the participation rate of British riders, by also allowing younger riders to ride modern bikes"

My faves for geometry/handling in standard trim are Honda TLR, Godden Majesty, Fantic 240, Italjet Piuma, SWM TL280 and any late model Bultaco

Are you referring to my OSSA (the green and white one as shown in the photos) or pmk's OSSA?

Thanks but that is how it came to me. Maybe MkII MAR brackets come like that. I think someone told me previously that I had those brackets fitted backwards. Again, that is how it came to me and I've had no reason to change it around yet.

The reason that there is no stator in the photo is because I took that photo when I had the stator out to drill the hole to run the wires out at the front/top.

and this photo is to illustrate why no-one makes a 9T front sprocket for OSSAs. This shows a 10T sprocket

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