feetupfun

The same can be said about early to mid 1970s 250cc vs 325cc Sherpa Ts. They both have a similar rated maximum output, but the 325 has much more power at low to mid RPM (from memory they are rated at 21 and 22 horsepower at their peaks) The reason for this is that the gas volume throughput of both motors is limited by them sharing the same size carby and the same size exhaust system. My Alpina M138 350 is also an amazing tractor like the M116 at low to what would be mid-RPM on most other big two stroke bikes of the era so I know what you mean. It doesn't stop you going fast in the bush though, because it has enough gears spaced apart enough to suit the power spread. I find that it is the suspension that limits how fast I am game to go in the bush (so I can still hang on) I'm currently rebuilding a M85 and am greatly looking forward to riding it too. I expect it will be much more like my Sherpa T 250 in handling and motor than the M138, because the early Alpinas share frame and steering geometry with the early Sherpa Ts, while the M138 is longer and has the Pursang triple clamps

If you mean increasing the offset so the kickstart clears the brake pedal fully, not a good idea because if the kickstart rotates much past the brake/footpeg, the kickstart mechanism stop punches a hole in the magneto case. Standard practice is to use the footpeg to stop your foot taking the kickstart lever past there, and don't mount the kickstart too far forwards either or the stop will hit the mag case even before the lever goes past the footpeg. Unless you are very tall, lengthening the kickstart will make it even harder to kick over because your knee and hip will be at even worse angles. Try putting the wheels on something to lift the bike a bit higher and see what you think Yes freight from the UK is horrendously expensive to here too. I usually minimise the cost as a proportion of the total by doing big orders, and making sure I buy all the big light things like mudguards at the same time. Also do joint purchasing with friends. I don't know how NZ sales tax works for imported goods, but here if the order is valued under $1000 it doesn't attract australian sales tax (GST) There is a lot of help available on the internet about cleaning up oxidised plastics and even discoloured fuel tanks. I usually break mudguards before they get too grotty-looking, and don't like plastic tanks so have never tried doing it.

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.