feetupfun

TY250 forks are actually shorter than TY175, but when run flush with the top clamps provide the same front height as TY175 forks set to the standard 20mm above the top clamps. TY250 fork tubes are stiffer in bending, hence they flex less (so the steering feels better in dry rocky riding) TY250 forks when standard have a higher spring rate than standard TY175 forks and alternate springs are available (so they suit heavier riders). Having used both types of forks on my TY175 and weighing 94kg, I would say it is a worthwhile mod for heavier riders on TY175s. ausy300pro is spot-on about the appearance. While later model forks would probably have a better damping action, the TY250 forks do look era-correct on a TY175 or Majesty.

The last bike I saw that had the primary drive gears damaged like yours was a TY175. The kickstart knuckle clamping screw head had punched a piece out of the clutch casing and the piece had fallen inside. Instead of taking the cover off, the owner patched the hole from the outside. Eventually the piece of casing went between the gear mesh of the primary drive, bending the gearbox shaft and knocking pieces off the gear teeth on both gears in the process. My guess with yours is that something solid went through the gear mesh zone and bent the crankshaft in the process. By the number of damaged gear teeth on the driven gear, the rider kept riding it despite what must have been quite a crunchy-sounding motor. Yamahas motors are truly tough.

Maybe the problem with the Venhill cable length has been fixed since Woody bought his cable because I recently bought a Venhill TY250 twinshock clutch cable (through John Cane) and it worked without having to use a spacer (on my standard lever, original clutch plates TY250A).

Standard problem on those hubs. Yes most get sleeved in steel or cast iron. If you are doing it yourself the thing to watch out for is that there is not much thickness available in the hub.

The clutch hand lever might be of a type that has the nipple hole in a different location relative to the holder for the cable outer - levers and mounts are not all the same You did not mention adjusting the actuating arm angle (the arm under the gearbox). It should be at 90 degrees to the cable when all the slack is out of the mechanism - the adjuster is near the sprocket Springs: The springs may be a later type that do not have the same free length the same as originals. Yamaha do that with their spares sometimes. Or they may be sagged due to age. If it was me and it was working fine I would leave it alone but everyone has a different approach to fiddling with their bikes. Some people (me included) run only three standard springs in TY250 twinshock pressure plates.

Nice photos - sad gear teeth on the clutch basket but I have seen worse Back on topic about the shifting, one thing you should check is that the hooks that pull on the drum pins are exactly the same distance from their pins when the shift lever sits where it wants to. To set it up to check this, put the indent plunger back in, fit the shift lever and make sure you have it properly in a gear. Once you have done this you can adjust the eccentric screw inside the centering spring to get the hook distances equal.

If you have used the same fork tube spacing on your C15 as on the Alpina, then a low mount made for Sherpa Ts should fit. Yes, some Alpinas came with high mounted guards, but the sliders are the same casting as Sherpa T sliders. Alpina forks are leading axle and have mudguard mount ears front and back of each side as on Sherpa T forks.

Yes I agree, but why are they more sensitive to pressure than the Michelin X11 tube types we used to use? When we ran the Michelin Tube Type X11 tyres, it didn't seem to matter if the tyre had 4, 3 or 2 psi, it didn't seem to flop around but the IRC goes crazy if the pressure drops a bit.

Yes Gordo the problem with the Dunlops is that the bead slips off its seat at low pressure. Dunlop do not even make tube type rear 803s. Whoever took your order was ripping you off. The only trials competition tube type rears are made by IRC and Michelin. The Michelin tube tyre is not the same (not as good) as the Michelin X11 tubeless type. The IRC tube type is good but requires accurate pressure monitoring to get the best without getting a floppy-feeling rear end. If you want to use that Dunlop on your rim, I have seen people fit four tyre clamps/grips/rim clamps whatever you call them - equispaced on the rim to hold the bead up on its seat. It is becoming more common to see tubeless rims on old bikes.

Tim thankyou for reminding me of some broken magnesium bike parts that should go off nicely next bonfire night

For the person who made a new key from a valve shim - I suspect that valve shims might be either a fancy grade of steel, or surface coated for hardness, or both. Woodruff keys are usually made from relatively easily sheared material (something like 250MPa mild steel), so that if the taper slips, the shearing of the key does not damage the crankshaft or flywheel in the process.

Hey Tim that one is one of my 79 YO Mum's favourite jokes

Stork the piston clearance on your mates large ship engines is not a good example to use because the pistons and cylinders on them are made of materials with matched rates of thermal expansion, so there is no requirement to allow for expansion from cold to hot. The Bultaco motor has cast iron sleeve and aluminium alloy piston which have quite different rates of expansion.

Remember that there is a reason for the clearance between the piston and the bore. It is to allow for the differential in expansion rates of the piston and the bore with temperature rise between ambient and with the motor at full running temperature. For this reason, the diameter of the piston and the materials that the bore and piston are made of will determine the ideal clearance. The piston Bogwheel is using is much bigger than the piston that would be used in a M198 and so the clearance should also be bigger. One point five thou might be OK on a 238 or 250 with a forged piston, but I think it would not be enough for a Bultaco 325 or 340 piston, and even more clearance would be required if it is a high expansion rate alloy piston (like a Wiseco).

I do know that the fork tubes are a different distance apart on these bikes so if you were thinking of using the KT wheel and mudguard it would require some mods. I'm one of the people who ran for a while with a complete TY250 front end on my KT a few years ago and it did make the KT front work very well in tight turns. However, I doubt that fitting TY triple clamps to a KT with KT forks would improve the KT steering. Both bikes have similar trail dimension as standard so by using KT forks together with TY triple clamps, there would be almost zero steering trail.

Not sure if it is a common problem with TY80s, but my TY80 just had a similar problem to yours and it was only a minor issue easily fixed. It was a pin in the shift linkage that slid out of it's hole in the inner case.

If the chain hits the shockie spring while you are riding and it annoys you, you can fit a washer or two behind the shockie eye to space it out a bit. Remember that the original shockies on that bike had the damper bodies at the bottom so gave more clearance for the chain than modern shockies with the bodies at the top (and spring going all the way to the bottom). I find that even 2-3 mm side clearance is enough to stop it making noise. When I run Falcons on my TY250 twinshocks I find that I do need to pack the shockie across about 3mm to give enough clearance. Some people run with less rear sprocket offset than standard to provide more shockie spring clearance, but this is hard on sprockets due to chain misalignment. Yes when the bikes were new there was a rubber chain rubbing block that fitted over the swingarm pivot tube end. They are no longer available from Yamaha but may be available from someone making replica rubber parts. You will find when you get aboard the bike, the chain will move away from the swingarm pivot tube and only touch it briefly when the rear wheel unweights on obstacles, hence why hardly anyone bothers about fitting a rubbing block there. The shockies you have may well be longer than the originals which will make the chain tend to rub more than with the original shockies on full droop of the rear suspension.

Montesa made versions of the 247, 348 and 349 that had bigger seats. In your case it would have been called Cota 247T. Have a search on this website, bikepics or search on google and you will soon find photos.

The airbox looks like Cota 348 to me

All you would have needed to do in 1982 to get those Yamaha TLS front brakes on your 1981 Bultaco was to find someone parting out their brand new YZ490J - no probs hey. By the late 1980s you might have done OK getting a YZJ front end because people might have started modifying their old MX bikes by fitting disc brakes to their MX bikes, or they might have been parting them out - but then again by that time you too could have fitted a disc brake to your Bultaco and wouldn't have wanted the TLS YZ front wheel - that is if you still were riding a 1981 Bultaco in the late 1980s! The main reason people didn't have YZJ TLS front brakes on their old twinshock trials bikes was because they didn't want to spend a squillion $ to improve the front brakes on a bike that would have already been superceded by later model bikes. The secondary reason for not using TLS front brakes on a trials bike is that they work very poorly going backwards.

Hi John. Welcome to the melee and I'd like to ask my own silly question. How old do you need to be to kicked out of Junior Motocross? Regards David Lahey (51 yrs)

Maybe because the YZ490 twin leading shoe front end came out in 1983 which was many years after everyone lost interest in riding KT250s.

For your weight I would suggest 50 pound springs. Yes that length for the Falcons is fine. Yes the top mounting pins are different diameter to the bottom pins.

Either mod will make the steering noticably heavier when riding slowly or stopped. If you want to try it you are welcome to ride my M138 Alpina, or I can lend you a set of Alpina/Pursang/Frontera triple clamps to try on your Sherpa T. Regards David

The Wiseco DT250 pistons are slightly different-loooking to Yamaha TY pistons, but the skirt is the right length, the pin to crown distance is right, the window holes in the rear skirt are the right shape and in the right place. The rings are standard Wiseco (two thin rings that are flat top and bottom) and piston kits are reasonably priced. Bought through SERCO (Brisbane, Australia) While the Wiseco DT250 pistons work well in the TY250 twinshock, for the TY175 I prefer using PRO-X, which are made using the same alloy and manufacturing process (and look the same as) original Yamaha TY175 pistons and are also reasonably priced.