feetupfun

The leverage ratio on twinshock type suspension never stays exactly the same throughout the travel. The mount locations on standard bikes are usually well chosen to minimise the variation in leverage ratio throughout the travel. When people modify only one mount location on a standard bike, the new arrangement can produce the desired increase in wheel travel but becomes less ideal as far as leverage variation is concerned. Laying down shockies by moving only the top mount forwards (or similarly by moving only the bottom mount forwards) usually causes the opposite of rising rate ie the leverage ratio on the shockies increases as the swingarm moves upwards.

Following on from the previous post about leverage, here is how to calculate how much you are changing the leverage if moving one or both of the mounting points or the compressed or extended length of the shockie or the length of the swingarm. First calculate the existing leverage ratio: Measure swingarm pivot to axle centre. (distance 1) At half travel position, measure from swingarm pivot centre to centre line of shockie at 90 degrees to shockie centreline. (distance 2) Divide distance 1 by distance 2. You should get a ratio of about 1.05 for a standard TY175 (Ratio 1) Now calculate the leverage ratio for the proposed changes: Mock up the proposed changes and do a similar set of measurements. If you move the top mount of standard length TY175 shockies to halfway down the plastic sidecover with no other changes, the leverage ratio will be something like 1.25 (Ratio 2) Now you can work out how much extra loading you will be applying to the shockies by the proposed changes. Divide Ratio 2 by ratio 1. (1.25 divided by 1.05 = 1.19) ie the new loading will be 119% of standard ie a 19% increase. Jay is telling you to make sure a suitable steering head angle is retained when you start fiddling around with the height of the rear of the bike (changing the height of the front or the rear of a bike changes the steering head angle). It is usually possible to achieve the ideal steering head angle when you do shockie mods without doing any mods to the front part of the frame.

You should expect about 30km/3 hours range gentle trail riding on a late 1990s trials bike with standard fuel tank in mountainous terrain.

I'm one of the people with a 250 MAR (MK II) fitted with a standard KT250 carby and it is still working very well. A riding buddy of mine has a Mk I MAR fitted with the standard Mikuni carby from an early 1970s TM125 Suzuki MX bike and it runs well too.

Yes the KT is not as easy to ride in sections as the more popular mid-1970s bikes however there are some redeeming features: Lots of people have never seen one and want to know more about them when they see one. They have great styling. Trying to agree on which design concepts were stolen from the other brands is a great discussion topic. If you are a bit of a gumby rider, riding a KT is a good excuse to use for your high score. The front brake arm is an amazingly distorted piece of steel and is way over-designed (another good discussion topic) They are a terrific trailbike - even better than a TY or a TL250. The straight cut primary drive gears make the coolest whirring/whining noise under load. With a few changes to the suspension, they really aren't too bad to ride in sections. Don Smith was involved with the development and I reckon that's pretty cool.

Yes you can buy decent shockies for TY175s in Australia Don Newell (07 33531874) in Brisbane imports Falcon shocks and usually has the right size in stock for TY175s. Peter Schoene in Victoria (03 51489191) (Bultaco Parts Australia) can supply the right Betors for the TY175. I suggest if you want to contact the Sammy Miller shop that you phone them rather than email. David

Scotty if you are having problems with the tiny standard Yamaha footpegs but don't want to spoil the original look of the frame, you can extend the standard footpegs to make them more comfortable. Have a look at the B&J Racing website. They sell kits for exactly this purpose and even offer a service to do the welding for you. The TY mono pegs aren't much bigger than the TY twinshock pegs. Why bother with them? That bit about using plates to use standard modern footpegs is really the best option. I have done that on my TY250B and the difference in comfort and balance is astounding. It looks fine if the plates are neatly made and colour match the frame. Email or PM me with your email address if you want photos. What is the problem with welding plugs into the bottom tube anyway?

My guess is that it is not likely to be interchangable. The TY250N-R motor is a different series to the motor used in the Twinshock TY250A-E.

If you are going to try the rocking technique, use top gear. It puts the highest torque on the clutch.

Well spotted. Great footage. Thanks

Copemech The Monty 348 and 349 do indeed use shimming on the top end of the conrod to keep the conrod in the middle of the big end. Not all bikes are designed the same. Vive la difference. Roberto I don't know what the minimum should be but mine seems to go fine with about 0.5mm clearance with a standard conrod. Note that some aftermarket conrods for 348s and 349s are wider in the middle section and may need very careful setting up (and maybe machining of the crank halves) to make sure the rod misses the crank wheels. David

Yes. It's a good one isn't it? Maybe Nick already knows where stump magnet lives.

What do the magneto cover dowel holes and kickstart shaft look like? If they are bent out of shape it might have been a crash into a solid object that did it. Otherwise I can't see the fracture surface well enough in the photo to see if it was fatigue failure or not. Typically 348 frames crack just under the steering head if the bike is ridden very heavily. There was a factory recall that added gussets at the steering head to the early model 348s. Later ones had the gusseting from new. I have three 348 frames and none of them show evidence of cracking or crack repair work there. Apart from that issue I reckon they are a great frame.

I bought that WISECO piston kit from my local (Gladstone, Queensland) Yamaha dealer. I'm sure any bike shop worth it's salt in OZ could get WISECO piston kits. An alternative for you would be to buy one from the US (say from B&J Racing). For us here in OZ the total cost of some items from the US is quite competitive with local supply due to the low freight rates from the US.

I used a 0,5mm oversize Wiseco in a rebore 3 years ago of my TY250 twinshock and the piston has been nice and quiet mechanically at all RPM ever since. I haven't even bothered to fit the rubber blocks to the fins.

Front sprocket 1 If you want to use 520 chain, the front sprocket from some Honda and Kawasaki MX/trail bikes will fit. Have a look at the JT website for spline dimensions of front sprockets and you will find plenty of matches for the KT splines. 2 PBI sprockets in the US make a 13T 428 chain sprocket that will fit the KT but according to their website won't sell outside the US. Part number 535-13 C/S. Rear sprocket Get a flat blank sprocket machined to the correct ID and hole pattern and have a spacer ring fitted to achieve the right offset. Use countersunk head screws to provide clearance for the swingarm. As far as I know no-one makes direct rear sprocket replacements. Fred Carter in New Zealand can supply lots of replica KT parts but has not been able to make dished rear sprockets yet. I hope someone does start making them for the next time I need one as they look much better than a flat one.

Seeing that you are asking about the clutch springs, I am guessing you have not actually looked at the M199 clutch yet. The spring preload should be adjustable on that model Sherpa. Each of the six springs has a nut which should be be positioned to ensure that the plates move apart in parallel as the clutch is disengaged. The nuts are also used to adjust the clutch spring preload. When you assemble a Bultaco trials clutch, a good starting point for getting the clutch spring preload correct is to adjust the springs until the (pre-lubricated) clutch is able to provide enough drive to allow the kickstart to turn the engine over compression without slipping.

Barnett clutch plates are available through motorcycle shops around the world. They are made in USA. Use standard Bultaco Sherpa clutch springs. Stiffer clutch springs are also available from Barnett if you have a hotted-up 360 Pursang with road-race gearing that is having problems with clutch slip in top gear.

Maybe there was something good done with Honda plates back then in Bultacos but the hot setup now is to use a Barnett friction plate kit in Bultacos. This allows the spring preload to be reduced, lightening the lever pull. Your mate may have heard of mods to some Montesa Cota clutches that used Honda friction plates to improve the action.

Fibreglass (polyester) resin is resistant to the petrol that was around in the 1970s. Some modern petrol intended for use in cars contains ethanol which can damage fibreglass resin. There are a few proprietary products for lining fuel tanks some of which are intended for lining rusty steel tanks rather than fibreglass tanks. Take care to choose the correct product. I recently used brushing grade epoxy resin (from a ships chandlers) as a tank liner for a Sherpa fibreglass tank and it worked very well.

Glenn, I have early 1980s Marzocchi shocks on my Cota 348 and have had great success using standard industrial oil seals in them costing a few dollars each from the local bearing shop. I have tried steel bodied 340mm Classic Falcons on the same bike to see how much better they were. I tried 50 and 60 pound springs on them. Yes the Falcons do have a slightly better action but the difference is pretty marginal. David

All this fuss about people making specials for Twinshock is a bit overblown. Some people like taking the hotrod route with their hobby and get great enjoyment making their bikes different, personalised and sometimes better than they were originally. Riders from the era can usually spot the differences made to a hotrod or home-made Twinshock and it does lend a hightened level of interest to the class which would not be there if we all rode standard bikes of the era. I've noticed that the sort of person who gets their kicks this way is not usually seen on the winners podium - maybe because they spend so much time modifying their bikes that they never get to practice riding. Have a think about who usually wins at a high level in your area and check out their bikes. Where I ride the winners are usually on very standard looking bikes of the type that were also (co-incidentally?) the best bikes to ride in their era. Just thinking about some of the top level twinshock riders where I ride brings up: Majesty 250, SWM 320, Fantic 240, Montesa 242, Montesa 348 and 349, TY175, TY250, Bultaco M198/199A and B. All of these are very standard bikes. There are also a contingent of hotrodded twinshocks which are greatly enjoyed by their riders but rarely win top level events. Maybe if an outrageously modified bike started winning there would be a protest but it just doesn't seem to happen here.

Why are you so sure it is an electrical problem? If it is 4 stroking, it may be the fuel/air mixture is too rich. There are many things that can go wrong to cause an intermittent rich mixture. Helping to diagnose the problem needs more info. Under what riding conditions does the 4 stroking happen ie throttle position, bike attitude, engine warm or cold, RPM sensitive, during accelleration or constant speed, anything coming out the float bowl overflow, does it go better after you turn the fuel tap off?

Q1 Is the rear suspension moving freely? (maybe there is a tight spot) Q2 Is there less rear suspension sag since the work was done? (maybe the spring preload was adjusted by the person changing the shock bush)

The TY250 number prefix changed to 493 starting with the 1976 model which was called TY250C in Australia. The paint schemes for the Aussie, US and Canadian TY250C and TY250D were different to the UK, German, Danish and French bikes so being an Aussie I can't tell from the photos what year yours would have been sold new. Even though they have different paint and stickers on the tank, side cover and exhaust heat shield, as far as I can tell, all the 493 prefix bikes around the world were mechanically the same.