feetupfun

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.

Jay is spot on as usual about riding the OSSA MAR in comparison to others of the era. What he didn't say was about how well the MAR frames were made. I love all the 1970 trials bikes and over the last 11 years have done lots of work fixing up some of my favourites ie TY250B, TY175B, KT250, Cota 348, 1974 MAR and am almost finished an M49 Sherpa. The simple and solid frame design of the MAR is something that stands out nowadays even if I didn't notice it when they were new. If you park an early 1970s Sherpa beside the OSSA and study the frames, you will see they are a world apart in design and construction quality. Later on Sherpa frame designs became more sensible but in that early 1970s era, the OSSA frame was quite advanced in comparison. The TY250 frame design was obviously influenced by the MAR design and is all the better for that. The wonderful styling of the MAR endears them to many riders. Personal opinion is varied here but I would rank the MAR second in the cool looks stakes only topped by the very sexy Cota 348. The MAR does have some unusual weaknesses though. The main ones I can think of are: The rear axle tends to bend in normal use. This can be improved by extending the effective width of the hub using an outrigger bearing on the sprocket side. There is a lot of drag in the rear brake bowden cable and torque rod arrangement. This reduces the sensitivity of the rear brake compared with bikes that use an open brake rod or open cable arrangement. The standard suspension was extremely soft and on todays sections would only suit a very light rider. This of course is very easy to correct.

Is this a trick question?

I suggest a good start would be to post decent photos of both sides, what country you think it was sold new in and the first six digits of the frame/engine number.

Sounds like one or both of them has a bent (sideways) swingarm and/or the rear section of the frame is pushed sideways. This sort of damage happens during a heavy sideways flip type crash. I've straightened quite a few old trials bike frames with this sort of damage. To have a better look at where the bending has occurred you could remove the rear guard, exhaust, airbox, seat and fuel tank and sight the bare frame tubes (the front section of the frame sometimes gets twisted too).

Try selecting neutral just before you come to rest. With no load on the gear dogs, it will shift to neutral much easier than stopped with the engine running and the clutch pulled in.

It is possible that there is runout at the primary drive gear which causes a noise as the crank rotates (primary drive meshing having no clearance at one point). Easily diagnosed by trying it without the clutch fitted.

I found the MITAS was fine except where a Michelin would conform well to the shape of the edges of hard objects, the MITAs would not conform quite as well. I wondered why until I measured the depth of the MITAS carcass and found it had about 1/2" less depth overall than a Michelin/Dunlop (ie lower profile than Michelin/Dunlop). Less depth = shorter contact patch. Grip on dirt, smooth large rocks and large logs was fine with the MITAS. Test bikes were 2003 Rev 3 200 and 1976 Cota 348.

The WES exhaust on a Yamaha TY250 twinshock frame is intended to have a rubber joint between the front pipe and the main exhaust section. If you don't want that, why not weld a steel sleeve the right diameter onto the front pipe so they will slip together? A photo showing the WES exhaust on a TY250 and also a photo of the footpeg mod you are asking about can both be seen on the B&J racing website. http://www.bjracing.com/ Your bottom frame tube has no threaded end because it is an A model TY250 frame. B and later model frames have a thread in the end of that tube which is there to allow fitment of an alternate footpeg kit made by Yamaha for trail riding.

Hi Scott Yes I got your return message but I thought something had gone wrong as it only had what I had written. Mystery solved. Good to hear Hilary rides Trials. My daughter Zara is 7 and still on trainer wheels on her bicycle. She's not allowed to ride her motor bike till she can ride the bicycle properly. See if you can work out how to send "Private Messages" (PM)s via this forum. That way the messages don't get posted on the public forum. To do that just click on the PM button on a forum posting of the person you want it to go to. That will open a new screen and you can type the message and send it. I'll will send you a PM later tonight. David

Twinshock era trials forks usually have "topping" springs that compress just as the forks top out. It is possible to overwhelm these springs by using a lot of preload on the main springs in combination with light oil. Another possible cause may be insufficient oil ie the oil level goes below the damping holes on full extension. The plastic bushes you should find in your monty forks are "bottoming" cones which generate a greatly increased compression damping rate as the forks reach full compression.