feetupfun

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.

If the backing plate can move after the axle is tightened, there is probably something amiss in the way the axle/forks/spacers/nut assembly is arranged. The backing plate should not be able to move around when you are riding or pulling on the brake. Check that the axle nut is not running out of thread (possibly caused by missing washer, wrong spacers or spacer in backwards or wrong axle or axle in the wrong way around) before it clamps the backing plate between the fork leg and the wheel bearing inner. If the backing plate can move, you will get very poor braking.

Thanks for the story on that bike JayLael. It's interesting about that Hodaka engined bike with the recently made frame being eligible for "classic" class in AHRMA. Are there any limitations on frame construction methods or materials for that AHRMA class?

Pinion puller photos sent by email to Roberto

Hi Scott Welcome to the Australian trials scene. What's this "46 years old!" bit. That is a pretty normal age for someone riding a Twinshock trials bike. Where in Australia are you? David Lahey "47 years old" Gladstone, Queensland

If you are talking about a 348 or 349 Cota, the primary pinion is on a taper and yes it takes quite a bit of force to get it off. Finger type universal pullers are not strong enough. I made a simple tubular puller which worked a treat. If you need help with how to make a puller, let me know.

Are you centring the drum on the shoes as you tighten the axle? Are the wheel bearing outers loose in their seats? Are the wheel bearings worn? Is the axle nut tight enough to hold the backing plate in position? Is the drum friction surface grooved or worn unevenly? Any of these things could give the symptoms you have mentioned (spongy feeling).

That certainly is a pretty unusual altitude to ride a trials bike. You will certainly benefit from reducing your jetting but I have no idea how far having only jetted for a two stroke riding up to 4000 feet ASL. For that I changed the main jet from a 240 to a 180 (Yamaha TY175). There was a fellow who used to post on this site from Indonesia who uses his TY250Z for mountain trail riding in Indonesia at very high altitudes. That being the same motor he may be able to give some guidance. I suggest you post in the Yamaha and maybe Techincal Help forums. It may also be helpful to someone who has jetted a bike for such low density air to know how cold it will be where you will be riding.

The cost of living must be different here. All the talk about how expensive modern trials bikes had me get out the calculator and do some figuring. Based on what a first year apprentice machinist is paid here now ($AU18000 PA) and what I was paid in 1977 for the same job ($AU2900 PA), a new trials bike ($AU1400 Sherpa in 1977 and $AU8500 Beta Rev3 in 2006) both cost 6 months pay. Depreciation rates on trials bikes here are generally $1500 for the first year and then $1000 per year until the bike is about 5 years old by which time bike condition is far more important than the age of the bike.

That frame in the Dirt Bike magazine test may well be a Sammy Miller frame made for a Sherpa and then modified in the US by <mention of this company is not permitted on Trials Central> Specialties to fit the TL250 motor.

The sections for twinshock bikes where I live provide a lower risk of injury with sufficient technical challenge. The twinshock sections are also less tiring which means I can enjoy riding all day.

There is always the option of having a replica frame made but the cost factor is considerable and there are very few people willing to pay. There is lots of labour required to copy a frame and if the production run is only a few dozen frames as it would probably be for people who want to build a TL250 motor Seely replica, you could expect to pay $1500 US to $2000 US for each replica frame.