feetupfun

If anyone seriously wants a new TY250Z I've heard that there are a few brand new ones left here in Australia that were never able to be sold due to the ridiculously high ticket price at the time.

A friend was practising full lock turns on his trials bike in his front garden and managed to crack his skull on a garden bed edge rock. He fell off at less than walking speed. He recovered OK eventually. You guessed it - no helmet. I have hit my head so hard on rocks landing from a riding fall that my ears were ringing for half an hour and my 1 month old composite fibre helmet was written off due to the depth of the scratches. Does anyone think I could possibly be able to write this posting today if I had been riding that day without a helmet? I don't buy high quality helmets because the insurance company gives us a lower premium, it's just a practical way to control a high risk situation.

If you can get hold of the standard Mikuni carby from a Kawasaki KT250, it fits straight onto the MAR that has spigot mount carby and the jetting is perfect straight from the KT.

An interesting experiment was arranged in the 1970s by Dirt Bike (USA) magazine with a dyno and a two stroke air cooled MX bike run on premix. All the fuels tested used the same petrol (gas) and oil type but they varied the petrol/oil ratio in a series of runs with the ratio going from one extreme to the other. I'm not sure but I think the oil was bel-ray synthetic and the petrol was "regular pump gas" which in 1970s USA meant leaded gasoline bought from a car fuel station. The results surprised quite a few people who had been advocating running as little oil as possible to maximise power. Contrary to this assumption, they found that the bike made more power each time they increased the oil to fuel ratio. I think they started at about 2% oil and stopped at about 7%. There are a few obvious things that prevent us from assuming that this experiment would produce the same results today with our different fuels, oils and bikes. I only brought it up as it may be of interest to people who are also interested in bike fuels. Differences from then to now Our "pump gas" is more variable in density, viscosity, octane rating and sensitivity to shelf age. Our oils are generally better at preventing carbon deposits and smoke. Our oils can maintain good film strength at lower oil ratios and higher temperatures. Our modern liquid cooled off-road two strokes are much less compromised on heat rejection at full power than a 1970s air cooled MX bike (ie a 1970s MX bike engine at full power would get very hot on a dyno even with huge fans blowing on it).

TY250Z forks (45mm) are much larger diameter than pinky forks and have cartridge damping. TY250Z front wheel and brake caliper are interchangable with TY250R (pinky). TY250Z and TY250R front ends would be very difficult to find as the bikes are very long lasting and have remained popular.

Forgot to say that if you had ordered a 493 TY250 or 1N4 TY175 brake arm, you would have got a steel one.

Some model TY250s and TY175s twinshocks had steel brake arms, some had aluminium. I always thought it strange that my TY250D with its special light rims (compared with A and B model TY rims), was the only one I had seen that has the heavy steel brake actuating arms! I haven't seen a twinshock TY made before 1977 with the steel arms so assume that changing to steel was a cost saving measure brought in when sales were slowing down.

Pre-1976 slim tank 247s have drop dead gorgeous styling but a white tanked 349 would probably make a more competitive ride. Is there any good reason for not buying both?

The first M51 Cota 349 usually identified by having a red plastic tank had a swingarm which was longer than the preceeding model M51 Cota (the last 348). The white tanked 349 (model following the red 349) has a swingarm which is the same length as the 348. The 348 does not have a problem with handling or a light front end. It is absolutely brilliant stock standard compared with other bikes of the late 1970s.

Standard is 10 front 33 rear

you 'avin a larf !! Overheating of air-cooled 4 strokes when ridden in the manner of a trial or during trials practice is a common problem here. Some 2 stroke air-cooled trials bikes also overheat in the warmer months. Boiling fuel in the carby and fuel line, pinging, flat spots, fuming engine oil, poor running and even refusal to run are the usual symptoms. One of the best things about riding liquid cooled bikes with thermstatic fans is that overheating is one less thing to have to deal with during an event. PS there are plenty of water cooled 4 stroke enduro bikes without thermo fans that boil the coolant under the same riding conditions.

Caustic soda (sodium hydroxide) solution is very damaging to eyes so make sure you protect yours before you start any mixing and heating. Caustic soda solution will dissolve aluminium (alloy) and zinc within seconds so keep it away from anything that isn't steel, stainless steel, cast iron, PVC or PE. Glass is OK chemically but easily broken.

Once the axle is tightened on a Cota, there is no way that the backing plate can move to allow the shoes to fit the drum better. Like 99.9% of drum braked trials bikes, you should tighten the axle and the torque link with the brake shoes holding the backing plate in the correct position.

If you want trials bars like the Renthals we used in the 1970s, the closest thing I have found are high-rise aluminium alloy trials bars made by AFAM. They have the same rise as the 1970s 6" rise Renthals. The Renthals that are sold today supposedly with a 6" rise have a much lower rise than 6" rise Renthals from the 1970s. I suspect Renthal changed their measurement method somewhere along the line. The AFAM bars come with a brace similar to modern Renthals and which can be removed if desired. They don't look quite as good to my 45 year old eyes with the brace fitted on an old bike, but it certainly prolongs the working life of the bars if you crash a bit or do lots of heavy landings. If you want seriously high rise trials bars, Fred Carter in New Zealand sells lovely replica KT250 handlebars which are a couple of inches higher again than the AFAM bars.

Here in sundrenched Australia, Betas come without the additional flywheel weight that is apparently standard in the UK. No one in my (mostly Beta riders) club even knew such a thing was available until recently. One day during a bike swapping session it was discovered that one of the Rev 3 250s felt much nicer than the others to ride and by process of elimination it was discovered that the cause of the difference was that the one that felt so much better had an additional flywheel weight. Not long afterwards most of the 250 and 270 Betas in the club had the additional flywheel weight fitted and all was well with the world again. We feel that even in our high traction riding conditions the heavier flywheel makes the bigger Rev 3s less demanding to ride. I have just bought a 2003 Rev 3 200 and am even considering adding flywheel mass to it. I suspect that young riders who enjoy the thrill? of quick engine response and have excess energy to burn may not think along the same lines as the "mature" riders in our club.

There are benefits from using the forks from a TY250 twinshock in your Godden Majesty. I tried every possible combination of fork oil weight and quantity, preload variation and front end height, even tried (single rate) springs from a 1974 TM 125 motocrosser in my TY175 forks but was never happy with the action on mid to large obstacles. You could set them up for large obstacles but then they were not very good for small obstacles. Possibly if I was lighter than my 89kg the standard forks may have been OK. I recently fitted TY250B forks with purpose made springs from B&J Racing in the USA to my TY175 and found that I could get the front end working well on large dry boulders and during heavy landings yet was still nice on small stuff. Yes both steering stems and steering head tubes are the same but the TY250 tubes being a larger (34mm) diameter come closer to the fuel tank on the TY175 than the standard forks on full lock. Both sets of triple clamps share the same offset geometry. TY250 clamps are simply TY175 clamps with bigger holes. Aftermarket tapered needle roller bearings for top and bottom are the modern remedy to the standard Yamaha steering head bearings. I use the same brand of tapered needle roller bearings in both a TY175 and TY250B and have found that this allows rapidly front end swaps between bikes if needed (ie no need to move bearings from seats). I don't know about where you live, but forks and triple clamps from TY250 twinshock bikes are readily available from bike wreckers here in OZ. I can post photos if anyone is interested.

I just measured the original Telesco shocks from my 1976 Cota 348. They are 360mm eye to eye. Axle travel to the point where the shock body just touches the bump rubber is 114mm. Axle travel to compressing the bump rubber 10mm is 129mm. You asked what the shockie length "should be". All I know is what works for me. I have 1980's vintage Marzocchi shocks on my 348. They are 352mm eye to eye and still work great. If I needed new shocks for the 348, I would buy Falcon Classic Trials shocks without hesitation.

The two stroke decompressor as on that Cota 247 works by opening only a small hole between the combustion chamber and the outside air. The small hole allows air to flow in and out in a controlled manner dissipating energy just like fork oil moving through the orifices in the damper rods also creates a drag which dissipates energy. A big hole in the head (as in taking the spark plug out) doesn't restrict the flow as much so doesn't provide as much braking effect. On a 4 stroke, the decompressor is there to allow the motor to turn over easier for starting so it works by opening a large hole between the combustion chamber and the exhaust header. In the case of a 4 stroke, an exhaust valve is held open continuously to provide the big hole. Please be aware that in dusty conditions, with the type of decompressor shown on that Cota 247 it is possible for dust to enter the combustion chamber via the decompressor hole (unless it has a filter), increasing ring, bore and piston wear rates.

BillyT You are spot on about the benefits of the pulsing effect on traction in marginal conditions. Modern (2-stroke) trials bike manufacturers design their engines to provide a strong pulsing effect at low RPM to maximise traction in slippery conditions. Being a regular rider of 1970s trials bikes since they were new, one of the things that is very obvious to me when riding a modern trials 2 stroke is the strong pulsing effect at low RPM compared with almost electric motor smoothness of the 1970s 2 stroke trials bikes. The physics is quite simple of why the pulsing helps. A tyre slipping on a muddy rock has less grip than a tyre that has the tread stationary relative to the rock ie static friction is greater than sliding friction. When drive is on the verge of being lost due to low traction, the part of the engine cycle when no drive is occurring (between when the exhaust port opens and when the next power stroke starts) allows the tyre to regain the benefits of static friction in time for the next power pulse. A four stroke engine simply has a stronger version of this effect because it has a longer period between power strokes for the same RPM. The effect is only a benefit when the tyre is on the verge of slipping. Once a tyre is spinning on a slippery rock, there is no benefit from the pulsing effect. The same effect is being used in MotoGP bike engine development. These bikes also operate at the margins of traction during acceleration and are being developed to maximise static tyre friction by optimising the pulsing effect. Another example is ABS braking systems on cars. They are are designed to pulse the braking to maximise tyre friction during emergency braking.

You will find lots of useful info on Jared Bates' site below and a good RHS photo of a 1972 Cota 247 in his photo gallery. http://www.southwestmontesa.com/ There are also many other sites with Cota photos. Have you seen www.todotrial.com It has good info and a good set of links to sites with plenty of photos of perfectly restored Spanish trials bikes. I'm glad you got a pre '76 Cota 247. They certainly are a fantastic looking bike.

It may indeed be the CDI box. If you have another Rev 3 there to use a test parts why don't you swap the CDI box first if you don't have a flywheel puller yet? London to a brick it's the stator coil though. Yes if the timing is right at idle it's not a sheared flywheel locating key.

Good to hear. I totally agree. I love riding my Cota 348 and OSSA MAR because in my eyes they are the coolest looking bikes I have ever seen. I suspect that the bike that someone likes the look of most has something to do with their association with that bike when they were at a certain age. I've found that unless a bike was made between 1972 and 1976, it really doesn't rate in the looks dept for me. That means that I was 13 to 17 years old when the strong impressions were made. How old were you guys when the bikes you love were made?

Well done checking with a strobe. Yes whet you are probably seeing is what MarkyG has suggested. To prove to yourself that it is the stator before you shell out for a new one, can you swap just the rotor with another Rev3 to see if the problem goes away? About half the early Rev3s in my club have suffered this failure so we are geting pretty good at diagnosing it now.

Dear Vintage Cota I can't beat your Cota 123 story because I don't have a bike that old but this story must come close. I discovered after taking the head off my 1976 Cota 348 for it's first ever decoke a few months ago that it still has the crosshatch pattern bore finish put there by the factory. The fork tube surfaces also have the original crosshatch pattern. Those Montesa fork gaiters do a good job protecting the forks. My 1974 Yamaha TY250A has never had the head off and still runs perfectly and is quiet mechanically. I guess one day it will also need a decoke? The speedo read 3000km and was still working when I took it off the bike nine years ago. David Lahey Gladstone Australia