feetupfun

Hi BJ downunder. What you have pointed out about the difference in the finning design of 1970s two and four strokes is interesting but I'm not sure that it means that the two stroke has more problems keeping cool on a hot day (which we certainly get plenty of here down under). The main reason that 1970s Honda XL250 engines have less finning than air cooled two strokes of the same era is that it is all to do with surface area available for heat transfer. Because of the location of the cam, rocker gear and cam chain case inside the 1970s XL250 Honda engine, the external surface area of the engine available for heat transfer ends up almost large enough even without fins - hence stubby finning. In the case of a two stroke motor, the surface area of the head and cylinder without fins would be way too small. Hence a two stroke of the same power output as the XL250 would need more fin area. In reality the comparison between these two particular engines is problematic because the H6360 Monty motor would probably be able to produce about twice the power of a standard XL250 so there is a need for additional finning by comparison for that cause as well (waste heat produced increases with increased power output). Remember that the sizing of the cooling surface area is not only designed to prevent overheating, but also to prevent the engine getting too COLD in service.

My Cota 348 came with air valves on the fork caps. They are intended to be used as a tuning tool for fork action. The standard steel springs inside are a bit soft on their own. About 3psi with the forks topped out works fine for me as far as spring rate goes, but it does cause a bit of additional friction at the fork seals. If I could buy stiffer steel springs and run without the air pressure I would. Unfortunately I haven't found a replacement yet. BTW is your Cota really a 349 or is it a 348? They are quite different in the frame, engine, triple clamps and seat/tank. Where I live the 348 was sold 1976 to 1978 and the first red 349 came out in 1979. Keep on trying to get the front brakes to work well. Its worth it. The 348 has very good brakes when set up properly.

The measured weight of my Cota 348 ready to ride trials (no lights and half a tank of fuel) is 93kg. Others in the same stable also ready to ride are: TY250B 93kg KT250A2 94kg TY175B 83kg I would have hoped that a bike made 10 years after these bikes (the TLR250) would be lighter than a Cota 348.

Yes the Barnett fibre plates work great in Bultaco Sherpas. The takeup becomes silky smooth and the clutch spring preload can be backed off to provide a lighter pull yet retain the required resistance to slipping (they have a higher coefficient of friction than steel on steel). I just wish Barnett made them for Montesas as the sudden and jerky takeup is the main weakness with my 348 Cota in competition.

Honda RS is spot on as usual. Some people use loctite on the damper rod screws so an impact driver may not unscrew them if that is the case. The top end of the damper rod on the TY175 is definitely different to on the TY250 twinshock. From memory the TY175 damper rod top has a circular tip approx 12mm diameter with two flats milled off. A piece of steel tube flattened on two sides will do the job holding the damper rod and if you use a good light and look with the forks bottomed you will be able to see exactly what is needed. Paul McLeod's technique also sounds good for loctited screws and is probably less work than making a special tube spanner.

Just use a rattle gun (air driven impact socket driver) on those screws. It works great.

If your bike is revving its tits off and the kill switch won't stop the engine and you can't stop it by changing up gears and using the rear brake, inverting the whole bike (ie with it resting on top of handlebars and rear guard) will stop it very quickly. It works by lifting the main jet out of the fuel in the float bowl. This procedure has saved three trials bike engines that were sure to either seize or throw the rod but I was able to invert them in time. However there are a few risks: Starting a fire from any fuel that leaks out before the motor stops. Getting caught up in the chain or back wheel. Upsetting the owner by doing something unexpected to their pride and joy at a time of high stress.

Bruce Smith aged approx 19 years, member of my local trials club, accomplished trials rider, on his 1976 Sherpa riding along a 4wd track while out practising trials in 1978. He rode into the front of a bullbar-equipped 4WD at a relatively low speed and like 99% of trials riders at the time was not wearing a helmet. Bruce died due to the head injuries he sustained. I'm sure others know similar stories.

The main reason that TY250Zs were a sales failure here was the pricing. A 1994 TY250Z was $10,500AUS in 1994. At the same time the first model Beta Techno was $6,500AUS. The Yamaha did not seem to be a more competitive bike overall than the Techno despite some brilliant components. No wonder that only a few were taken up by privateers.

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.