feetupfun

Adjust the pushrod till the cable makes a 90 degree angle with the arm at mid travel

You may have bent the crankshaft while changing the crank seals in which case the knocking is the primary gear mesh running out of clearance and then when the nut jammed in the primary drive its possible that you may have bent the gearbox shaft. You should be able to tell if the crankshaft is bent by taking the spark plug out and turning the motor over by hand, feeling for tight spots. If there are tight spots then the primary drive mesh is running out of clearance. If the tight spot is once per revolution, the crank is bent. If it is once per clutch revolution, then it is the gearbox shaft that is bent and you may even have a combination. Having the nut jam in there may also have deformed some of the teeth on the clutch or on the pinion, which can also make a knock.

US bike magazines often ran trials bikes on a dyno as part of a testing for magazine articles so you may have luck getting torque and power curves by looking for old magazine tests. I remember the Aussie magazines had a thing for it in the 1970s too and one such comparison in Trail and Track mag put both the Suzuki RL250 and the Yamaha TY250A curves on the one chart for comparison. The RL250 motor performance was well above the Yamaha in the mid RPM range and was about the same at top and bottom. I don't actually remember ever seeing a curve for a Cota 348. If you are keen enough, just put yours on a motorbike dynomometer and get it tested. I'm sure we would all love to see the result. David Lahey

Belldane, I think you are mixing up the terms "low down torque" and "torque". Torque is a moment about an axis. Units of torque are force times radius as in newton-metres or foot-pounds. Internal combustion engines produce a range of torque over the operating RPM range. Motors are designed to produce their torque peak (and how quickly it drops off on both sides of the peak) at an RPM range suitable for the purpose. If a motor generates good torque at low RPM, then it is comonly referred to as having good low-down torque. People commonly leave off the "low-down" bit and call it a torquey motor. Power is torque times RPM or newton-metres times radians per second, or Watts. The RPM where peak power is generated is usually higher than the RPM for peak torque. A Cota 348 produces its peak torque about where Wayne said and its peak power at slightly higher RPM. The important performance characteristic for any trials motor is that the torque curve is relatively flat, compared with a racing motor. Motors are commonly described as "peaky" if they have a sharply rising and falling torque curve. The Cota 348 has a lusciously flat torque curve. David

Head aluminium Cylinder aluminium with ferrous liner

And here is what a 348 gearbox breather looks like (small shiny hole). Photo taken fron underneath

I think that a more common carby replacement for the 348 would be to a Mikuni, if only because so many people are already familiar with jetting Mikunis.

By the way, the cream colour of the oil means that it is doing its job of preventing water contacting metal. The properties of the oil cause water to be broken into microscopic droplets (surrounded by oil) so that only oil is in contact with metal parts.

I'm never surprised at what comes out of a second hand bike gearbox. One 348 I bought had forks that didn't move up and down. When I took the caps off I found why - they were completely full of water (the air valves had let water in from rain and the forks were jammed from the steel bits rusting up). As well as that someone had fitted fork spring preload spacers so long that the springs had been coil-binding severely and been ruined in the process (bent springs). 348 and 349 gearboxes do have breathers, but they don't have hoses on them to reach above water level! Have a close look at the rear of the gearbox casing about where the swingarm pivot is and you should see a little hole there which is the breather. Another 348 I bought had that hole sealed up with araldite (and no other way of preventing pressure/vacuum issues within the gearbox)

There probably should be a heat shield in there to protect the fibreglass, but they came standard without a heat shield. My exhaust is in contact with the fibreglass, so I suspect a shield or lagging would have to be quite thin to fit in there.

Stu I expect a set of batteries will probably last 3 to 6 years. The originals that came with our OSET are about 18 months old now. I bought a second set so my kids can ride the bike for longer in one session and it cost about $AU90 for the second set from the local battery shop. They are a common size sealed lead-acid battery, like the batteries you would find in an electric lawnmower, or a domestic security system. David

Do you mean a CT3 or CT2 (as in a 175cc Yamaha trail bike from the early 1970s)? If you do, they would be quite difficult to convert to a trials bike, and the cost of conversion would probably put the eventual cost of the bike well above the cost of buying a cheap trials bike. If you can buy the CT3 for a low price, you could probably on-sell it to a collector for a decent price and make some money. They would be much rarer than a TY175 and probably quite collectable, depending on how complete it is.

If you use 428 chain (which is what came standard on all TY250 twinshocks), the standard rear is 53 teeth. tt5th, I suspect you are talking about sprockets that use 520 chain, which of course have less teeth for the same diameter than sprockets for 428 chain.

Alan, the B, C and D model flywheels have an even bigger external steel band on them than the A model. I've ridden them with and without the band and find that while they are quite zippy with no band fitted, you need to have a very quick clutch hand to avoid stalling when riding serious dry rock sections. I have ended up with steel bands that are about 2/3 the size of the B,C and D model bands (or 3/4 the size of the A model band) on my B and D model motors. David

200 may be OK depending on altitude and if you do road riding or not. When my TY175 was 175cc and was being ridden only in trials sections, I found that a 185 main was just the ticket. If you do a bit of sustained high speed, I wouldn't go much leaner than a 220.

Standard they have a steel band fitted to the OD of the magneto flywheel. Are you saying that yours has a weight fitted to the face of the magneto flywheel as well as the band on the OD?

I'm not saying it is a blocked pilot jet, but to answer your question, a bike with a partially blocked pilot jet may run and start OK when warm, but not start cold, because motors need a richer fuel/air ratio to run when the metal parts are cold than they do when warm. If you have the MK2 AMAL on yours, check both idle mixture routes where they exit the body. One hole is under the rear edge of the slide and the other is about 5mm further along towards the motor and is a very fine hole. Why is using the choke a problem for cold starts?

The Scorpa SY250 is also makes a great learners trials bike. Of the three you mentioned, the Honda is the most dated design but is still a robust reliable bike and perfectly capable in competition at beginner and clubman level. Where I live none of them, even the SY250 long ride, is road registerable. Very few people want rego either because even with a long-ride type seat they are very uncomfortable to ride sitting down, and competition tyres wear out in a flash on the road.

Alan yes I have tried ATF in TY250 twinshock forks and while it functions well (good lubrication and doesn't foam), I find the damping is too light for my weight and riding style. I use either 15 or 20 wt fork oil in TY250 twinshock forks, depending on what bike they are on. I would have put ATF somewhere between 5 wt and 10 wt as far as damping goes. David

I haven't tried this yet myself, but if you are serious about fork performance, you could fit a set of cartridge emulators. There are a couple of manufacturers that make them the right size for those forks and many are now in use in 1970s MX bikes due to the performance benefit. I have not heard of them being used in trials bikes yet but there is no reason why they wouldn't provide a better action if set up properly. In case you haven't heard of them, they fit between the tops of the damper rods and the springs and take over the control of oil movement from the damper rod holes, which are enlarged as part of the installation. They use a shim stack to control compression damping and fixed holes for the rebound damping. I would love to hear if someone has tried them in an old trials bike. If you need stiffer-than-standard Yamaha springs for the TY250 forks, they are available from B&J Racing in Tennessee.

I'm with tony283 on this - you really should set up your fork oil height to suit your weight and riding style, rather than the manufacturers specified volume. To ensure the forks behave consistently from one oil change to the next, I only ever use the measured height method. In the case of both types of TY250 twinshock forks and my weight and riding style, I set the oil height to 125mm from the top when fully compressed. I'm not recommending you use the following figures, but here are the Yamaha numbers you asked for: TY250A (holes at side) 175cc of SAE 10W30 Motor Oil SE TY250B,C,D (holes at rear) 162cc (5.5 oz) 10, 20 or 30 wt Fork Oil David

Which TY250 twinshock forks are they? The location of the drain holes differentiates A model sliders from later models and I think there was also another internal change between B model and C/D models.

And here is a standard LH peg on a different M49

I just read your post again. Here is what the standard pegs look like.

Here is what my series 1 M49 brake lever looks like. ID number is M4900100. This bike has been fitted with folding pegs to meet safety standards in competition, but it came standard with the fixed footpegs. I'm assuming you know what the fixed pegs look like. If not I can post a photo showing the original pegs.