feetupfun

I don't think the drive side crank seal is gone, but if you really want to get that drive gear off, you will benefit from a very strong puller that doesn't damage the gear teeth, and cycle it hot and cold while it is loaded up. If you search in the forums you will find many discussions on the subject of 1970s Montesa Cota drive pinion removal. Black oil dribbling out the exhaust is definitely not a leaking crank seal. It is just part-burned two-stroke oil. If you do more than just tiddling around on it you will burn more of the premix oil and lessen the goo production rate and might even clean some of the goo out. Also make sure you don't have too much oil in the premix or a too-rich fuel air mixture Gearbox oil leaking into the crankcase forms a dense white/light grey cloud of exhaust smoke which stinks and the oil level goes down Normal exhaust smoke is clear pale blue and smells like normal two-stroke exhaust

Please let us know how you go. I plan on having an M198 rebored soon

The (very high) standard gearing on the TY125 may have been chosen to help meet noise level standards rather than be ideal for road performance. It was common practice in the 1970s

12/53 is what I use. 13/53 is OK too if you don't mind using the clutch here and there

For all the years that Scorpa trials bikes had Yamaha motors, Yamaha badged them as Yamahas in Japan, so it is nothing new

They can run backwards, just like many other two strokes can. The tendency to run backwards can be almost eliminated if you set the spark timing correctly and get used to using the unusual kickstart. Yes it is a common occurrence for the 348 Cota to shift poorly when the motor is stopped. Mine won't shift down gears at all unless the motor is running (ie rocking the bike to move the gearbox output shaft does not help). They also have a tendency to drop out of second gear if it is not properly engaged

castor oil exhaust smells great and is good for racing two stroke engines that get worked hard and get rebuilt frequently, but in trials engines it has a tendency to cause the rings to stick in the grooves, and will cause carbon deposits to build more quickly than mineral and synthetic oils do, and the deposits from castor oil are much harder to remove. Castor is fine and works well if you don't mind the reduced intervals between top end work sessions on your engine. One of my engines (TY250) has not had the head off in it's 42 year life and the top end is still working beautifully so there is something to be said for using oils that don't cause problems

I noticed something funny when using a timing strobe light triggered by the HT lead. On one bike I was checking the timing (a DT175 with electronic ignition) and found it had a pulse in the HT lead at a few other positions (as well as the usual one that fires the spark plug) strong enough to trigger the strobe That could be why your tacho is being tricked

You probably need to check out the shift mechanism that is behind the clutch and see what has gone wrong when you tried to get the shift shaft to slide out. You can loosen the clutch nut using a folded rag jammed in the primary drive gear mesh point Probably a stupid question, but did you put the circlip back on the shift shaft?

http://www.ebay.com.au/itm/1974-YAMAHA-TY250-OEM-12-1-2-INCH-REAR-SHOCKS-BACK-SUSPENSION-ABSORBERS-AHMRA-/331752144706?hash=item4d3df7ff42:g:QVMAAOSwZG9WlBZg

If you want it to look like it has original chromed TY250 shocks, there are shocks available for that and they are quite cheap, but they are terrible for riding the bike with. the springs are way too stiff and the damping is very poor action If you want "non-gas" shocks for riding trials, they are available still being made by Betor, but are 1970s technology so the action is not as good as modern design shocks If you want it to ride as good as it can, you need to get modern design trials shocks, and all the ones I know that have a great action are pressurised with gas If you really want to understand why the gas is there, you could study the historical development of hydraulic suspension

A definitive test for the crank seal letting oil in is to look for a reduction in oil contents in the gearbox with use It's normal to have oil on plug threads It's normal to have all sorts of gunk, yes just like that, come out the end of a 2 stroke exhaust

You have to treat trials tyres differently. If the rear starts to spin, you have to back off the throttle to get grip back. A spinning trials tyre=useless, which is quite different to a spinning knobby. It's a common mistake for people moving from enduro bikes to trials bikes The rear tyre grip thing is why trials bikes engines have more flywheel effect built in than MX or enduro bike engines

http://www.yamahaty.com/english/tyusineen/milleren.html

Photos please

Can you post a photo of the forks so it can be determined if they are the original forks?

I can weigh a GasGas 50

Good philosophical question there darms, but nope, I love those modern tyres and get great pleasure riding my old bikes with modern tyres and modern shockies, and pleasure is what I ride for

A few years ago Yamaha stopped people from selling retro Yamaha clothing and recently, Yamaha released their own retro Yamaha clothing range to celebrate an anniversary.

The hot setup for these bikes is to fit the whole front end from a MT125 or CR125. The fuel tank would need to be reshaped to clear the fork tubes with this change, but it makes the front end suspension and steering much nicer. Another alternative would be to try and find a set of forks from one of the little Honda trail bikes from the same era that had the same diameter fork tubes (SL125, SL100, XL125) and use the springs out of them

Most people increase the preload a bit and use heavier oil. If you still want different springs, what size (outside diameter of coils) are your TL125 springs and how long are they?

Is this a wind-up?

about 25mm on a TY175 frame, if you make the bend at the front engine mount. If you are not confident, just draw a scale diagram

What was the issue that stopped you using a 69mm KDX220 piston?

have a look at the bike that the front end came off and you need to have less rake than it has