feetupfun

you said past 20 years, so that is since the Beta Techno and Yamaha TY250Z era Since those bikes Bike mass progressively reduced (approx 15 kg difference to 1995) I think that four pot front brakes in about 2000 was the biggest "step change" in performance. Suspension technology progressively improved - a big difference from 1995, but not a step change Engine response has become progressively faster, not a step change Step change in ease of significantly changing the motor performance characteristics by the owner with introduction of programmable fuel injection with 4RT in 2005 Same for engine ignition map change via handlebar switch being a step change, but probably not important for 99% of riders

Around that time some Montesas had air valves and very light coil springs in the forks. Springing was air+coil. Mine was like that and I found it was an improvement to get some springs that were the right stiffness so I could run without air pressure in the forks. I can't remember for certain what springs I put in but think they were either Sherpa T fork springs or OSSA MAR fork springs. A weakness in those forks is the plastic material that the anti-bottoming cones are made from. It spreads/swells over time and can cause the forks to be sticky on full compression. Replacing the parts with aluminium replicas fixes that forever, or if you just want to try to see if it makes a difference, they can be machined back to size. I find that the damping action of the 348/349 forks is fine with them completely standard, and would not recommend changing the damping action. 15 or 20 WT fork oil works well for me Magical springs and adjusters is one way to get the spring rate and preload right, but do cost $$$

Yes a TK carby was standard on the first model TY250. If it still in good condition, you will find it works very well. You can buy some parts through Yamaha dealers but they are usually very expensive. There are new carbies available if you need to replace it. Keihin, Mikuni, Dell Orto and OKO are popular choices

just turn the sound off

other things that might be related are: People have been known to add extra fork spring preload, which will increase the likelihood of topping out There should be anti-topping springs on the damper rods, which may have been removed

It's not the angle that counts. It is the leverage ratio You need to measure two distances and divide one by the other to get the leverage ratio distance one: distance between swingarm pivot and wheel axle centres distance two: distance from swingarm pivot centre to centre-line of shockie (measured at 90 degrees) If you divide distance one by distance two on a twinshock bike, you will usually get a number between 1 and 1.5 A standard 1960s Bultaco trials bike will have a number close to 1, and a 1980s trials bike will usually have a number near to 1.4 I weigh 95 kg and on my bikes that are near 1, I use 40 pound springs. On my bikes that are around 1.2, I use 50 pound springs and on my 1.4 bike, I use 60 pound springs.

I'm 15 stone and reckon 360mm with 40 pound springs is perfect for Mk2 MAR

The grooves are to improve the sealing of the head-cylinder joint. No O rings or gasket or sealant are needed. You could fit a copper or aluminium gasket if you want to reduce the compression ratio, but it is not needed for sealing. Having the grooves reduces the force required to create a seal.

http://www.mid-atlantictrials.com/OKO.html

Yes lots of piston slap when cold and warm. The modes of piston rattle are different at different RPMs and temperatures which is why the noise changes Are the rings freely moving, or gummed up? The cylinder fins sounded like they were ringing. Are the fin dampers OK? It might just be the way the phone records the sound Possible noise from crankshaft main bearings Could not hear any conrod or primary chain noise

thankyou the_oil_baron

What did you do to the brake pedal arrangement to suit the new peg location?

They are similar enough in design that it depends on how well each bike is set up, as to which one is better to ride

The KT has very cool sounding primary drive gears though

Possible reasons: Maybe not enough people want them. The standard seat is much more comfortable than what you are seeking. It would be quite easy to make a seat like the one in your photo so if someone wanted one they could easily make their own. The TY175 is a far more popular choice for competition, so the market for them would be bigger

Get a new air filter. The old one will be very fragile, and may get sucked into the motor, or be restrictive 25:1 is a lot of oil in the premix If you have put oil down the spark plug hole a few times, it will be very smoky when it starts The crank seals may need to be replaced due to old age May need to disassemble the clutch to free the plates if they don't release when you have a test ride Go for a test ride long enough to fully warm up the motor before you drain the oils

Montesa Cota 247, 348 and 349 also drive the primary gear with a taper

If its sustained riding - like along a road - that you are talking about then the cause might be poor tank tank venting creating a slight vacuum in the fuel tank

John what is that light coloured thing visible through the bearing hole?

TY175 does have a weakness in the design of the return stop peg on the shaft. It can slip around on the shaft. Common problem. If it has slipped, work out where it should sit, move it back and weld or bronze weld it in position. The original design is a press fit that can slip if the kickstart is allowed to slam against the return stop. If that has gone wrong, you are lucky not to have holed the clutch casing Before you start pulling it apart though, a TY175 kickstart that is working correctly engages when the lever is horizontal (about level with the footpeg) They also develop looseness in the knuckle joint which causes the end of the lever to be lower

electronic ignition supplier should tell you what gap to use, but there will be no harm done using 0.4mm. I use 1.0mm plug gap on my bikes that have modern aftermarket electronic ignitions.

A local engine rebuilder here makes replacement crankshaft halves as an alternative to the build-up/grind back technique. Both will work

Probably depends on who does the welding, but the ones I've seen done with welded nuts still work fine.

You can get aluminium nuts welded to the WES and fit the standard heat shield. I think this looks great