feetupfun

Shockie spring preload is adjustable over a wide range and you set it to give about 50% sag with full rider weight on footpegs. If you have the correct rate shockie springs, after setting the preload, there will be a few mm of sag with no rider aboard. TY175 axle plates are mild steel

Yes springs are pounds force per inch of overall length change. If you use standard shock mount and swingarm length (approx 4 inch axle travel), use 40 pound springs. If increased wheel travel (5 to 6 inch axle travel) suspension design use 50 pound springs. Oil level is with springs out and fully bottomed. The last bit of travel when bottoming is very slow due to the hydraulic anti bottoming, so keep pushing until you feel metal-to-metal I did already suggest shockie length of either 340mm or 360mm, depending on what steering head angle and shock mounting arrangement you want to use. If it was me and using TY250 forks, I would use 340mm shocks, retain the standard shock mounts and extend the swingarm behind the lower shock mount. If you want more ground clearance at the expense of steering quality, use TY175 forks fitted flush with the top clamp and fit 360mm shocks.

There is no cover from another Yamaha model that fits the TY250 because the TY250 motor has a unique engine inner casing design. I have a theory that the TY250 design was arrived at to have the kickstart shaft as high as possible, to allow for a decent arc before the kickstart lever hits the footpeg. The other Yamaha 250 dirt bike models at the time (which were designed years before they started thinking about making a trials bike) had their footpegs much further forward and lower down relative to the motor. Having a high kickstart shaft meant they could have a reliable and long-lasting kickstart lever and knuckle arrangement on the TY250 that greatly outlasts the TY175 (compromise) design

I think you would sell plenty if they were $200. I just paid a lot more than that each for two NOS YZ125 covers

the hose nipples under the words AMAL are the bowl vents. Some people join them and leave a slit in the hose, like yours was. It's also OK to fit a short hose on each one. The layout of the vent hoses ideally minimises the amount of petrol that flows out of the bowl if the bike takes a tumble, and prevents water getting into the bowl via the vents. Yes you are correct about the two screws on the side The starting lever handle has been bent downwards maybe to clear the frame brace, or to make it easier to reach The needle jet and needle wear out extremely fast, so if it seems to have poor jetting that would be a good thing to have a look at replacing.

It is fairly sticky stuff so maybe

I'm about 210 pounds and found that standard TY250 forks had a pretty much perfect action on a TY175 after slightly reducing the spring preload (5mm to 10mm less) and using 5wt fork oil set at 125mm from the top. I find the standard damper rod oil hole setup works very well for me. If you don't mind the expense you can buy aluminium damper rods and a lightweight front axle which will reduce unsprung mass. I run Gold Valves in one set of TY250 twinshock forks and after proper setup the action feels a bit better in rocky streams and no better riding anything else. While it was a fun experiment, I'm not planning to do it on any of my other bikes with TY250 forks. If you want more fork travel for some reason you can leave out the anti-topping springs, but you may get some topping out on steep ascents. I leave the anti-topping springs in all of mine because I find that the standard fork travel keeps the steering geometry just how I like it. Same thoughts apply to the length of the damper rods. TY250 forks are shorter than TY175 forks so if you want to preserve the standard height at the front, they need to be flush in the top clamp to be the same as TY175 forks with 20mm protruding. If you fiddle with the height of the rear end you will probably need to experiment where you have the tubes in the clamps to get the steering sweet. Where you move the top shock mounts to depends on the steering head angle you want. 340mm is good with standard mounts if you want a modest rear end lift, and if you relocate the top mounts to the frame upright you can please yourself how much rear end lift you get by choosing where (fore and aft) you put the mount relative to the frame upright. 360mm shocks raise the rear end quite a lot using standard mounts. Some people like that effect. Using 360mm shocks combined with standard lower mount and top mount relocated to the frame upright will give too much rear end lift. You can move the rear frame upright forward to correct for this, but because 340mm and 360mm shocks both have 4" shaft travel, there is nothing to gain except mass by using 360mm shocks if you are going to move the top mount. A rear end mod that is popular for TY175s is to lengthen the swingarm behind the bottom shock mount by lengthening the axle plates. This increases rear wheel suspension travel and lengthens the wheel base. Swingarm length is usually increased by 30mm ~ 45mm. If you do this mod, 50 pound springs. There are no decent trials twin shocks made in the USA. There are a few good brands to choose from, all made in the UK. I use Falcon Trials Classics which I think are imported into the USA by B&J Racing, Tennessee, A bigger carby is no better for power unless you are also hotting up or enlarging the motor. Standard is a 22mm Yamaha-Mikuni. A pre-jetted OKO would be a good thing if you want a modern design carby. Google mid-atlantic vintage trials for info on pre jetted OKO carbies for twinshock era trials bikes. Need more info about what the problem is with the clutch. It could be notched basket fingers, cratered cam surface, shiny plates, bent plates, wrong oil, water in oil, old friction plates, cheap friction plates, uneven spring length, wear in the basket bushing, axial movement of the gearbox shaft. They have a pretty good clutch action compared to other bikes of the era when it is working properly.

You can still buy genuine Yamaha fuel taps if you want a top quality item. The mounting thread on TY tanks is incredibly popular so there are a vast array of aftermarket (made in China) fuel taps that will fit and function but as with anything you take a chance. They are so cheap so why not buy two and have a spare, or buy two different types and keep one as a spare

Standard TY250A frame swingarm and forks. 340mm shocks fully extended. Forks extended to where the anti-topping springs engage Axle centre to axle centre 51 inches with rear axle about halfway along the slot Front axle centre to front motor mount bolt centre 21.75 inches (1' 9 3/4")

Well I know that they didn't change the steering angle from go to whoa and will go and measure soon

New aluminium under tanks are available for the slimline Bultaco tank

UK Montsas usually have an aluminium tank inside a fibreglass cover. I'm sure you could get one of the aluminium tanks and use your tank to make the cover from. Sorry I don't know the road-legal status of fibreglass tanks in the UK in 2017

Keep the spokes tensioned nicely with that hub because they are known to crack around the spoke flange if the spokes are not kept tensioned properly. I'm not sure if it is the Honda hub from that photo, but the later model Montesas had them and it looks like one from what you can see.

The bashplate is not the original one and doesn't fit very well. I wouldn't worry much that it is not working as a stressed member with those rear mounting clips like that. The motor castings are easily strong enough to act as the frame member as long as the engine mount bolts are kept tensioned and you are not planning to ride at world trials championship level or race it over the jumps on a modern MX track.

Front wheel looks like a later model Montesa Cota that has a Honda hub (from after Honda bought part ownership of Montesa~1985 maybe)

Carby and forks and frame look right for a 1981 349. Front wheel having a gold rim means it is from a later bike. I'll have another look at it

I thought the fork sliders changed too

Probably better to measure the distance between the axle centres with the steering straight, because some people move the mounting points for the rear shocks

434 -00*** is A model (1973/74) The frame geometry on twinshock TY250s was unchanged through all the models. I would measure one of mine that I know is completely standard except I can't work out what you are calling the lower front shock mount

Test for it being a "too-lean" issue by next time it won't restart when hot, engage the starting circuit (the "choke" lever). This will help it to hot-start better if it is a little bit lean on the pilot jet. Similarly kick it with throttle fully open and "choke" off to test if it is flooding the engine somehow when stopped. Another thing to do is to fit a clear fuel line or a clear in-line fuel filter so you can see if there is a problem happening with fuel supply to the carby bowl when it is hot and stopped. Beware that sometimes a float valve will work fine while the motor is running but stick open or shut when there is no vibration. Another thing is that a leak at the ignition side crank seal will not reveal itself as smoky exhaust

My favourite is someone saying their TY250 is totally original and never been worked on, and the aftermarket replica front mudguard is fitted backwards

Do the papers have to be for a MH200 that is registered in Germany or would rego papers from another country be OK?

Do you mean you want identification papers as a way to prove the age of the bike?

Manuals for these are available from http://www.southwestmontesa.com/manuals.html

Have the drive dogs on the gears ground to help stop it jumping out of gear (if they are worn)