feetupfun

Your explanation would make sense if front wheels were run tubeless. The original post was about a front wheel, not a (tubeless) rear wheel

I'm thinking your front wheel might be from one of these https://thumpertalk.com/forums/topic/899705-my-76-yamaha-it400c/

Thanks for the wheel photos. It's a nice-looking wheel. TY front brakes are 110mm as are MX YZ125 B, C, X so your wheel with its 130mm brake drum is from something else. I see your front hub has speedo drive lugs on the opposite side to the backing plate so your backing plate that has no speedo drive may well be the one that originally went with that wheel. My memory is telling me that DT250/400 speedo drive is on the backing plate but I might be wrong. If I'm right then your wheel is probably IT175/250/400/465/490. If you have a close look at the wheel rim you might see a manufacturing date code. I also have a mystery front wheel. It came on a TY175 I bought and I've never worked out what it is from, but it is some sort of off-road Yamaha with 110mm brake drum and alloy rim and heavy spokes. The hub has equal size spoke flanges and looks like TY175/250 except it is painted black and it looks to be the original paint. The date mark on my rim is something like 1981. I'm thinking mine might be from an IT125G but I'm open to suggestions. My wheel works perfectly matched up to a TY175/250 backing plate although it is a bit heavier than a TY wheel due to the heavy spokes and stronger rim. It will be good to hear how you get on with the forks and tyre/mudguard clearance.

I'm pretty sure that fork slider sleeves for those forks would need the sliders machined out to allow them to be fitted because those forks don't have removable bushes. The fork tubes run directly on the bore of the sliders. Your brake backing plate is not DT. If it was DT it would have a speedo drive built in. The backing plate looks like MX B or C or YZ 125 B or C or X to me. It could also be an MX YZ 250/360/400 backing plate. If you measured the OD of the brake shoes it would tell you if it is MX YZ125 or MX YZ 250/360 backing plate. Same for the hub which I still haven't seen. If it is a 125 backing plate the brakes would marry with a TY front hub. Race Tech Gold Valves are sometimes called cartridge emulators and are designed to be fitted to old bike forks and attempt to mimic the way that the damping works in modern forks. Google search on Race Tech and there will be lots of info on their website. I'm always happy to help and have a fabulous birthday tomorrow. It's a great time to be alive

Your fork sliders look like DT250/360/400 B/C. I've posted a link to parts diagram in a separate posting. Race Tech gold valves are fun to set up if you like suspension tuning and if you get it right they work pretty well. The brake drum might be heavier (haven't seen a photo of your front wheel yet). The rim and spokes from MX YZ DT IT might be a little bit heavier. That brake backing plate in the photo will be pretty much the same weight. Most Falcon shocks require the body at the top to work properly. Some Falcons work either way. Should be easy to get that sag with you aboard if you weigh more than 60 kg (but then I haven't seen a photo of your shocks yet). On my bikes with 50 lb springs and 340mm Falcons the clip is nowhere near either end of the range. Something like third groove from the top from memory. There is no real world benefit in using a TY front hub unless what you have is a lot heavier (I haven't seen yours yet). The backing plate needs to match the fork leg or there is more work to do. As for buying TY forks, why not just try what you've got first. You can always change things later if you want. If the chrome is worn through on a fork tube, the sliders will probably be worn out too. Seeing you have a lathe it wouldn't be hard to sleeve the top ends of the sliders back to the right shape/size.

https://www.yamahapartshouse.com/oemparts/a/yam/500418cdf8700209bc785182/front-fork-250b-c-400b-c DT250B DT250C has the two stud clamp

Here is the forks page from the TY250A parts book. The graphic artist who drew the anti-topping spring has drawn it to be about 100mm long when in reality the anti-topping spring is only about 25mm long. I'm talking about the spring that is shown fitted below the piston of the damper rod (item number eight)

I didn't say to buy a trials bike. That was someone else. The TY250 parts book does show two springs but they really only came with one main fork spring. I'll have a look at the parts book tonight and that will remind me of what you are looking at. Many people have said the same thing as you previously about the parts book. Yes TY250 fork springs for 34mm Kayaba forks will fit in 34mm Kayaba MX YZ forks. You would need to make custom length preload spacers. Trivia fact No 1 - you can fit Race Tech Gold Valves designed for MX YZ DT 34mm forks in 34mm TY250 forks. You say your forks are similar but not identical to 1975 YZ250. What do you see that is different? Trivia fact No 2 - if you have a look at photos of some of the early prototype bikes that Mick Andrews rode while developing the TY250 you will see what looks like your front hub and fork sliders in use - straight out of the Yamaha parts bin.

You could use a plank as a balance beam, put the motor at one end and add mass (water in a bucket that has litres marked on it) to the other end. Measure the distances from the fulcrum and multiply the distances by their mass to give you the mass of the motor. Motor mass x distance to fulcrum = mass of water x distance to fulcrum Motor mass = mass of water x distance from bucket to fulcrum / distance from motor to fulcrum

From what I've read, he's got a TY250 twinshock that has the trail riding kit that was available from Yamaha (and popular in the USA) when they were new. It sounds like someone has fitted a YZ 250/360 or MX 250/360 front end from the mid 1970s to the TY.

The writing on your MX YZ forks is not TYB, it's KYB, which is the company that made forks for Yamaha at the time. KYB is the trademark for Kayaba. Yes you will need to have the fork tubes sticking out the top for it to steer properly. Yes it is best to leave the springs out when testing fork travel. Those Falcon shocks are a good length if you are not lowering the footpegs. Original shock length was 325mm. The ground clearance with those Falcon shocks will be fine. Set the sag with you aboard to 40-50mm. If you want more ground clearance you can fit longer shocks but with longer shocks, standard position footpegs will be too high for the bike to handle well with you standing up. 15WT oil in MX YZ forks may be too heavy damping for a trials bike but try it first. It is easy to replace. The MX YZ springs will probably be too stiff for trials riding but try it first and you can fit TY springs later if you need to. The top hat washers should sit on the top ends of the fork springs. Yamaha use that sort of washer so that the cylindrical preload spacer that usually fits above it can be made very thin-walled yet not slip past the end of the spring.

Your triple clamps are TY Yamaha. Your fork sliders are MX or YZ Yamaha and your brake backing plate is MX or YZ Yamaha. The lines on the fork tubes are there as a reference for where to clamp them on the bike that they originally came from. If they are longer tubes than TY250A then they are not TY250B, because all the TY250 twinshocks have the same length fork tubes. I would expect that the fork tubes are also off a MX or YZ Yamaha. To maintain the standard TY steering geometry you should set the tubes so that the axle is in the same position as it would be with TY forks. If the travel of your forks is longer than TY forks you may need to fit travel limiting spacers to the damper rods, to avoid the mudguard or tyre hitting the lower triple clamp.

A very hot exhaust header and poor running can be caused by the ignition timing being set too late

smells nice too

From what you have written it sounds like you have non-standard forks, so to know how high you can have the mudguard above the tyre, you will need to test where the tyre OD is with the forks fully compressed. Standard TY250 forks don't have much room between the tyre and the lower triple clamp and yours with different forks might have even less.

I've not seen a front wheel rim tape glued on. Do you know why it had been glued?

On my SM TY175 pegs, the holes for the pivot pin elongated. I do weigh 95 kg though. I welded on some thick washers and they have been OK since.

reminds me of: Hey. You. Get offa my cloud

If spring steel yields over a long time period when loaded below it's yield stress limit, then yes. Bear in mind that clutch springs only carry slightly more load when the lever is in than when it is out, so it probably doesn't make much difference to how long the springs last storing the bike that way.

Some (not many) MK4 Matador components are shared with Alpina and Frontera. I would say they are no more or less valuable than any other early 1970s Bultaco.

Depending on the price of a new basket, it may be cheaper to pay someone to machine the finger edges flat with a milling machine. That would be about 30 minutes work on a milling machine

Yes I've done it but it was a few years ago so the photos will be a fair way back in the forum. It was the first time mine had been apart. One of the boxes had metal wool packing and the other was mineral fibre. Can't remember which was which. I used mineral fibre in each and made new perforated tubes for one of them. Mine didn't run any better after the repack but was lighter and quieter. I notice that later model Montesa trials bikes of Spanish origin had a much longer header pipe than the 348 which may mean that having the front section packed at the right density where it is in contact with the perforated tubes could be important.

YZ125C and YZ125X had this cover

I just bought a bike that had been sitting for 15 years straight with the clutch lever held in with cable ties because the owner didn't like it to stick. The clutch works perfectly

If they have falcon springs, the rate is usually engraved on one end of each springs, on the flat bit. They usually have just the first digit ie 5 for 50lb, although I have some that have the full number 50 for 50 lb. If there is nothing engraved you can work out the rate using a coil spring calculator of which there are many available free on the internet. Falcon springs are not colour or finish coded. Standard used to be chromed and now standard is powder coated black.