feetupfun

The timing will depend on what the advance curve for that igniton looks like and what you are looking for. I fitted an electronic ignition (not the same as yours) because I had just converted my TY175 to 210cc. With the 210cc, the motor response was a bit too fast for me at low RPM. The fixed timing (points ignition) was a compromise between low RPM and high RPM performance. With the electronic ignition with an advance curve, I could get just the right timing at low RPM to get rid of the snappiness and still not compromise the high RPM performance. By the time I was happy, the timing was a bit less advanced at low RPM, maybe 5mm less advanced on the OD of the flywheel compared to standard timing. I would suggest you put yours in the middle of the slot and try it out. If you like experimenting (it sounds like you do) then try different stator plate positions and see which you like the best. A strobe will provide visual feedback for where the timing is, but it will just be moving by the same amount in degrees as you move the stator plate.

Shedworks (UK) for a new fuel tank

With electronic ignition, there is no way to determine the timing without a strobe and the motor running. Yes you have to take the flywheel off to adjust the stator position.

Yes your magnesium MX/YZ/IT backing plate is cool and most importantly the brake torque slot matches the lug on your fork leg. I get my brake shoes relined with oversize thickness linings and then mount the shoes on a backing plate and machine the linings to suit the diameter of the drum they are going into, using a lathe. Most of the brake drums in my trials bike hubs have either been remachined at least once or are worn and need remachining so buying brake shoes with standard linings is not an option. Racetech do make gold valves to suit 34mm DT forks, but they don't list them as such in their on-line catalogue, nor do they list gold valves for 34mm TY250 forks. I bought the gold valves they list for 34mm YZ MX forks and used them on TY250 forks. The Racetech website explains where they sit and how they work. You do need to machine the end of the damper rods and drill extra holes in the damper rods to fit gold valves. Then buy much stiffer fork springs and spend about 20 hours getting the setup working well.

TY twinshock front backing plate vs MX YZ 125 backing plate. Both use 110mm brake shoes

This brake plate came with the mystery wheel. The arm on it in the photos is TY175. I use this backing plate when I'm machining TY twinshock front wheel brake shoes because it doesn't have the arm sticking out like a TY twinshock backing plate does.

Photos of my mystery wheel. In the photos it is holding up the front end of a derelict TY175JC

Yes that is a cool way to help ID Yamaha parts if they have the prefix on them. That list can also be found using google. I was told by a Yamaha dealer that there is a system that records each time someone asks for a Yamaha part that is "no longer available" and once enough people have asked, they start making them again. I suspect that TY175 kickstart levers might be one such part that they now sell. That prefix on your brake shoes doesn't help much with identifying your backing plate though because many Yamahas models use that same pattern of brake shoe.

You didn't actually mention the mode of failure of the wheel bearings on the 4RT. Without knowing why they failed it is hard to know how to make them last longer. In my experience, trials bike wheel bearings last for many years.

That's them yes those three little lugs. The speedo drive casing normally fits between the hub and the fork leg. It's a good design because you can take the wheel and backing plate away from the bike easily without having to undo the speedo cable. The speedo drive and cable can just hang there waiting for reassembly. I've been trying to take some rim stamping photos of my mystery wheel but my phone battery went flat. Photos coming later

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.