feetupfun

I haven't ridden a 4RT since 2005, but would get the Repsol if I was getting a new 4RT. What's strange about that? Where I live the new year models of most brands only come in one or two shipments and most bikes are pre-sold anyway. The only time I have ever had a chance to have a go on a demo bike of a new model was the 4RT when it came out, and the Rev4 250 when it came out. If you wait to ride a mate's new bike of the type you are interested in, there are none left in stock, so you have to wait till the next year's model, and then it will be different anyway. One of my riding friends wanted a Rev4 300 last year and it took about 6 months for him to get it. Is that enough reason to buy a bike without test riding?

Ok just pulled a seal out and measured. The distance from the seal seat to the circlip groove is 12.5mm and the seal is 10.5mm thick

Something sounds fishy with the 15mm dimension in your forks. I would have thought that standard was about 12mm. I will measure some TY250 forks in the morning. Be aware that the circlip is not really essential unless you are going to run with air as the spring. Plenty of times when I've gone to remove the circlip there has still been clearance to the washer that was there when I fitted them - ie the seals have not moved away from their seats in use, and I use anti-sieze on seals so they are easier to get out when I change them. I also run sealed fork caps in my Bultacos that do not have seal retainers and have never had one of them have the seals move either.

It's not unusual to need a few basket cases to make one complete bike. I have a TY250B that is made up from three bikes that were all worse than what you have there in their own way. You can always sell off the excess bits when you have a complete bike, to recover some costs I like that saying Guy

I thought it was common to have a steel backing in TY250 fork seals

your photo shows what looks like the steel innards of a seal with a piece cut out. You say you have tried all methods but did not mention which methods. To loosen the steel "flange", gentle heating of the aluminium slider around the "flange" would be a good idea. After that, a slide hammer with a 90 degree hook on the end of the shaft should get it out without damaging the aluminium. If there was a seal under the "flange" then what you have there is not a standard arrangement. I'm thinking maybe someone replacing the seals previously could only get seals that are thinner than standard, and used the steel part of the old seal as a spacer to make up the difference.

there is a chamber inside where that hole is, and that chamber is connected to the gearbox by a small passage. The passage between the chamber and the gearbox is located in a way that avoids oil going from the gearbox to the chamber If you don't like that design, you can always block that hole and install a vent in the filler cap. The Montesa design actually works pretty well (on my bike anyway)and I've not heard of anyone complaining about it.

That hole is the breather hole for the gearbox

have you checked the timing with a timing strobe light? Kicking back when you try and kick start usually means the spark is happening too far before TDC. Just because it is a CDI does not mean that the spark timing will be correct - may have happened when the camchain was off to replace the rings

I've never seen official instructions for this, but have had success with: Fix a piece of thin card or similar spacing shim to the outside surface of the laminations, set the stator coil towards the outward limit of its travel, set the screws with just the right tension so the coil can move if pushed, then fit and remove the flywheel carefully with no points (to make fitting the flywheel as easy as possible) but with the key in place, then tighten the stator coil screws

would help to know what bike

Lobito can be made into a reasonable twinshock trials bike with many modifications, but if it is still standard, the footpegs are way too far forwards, and the steering geometry is wrong. I think to ride an old bike you have to actually like riding old bikes. My wife started riding trials on an OSSA MAR in the early 2000s and thought that was OK until she rode a 2003 Beta, then wouldn't touch an old bike. Like twinnshock's local rider, I also started riding trials on an early model Alpina back in 1975, and thought it was OK until I rode a proper trials bike for the first time, and a Lobito is worse to ride in trials than an Alpina

thanks Bondy I was wondering what they look like on a bike. Is the fuel tap well located (with reasonable clearance between the hose and head)?

if it's only on overrun when the throttle is fully closed, and continues for as long as the overrun is happening, then changing the jetting will not make any difference. If it is a transitional effect as the throttle is being closed, then a jetting change (pilot jet size usually) will help The bong, bong, bong on continuous overrun that some 2 stroke bikes do is usually due to the fuel/air mixture going too weak to burn during overrun. Air ingress will usually be worse on overrun because the crankcase vacuum is higher when the motor is being driven by the rear wheel. It can be a bit distracting. Four stroke motors are susceptible to exhaust bang if they have air ingress into the exhaust system near the head end of the pipe. On a TY, the bong, bong is caused by air ingress which causes the air/fuel mixture to go too lean to burn for a number of strokes of the motor, until enough petrol accumulates to get the mixture up the point where it can burn in a single violent stroke (the bong stroke)

The only difference I have seen is in the branding on the lanyard part that goes around your wrist. All the magnetic ones I have seen are made by Leonelli (and are the same device).

Totally agree with Woody comments and suggest you have a ride of one before you go ahead. If you can't find one to test ride, that should tell you something The TY250 head will be the wrong shape inside because the bore diameter is different. It may also need machining to get the compression ratio right. The different model TY250 cylinders can be identified externally. If after all the advice, you still want to ride something with too much capacity, did you know you can make a 360cc TY250 motor? You can use an RT2 or RT3 or DT360 cylinder, head and conrod and lengthen the stroke. The downside is that it needs more machining of the crankcases than the 320/340 motor due to the stroke and conrod differences, and will not fit in a Yamaha based frame Majesty due to the extra height of the cylinder and head. It also needs a flywheel approximating the weight of a Volkswagen Beetle to make it rideable

Maybe it's just my eyes but in the lower photo it looks like the steel cylinder that the plastic anti-bottoming cone fits into is on upside down, because I think I can see a short recess inside the end of it. I think the end with the recess should face up not down. If the cylinder is the right way around, you should be able to slide the whole plastic cone into the cylinder.

centre of clevis pin hole to far end of threaded section 411mm threaded section 70mm

Twinshock or mono TY250?

I bet that magneto cover would not fit on the inner cases of a YZ

The bike with the dell orto carby that Craig is sitting on has a cylinder which displays a part number in the casting, which identifies it as originating from a TY250B The engine inner cases and engine covers on that same bike look they are from a 1970s Yamaha works motor that Mick Andrews used The kickstart lever on that same bike is TY175

There are a few different bikes shown in this forum string. Please say which bike you are referring to when you post as it is getting to be quite confusing

That was a Kato 175 replica owned by Craig Mawlam

If you want to stay around 250cc, then get the bore measured by the person who will be reboring it. They will tell you from that what size oversize piston to buy. You should always get the smallest oversize that will clean up the bore, so you then have more options for the next rebore. If you want a bigger displacement, you can get pistons up to 2.0mm (0.080") above standard diameter off the shelf made by Wossner, which increases the capacity. TY250 motors can also be made 320cc but this is a lot more expensive because there is a new sleeve and more machining required, and rejetting, and a heavier flywheel

Bushfires one corner, cyclones and flooding in another. Jon I hope your brothers place is going to be OK