feetupfun

For the person who made a new key from a valve shim - I suspect that valve shims might be either a fancy grade of steel, or surface coated for hardness, or both. Woodruff keys are usually made from relatively easily sheared material (something like 250MPa mild steel), so that if the taper slips, the shearing of the key does not damage the crankshaft or flywheel in the process.

Hey Tim that one is one of my 79 YO Mum's favourite jokes

Stork the piston clearance on your mates large ship engines is not a good example to use because the pistons and cylinders on them are made of materials with matched rates of thermal expansion, so there is no requirement to allow for expansion from cold to hot. The Bultaco motor has cast iron sleeve and aluminium alloy piston which have quite different rates of expansion.

Remember that there is a reason for the clearance between the piston and the bore. It is to allow for the differential in expansion rates of the piston and the bore with temperature rise between ambient and with the motor at full running temperature. For this reason, the diameter of the piston and the materials that the bore and piston are made of will determine the ideal clearance. The piston Bogwheel is using is much bigger than the piston that would be used in a M198 and so the clearance should also be bigger. One point five thou might be OK on a 238 or 250 with a forged piston, but I think it would not be enough for a Bultaco 325 or 340 piston, and even more clearance would be required if it is a high expansion rate alloy piston (like a Wiseco).

I do know that the fork tubes are a different distance apart on these bikes so if you were thinking of using the KT wheel and mudguard it would require some mods. I'm one of the people who ran for a while with a complete TY250 front end on my KT a few years ago and it did make the KT front work very well in tight turns. However, I doubt that fitting TY triple clamps to a KT with KT forks would improve the KT steering. Both bikes have similar trail dimension as standard so by using KT forks together with TY triple clamps, there would be almost zero steering trail.

Not sure if it is a common problem with TY80s, but my TY80 just had a similar problem to yours and it was only a minor issue easily fixed. It was a pin in the shift linkage that slid out of it's hole in the inner case.

If the chain hits the shockie spring while you are riding and it annoys you, you can fit a washer or two behind the shockie eye to space it out a bit. Remember that the original shockies on that bike had the damper bodies at the bottom so gave more clearance for the chain than modern shockies with the bodies at the top (and spring going all the way to the bottom). I find that even 2-3 mm side clearance is enough to stop it making noise. When I run Falcons on my TY250 twinshocks I find that I do need to pack the shockie across about 3mm to give enough clearance. Some people run with less rear sprocket offset than standard to provide more shockie spring clearance, but this is hard on sprockets due to chain misalignment. Yes when the bikes were new there was a rubber chain rubbing block that fitted over the swingarm pivot tube end. They are no longer available from Yamaha but may be available from someone making replica rubber parts. You will find when you get aboard the bike, the chain will move away from the swingarm pivot tube and only touch it briefly when the rear wheel unweights on obstacles, hence why hardly anyone bothers about fitting a rubbing block there. The shockies you have may well be longer than the originals which will make the chain tend to rub more than with the original shockies on full droop of the rear suspension.

Montesa made versions of the 247, 348 and 349 that had bigger seats. In your case it would have been called Cota 247T. Have a search on this website, bikepics or search on google and you will soon find photos.

The airbox looks like Cota 348 to me

All you would have needed to do in 1982 to get those Yamaha TLS front brakes on your 1981 Bultaco was to find someone parting out their brand new YZ490J - no probs hey. By the late 1980s you might have done OK getting a YZJ front end because people might have started modifying their old MX bikes by fitting disc brakes to their MX bikes, or they might have been parting them out - but then again by that time you too could have fitted a disc brake to your Bultaco and wouldn't have wanted the TLS YZ front wheel - that is if you still were riding a 1981 Bultaco in the late 1980s! The main reason people didn't have YZJ TLS front brakes on their old twinshock trials bikes was because they didn't want to spend a squillion $ to improve the front brakes on a bike that would have already been superceded by later model bikes. The secondary reason for not using TLS front brakes on a trials bike is that they work very poorly going backwards.

Hi John. Welcome to the melee and I'd like to ask my own silly question. How old do you need to be to kicked out of Junior Motocross? Regards David Lahey (51 yrs)

Maybe because the YZ490 twin leading shoe front end came out in 1983 which was many years after everyone lost interest in riding KT250s.

For your weight I would suggest 50 pound springs. Yes that length for the Falcons is fine. Yes the top mounting pins are different diameter to the bottom pins.

Either mod will make the steering noticably heavier when riding slowly or stopped. If you want to try it you are welcome to ride my M138 Alpina, or I can lend you a set of Alpina/Pursang/Frontera triple clamps to try on your Sherpa T. Regards David

The Wiseco DT250 pistons are slightly different-loooking to Yamaha TY pistons, but the skirt is the right length, the pin to crown distance is right, the window holes in the rear skirt are the right shape and in the right place. The rings are standard Wiseco (two thin rings that are flat top and bottom) and piston kits are reasonably priced. Bought through SERCO (Brisbane, Australia) While the Wiseco DT250 pistons work well in the TY250 twinshock, for the TY175 I prefer using PRO-X, which are made using the same alloy and manufacturing process (and look the same as) original Yamaha TY175 pistons and are also reasonably priced.

I've used Wiseco DT250 piston kits in the last two TY250 twinshocks I've had rebored (5 years ago and 1 years ago) and they have both been been fine since - they haven't rattled and everything fitted perfectly

They stop the bearing from moving out of position

Yes the exhaust comes off without having to move the motor but it is a bit of a trick Yes a removable hatch is a good idea if you are going to ride the bike for a few years. Please ask your friend what benefit come from using steel wool compared with mineral fibre. I'm just about to repack my 348 exhaust too and might give it a go.

I couldn't see any model ID on either of my KT carbies. They do have the Mikuni symbol (MIC) on the side of the slide casing. The Owner's manual calls the KT carby a VM26SC

If you can post a picture the carby may be recognisable to someone. If it is from a KT250 Kawasaki, it will be a snap to identify! I'll have a look tonight to see if there are any ID numbers on my KT carby.

Yes the Rev 3 carby is leaned forwards lots and a front down attitude makes it worse. If you do a search on the Beta Forum you should find hundreds of postings on this issue with many suggestions for how to avoid losing petrol out the overflows. It might even be a pinned topic. What is "1 1/2 to 2 1/4 air bleed"?

I have seen this problem in my friends 2001 Rev 3 and his problem was piston ring blow-by. Fixed with new rings. Hard to diagnose yours via forum postings though - yours may be stator failure. The stator problem can cause an effect very much like rich mixture above a certain RPM because the ignition cuts above a certain RPM, causing misfire. Standard carby settings are fine. Most people go through hours of changing things on the carby, only to find that it was not the problem. There was a good reason the float height was set low and you will find out why when you ride the bike more. Set the idle mixture screw to suit your bike. The standard setting or anyone elses setting is only a guide. You didn't list the pilot jet size and that is the thing that people commonly change.

Alan DT forks are not old enough to be eligible for pre-65 in the UK

Provenance is an important component of pricing old bikes for many people

I can suggest trying somewhere that sells parts for Chinese dirt bikes because I recently needed a fuel tank cap for my PW50 Yamaha and the local Yamaha shop told me the cap for the Chinese copy of the PW50 would fit, and it did, and it was very cheap, and from memory the cap for the TY250/350 monos is very much like the PW50 cap.