feetupfun

I think the red 99 also has Dougies name on it

Fork oil weight is a personal thing so 10WT might be fine for you and those springs. I use 20WT oil and run the level higher than the standard oil quantity provides (125mm below the top of the tubes with forks bottomed and springs removed) You should set the spring preload to suit whatever springs you fit so that the forks are at about 50% of their travel with you on board and with your weight only the footpegs. Take care not to use so much preload that the springs coils touch before the forks bottom (the more preload you give the springs, the smaller the gaps between the coils) Another thing is that by using solid spring spacers you are effectively reducing the air space inside the forks

Jaan the free length of a spring is not particularly important. The spring rate is determined by the other properties of the spring (number of active coils, coil diameter and wire diameter). The free length can change over time. Another thing about 348s is that there was a range of springs used in them so it is no surprise to me that what came out of your forks is different to what someone sold you for your bike. I have three sets of 348 forks and they all had different springs in them when I got them. Without meaning to confuse things further I have found that to get a high enough spring rate for my 348, I have used aftermarket springs made for TY250 twinshock forks.

Mate if you really want to know what Monty you have there, you will need to do a bit better describing it or post a photo somewhere so we can see it And Montesa has only one s (in your profile)

If you are asking about mine, the squish area in the head on mine was machined to make the OD of the combustion chamber the same diameter as the bore. That change combined with the bigger piston displacement changes the compression ratio, so the sealing surface was machined to bring the compression back close to but slightly higher than standard. No I don't remember how much was machined off the sealing surface and it would probably not be much use knowing anyway unless someone was to do one exactly the same big-bore job. No the timing works great at the standard setting but low RPM throttle response is more sensitive to the timing setting than before. I think an ignition with an advance curve would provide further benefits.

If the sleeve fits the standard crankcase mouth, it will be less than 200cc. Mine has a 72.5mm bore, very thin sleeve and the studs are just touching the OD of the sleeve. You don't machine the ports. You cut them by hand undersize before the sleeve is fitted, then enlarge them back to the right size with a porting tool to match the ports in the aluminium casting. That is the same with any new sleeve, big bore or standard. Yes raise the compression, fit a good electronic ignition, and lighten the flywheel a bit. If you go up in compression much though, don't lighten the flywheel as my standard flywheel is now a bit on the light side with the higher compression. You can machine the head for more compression without affecting the squish height by machining the squish area back as well as machining the sealing surface. Most people just run a very thin head gasket to get the compression up a bit. Yes I would agree that if you want to retain full reliability, then retain the standard sleeve. My sleeve is scary thin, and I don't expect to get the life from it that I would expect with a standard motor.

MITAS make a road-legal rear competition trials tyre that is much better for off-road use than the normal road-legal rear tyres, but it is made for tubeless rims. I have had one on my 348 rim but it took lots of perseverance to get ot to stay on at low pressure and I do not know if it will fit properly on a standard Yamaha rim. These MITAS tyres do wear lots better than IRC/Michelin/Dunlop comp tyres, but are lower profile so will always be a bit performance limited on edges (sharp rocks, slippery roots etc) compared with the favorites

I've been riding my 205cc TY175 since 2004. It is not the same conversion as John Cane or Craig Mawlem. It uses a piston made for big-boring the DT200/WR200 motor. It happens to have the right pin diameter and piston height to crown dimension that matches the TY175, so no barrel spacer or gudgeon pin bushes are needed. I use a standard carby, standard front exhaust with WES rear muffler and standard airbox arrangement and standard port and ignition timing. The barrell was resleeved bigger using a second hand Suzuki GS1000 sleeve. The cylinder head retaining stud holes were welded up and redrilled 8mm and plain 8mm nuts used hold it together. This was to provide for the increased ID of the head gasket hole. The head was machined to suit the bigger bore diameter and to provide slightly higher compression ratio than standard. The crankcases were machined out to suit the OD of the bigger sleeve. The flywheel is standard. The positives It runs fine and has more torque at all RPM than standard but only as much as you might expect with the extra 30cc. It is more responsive to throttle input (same flywheel and more power) The negatives It vibrates noticably (due to the increased mass of the bigger piston) It stalls easier It is easier to break the standard flimsy kickstart lever when starting it The motor mount bolts have to be removed and motor inclined to provide clearance to get the barrell on or off the (longer) studs.

If you are used to riding modern bikes, there may not be anything "wrong" with the TY mono. It will probably just be your perception of what a motor should respond like.

Why would you fit a one-way valve to a vent line when it needs to vent both ways? I know some tank vents have a device to stop fuel pouring out when the bike is upside down, but that is not a one-way valve.

The oil in the exhaust is because it is a two stroke and you are using oil that resists burning. Clean the muffler out and get used to doing it. Don't fiddle with the jetting if it works well within sections. Your problem sounds more like: Possibly might have too hot a sparkplug heat range for high speed work Possibly a part blocked fuel tank/float bowl venting system Possibly a crankcase air leak Possibly fuel supply blockage (tank valve filter?) Possibly old fuel Possibly carbon deposits on piston crown and inside head Keep your finger on the killswitch until the motor has stopped turning. It's only backfiring because you are effectively turning the ignition back on before it has completely stopped. How do you know you are getting reed flutter?

Unless you know the engine number that the barrel came off originally, no you can't date them any better. What does it matter what year it was made if they didn't change the design between C, D and E models? From memory here is a guide to the 493 models numbers: The unique engine number starts after the 493- . If the number starts with 1 that means C (1976) model. 2 is D (1977) and 3 is E (19??). Check out John Cane's guide here and see if I remembered it right! http://www.tytrials.co.uk/technicalarticles_302159.html I had problems fitting a Venhill KT250 clutch cable to my KT250. It was the first Venhill cable I tried and I haven't bought any more since and so haven't tried the Venhill clutch cable made for the TY250. Apart from the KT, my bikes have clutch cables made by myself with inner and outer bought from the local bike shop and from some imported universal clutch cable kits and they work great. While Yamaha front brake cables are available (in black) and are very good, Yamaha TY250 twinshock clutch cables are not available any more/yet.

What does your artistic mind say? I personally don't like the look of the fibreglass M159-tanked Sherpa T and do like the look of the plastic M198/199 type tank in both red and blue so for me it would be easy (thats why I got a M198 in the first place - because they look so cool) As far as longevity goes, the plastic tank will probably discolour over time and they can be tricky to make look good again with paint (which is probably why they are being made again 30 years after they were originally made). The fibreglass tank will be able to be repainted ad infinitem with ease, but will need to be lined with epoxy to protect it from the ethanol that will soon be in all our Aussie car fuels. I reckon the best Sherpa T tanks are the aluminium ones that were fitted to UK bikes. The older bulbous ones look especially good. The later ones that look like the plastic tanks are sensible and long lasting but don't really look any better than the plastic ones. It is a great debate

Oh well it was worth checking the barrells. Yes except it is the other way around. Opening the points gap makes the spark happen earlier.

Yes change the timing by changing the points gap. It doesn't take much difference in gap to move the timing a lot. Are you sure that both your TY250As have TY250A barrels? http://www.trials.com.au/phpBB3/viewtopic....f=16&t=3938

I think Alan's bike is a TY250A and I know from back to back testing of my bikes, that standard TY250A motor performance is very different to the B and later models. The weight band on the A model is a bit narrower than the band on the TY250B and later models, but if you put a B or later weight band on the A model, it actually makes riding the bike harder, not easier. I know because I tried it. The problem with the A model comes from the internal differences between the A model and later models. I suspect it is the port timing but my early TY250A motor has never had the barrell off so I haven't been able to compare it with a later model barrell. The effect I'm talking about is that at extreme low RPM, the A model motor responds to throttle input quite weakly, but at a bit higher RPM the throttle response is fine. This makes the A model a bit exciting in tight riding (when the RPM moves through the transition in either direction). Alan the guys at B&J Racing in Tennessee have a handle on this and have some sort of fix available. Another alternative is to find a B or later barrell for your bike.

http://www.yamaha-majesty.com/product_info...products_id=145 http://www.yamaha-majesty.com/product_info...products_id=143

The degree of restriction that the airbox creates, whether from the cross sectional area of the airbox entry, or from the type/thickness/area of filter media will also affect the jetting. Depending on whether your TY250 Mikuni carby is from a B model (two big round holes in the side of the airbox), or from a C or D model (air entry is via a baffled slot in the airbox lid), the ideal size of the air entry to the airbox will be different. The standard jetting in the two different TY250 carbies is quite different.

http://www.trials.com.au/phpBB3/viewtopic....f=16&t=4587

Yes thats it. The "check valve" for compression damping is a round thin steel shim designed to provide rate-sensitive compression damping by flexing out of the way of the oil.

Yes Tony the standard carby on the TY250 uses an external air supply to the emulsion tube. I think you tricked a few people saying it is a standard VM26 carby. It is a Yamaha VM26 Mikuni which is quite different to the off-the-shelf VM26 Mikuni. It is a bit of a weak point for the bike because the emulsion tube requires frequent cleaning out due to dust ingress up the hose. Yes the end of the hose needs to be at atmospheric pressure. It has nothing to do with the pilot/idle circuit.

If you shorten the spring, there will not be enough travel left in it. There is barely enough intercoil space already for the fork travel. I find that both the rebound and compression damping is too light even with 20wt fork oil. I weigh about the same as you. When you push the front down and up by hand, the forks sound like there is not enough oil in them (ie they snore) even if you fill them up to 125mm below the top when bottomed with the springs out, which is a fair bit more oil than the specs list. Heavier springs are available, but while they resist bottoming better, they exacerbate the lack of rebound damping amongst rocks. They are good except for the effect when riding rocks. I want to try cartridge emulators, but will wait till I need to change the fork seals next, as the forks are tolerable the way they are. YSS make the right size emulators for the TY250 forks (emulators intended for YZ250/360 from the 1970s)

Here you go Jaan http://www.francetrialclassic.com:80/fr/pr...348-et-349.html

No one twinshock trials bike is best for everything/everybody, but many people have a favourite.

How can anyone riding a modern bike sitting down and cover the brake pedal at the same time? Twinshocks are fine, but on moderns my foot won't bend up enough if I'm sitting.