feetupfun

To make the head angle steeper, I lengthened the frame backbone 25mm and made a bend in the front tubes at the motor mount To lengthen the swingarm, I extended the axle plates 45mm Mine actually looks quite standard unless you look closely, which was my intention If you send me your email via PM I will send you photos of a TY175-based bike that you would probably be interested in seeing for ideas for your "far as I can" bike

Its not the carb size or the air filter design that limits the performance

Its hard to tell from what you have written, but it sounds like you might be measuring voltages of open circuits. If the ground wire in the headlight circuit is 19V above the frame voltage, then the "ground" wire is not grounded. To find out what voltage the bulbs are seeing at different RPM, measure the voltage difference between supply and ground with the headlight and taillight both on. Some bikes do make too much voltage at high revs even with the correct bulbs in service and good earths. I used to use 12 and 24 volt bulbs in my (nominally 6 volt) 1970s Japanese trials bikes and trail bikes to avoid blowing bulbs. I have seen many other techniques successfully used too.

Yes you have described a sticking clutch. To free up clutch shift to top (5th) gear, pull clutch lever in and rock bike forwards and rearwards. The tyre will grip better in the rearwards direction so concentrate on maximising your effort when moving it rearwards As previous person said, clutch issue has nothing to do with motor not starting. If it ran, now it doesn't, ask yourself "what is different?" Usually that is engine flooded with petrol or no petrol getting to combustion zone.

I use automotive acrylic lacquer for frames but I'm sure there are many other suitable paints.

Gloss black is the right colour for the Cota 348 frame. The brace pieces that go from the swingarm pivot to the cylinder head are plated decorative chrome

Looks like the 4 stroke has a non-linear cable pull ratio. This is usually done to smooth out the action of the throttle when it is near closed. Trimming the ramp to make the action more linear will not help you with off-idle throttle sensitivity because the ramp is the bit that affects throttle opening at the other end of the throttle action You need a tube that has the cable end holder following a smaller diameter circle - like the other one in your photo

One TY175 I have came with a 26mm Mikuni from a DT175. It now has a standard TY175 carby on it because with the 26mm carby it did something that put me off a bit. It felt exactly like a normal TY175 below about 6000RPM. If you accellerated at full throttle through 6000RPM it would suddenly accellerate faster at that point, much like the way a two stroke MX bike does, but much milder. It was a bit disconcerting for me when riding obstacles that require full stick. Because of this, and because the bike is a loan bike for trials learners, the standard carby is probably better. For trail riding on that bike, the 26mm carby is definitely a hoot and helps with big (sustained) hill-climbs.

Earlier this year, I actually went through the process of modifying a TY250 frame to make a replica of a bike which was built in 1976 and which used very similar ideas to what was done years later with the Yam frame Majestys. It was quite a learning experience and as people say, a lot of work to do it right. It took me 4 full working days to make the jig and do the basic frame mods. It doesn't sound much but that is 32 hours work and that does not include the myriad of other work that is needed to be able to complete the bike. It is definitely not easy and I would agree quite impossible to do properly without making a jig first. In the case of the bike I did, the target was to raise the motor 1 inch, and from all I have seen and read about the Yam Frame majestys, I would say that was the case with them too. The claimed 2 inch increase in ground clearance and steepened steering head angle were achieved via the combination of raising the motor close to one inch and the increased rear end travel raising the rear end of the bike. Having studied photos of Yam framed majestys with tank removed, I know by comparing the locations of the sparkplug tips relative to the horizontal brace behind the toolbox, that they had the motor raised the same amount or very close to the one I did. I'm not keen to post up photos of how the work was done yet because that bike was built for a feature article in a magazine and it hasn't come out yet. I do plan on posting the photos up somewhere accessible later on.

It is an older Beta motor but does not look like a Zero motor to me. They had cylinder castings with flats. This one is round. Cylinder doesn't look right for the Beta Synt models either

From what I have seen, the other Montesa trials bike square section fork bottoms from the same era look the same as the 348 and probably are

As well as those things Tim said, if the fuel evaporates out of the float bowl over time (fuel tap off and does not pass), it leaves a viscous oily residue coating on everything that it was touching that takes a while to wash off when you turn the fuel on next time. I find that I get restriction in the pilot jet from this residue that makes the bike run poorly for a few minutes after not being run for a few weeks. With longer term storage (with the fuel tap left on or passing) a solid polymer film can form on the surfaces that the petrol touches and it can be very difficult to remove the film.

1974 was the first well-known Yamaha single-shock trials bike. It was a Tilkens (non-linkage) design rear end like the early YZ and MX Yamahas. Mick Andrews rode the SSDT on it that year. The first Yamaha linkage-type single shock trials bikes were in existance in the early 1980s as development bikes but the design was not "revealed" until 1983. There were probably home-made single shock trials bikes before Yamaha started making them.

Not a dumb question at all. Yes some people run with less spark advance than standard on trials bikes, especially on large capacity motors. The standard setting is only a guide. Why not try it and see what you think. The motor will run even with the spark after TDC so you can experiment to your hearts desire. To start with I would try 10 degrees BTDC and see if you can feel the difference. Remember to test for the effect on high RPM performance as well as low RPM response.

Wossner are making pistons 4 mm oversize for some patterns so that might be the source for a 74mm TY250 piston.

Replacement (unobtainable) magneto covers for Yamaha TY250 twinshock or TY175 Twinshock bikes would probably be suitable parts for the case study although they would be less of a challenge than a one-piece airbox.

Those KLR 650 motors are fantastically long lived. You can expect 50,000km before it will need rings and a cam chain. If they stil have the CV carby, the fuel economy will be good too. The only downside I know of is that the suspension is 1980s trailbike technology so while it will be smooth riding don't expect it to be up with the latest bikes on broken ground. The hot ticket I remember for improving the front end for off-road stuff is to fit a TT600 front end (clamps, forks and wheel)

Front uprights shorten at bottom end. Rear uprights shorten at top end. Middle uprights shorten at top end.

My IRC fronts have been very slow to wear. The rubber feels same softness as Dunlop to me when new and carcass stiffness is also very similar to Dunlop ie carcass not as flexible as Michelin given same pressure. All proper trials competition tyres have softer rubber and are more flexible than the Pirelli. I have never been able to wear out a front trials tyre before the rubber goes hard so maybe I'm not the right person to give an opinion about which competition trials tyre is best for intensive long distance trail riding.

IRC fronts are good value in Australia and are much better to ride with than Pirelli fronts. I rate IRC fronts along side Dunlop fronts but not quite as good as Michelin fronts.

TY250 twinshock gear change shaft replacement does not require the cases split, but does require removing the clutch cover and the clutch basket. There is a circlip on the LH end of the shift shaft and once it is removed, the shaft slides out towards the right. Replacing the shaft is probably less work than repairing the cut-off shaft.

You could try a few more different solvents. There is bound to be one that works (alcohol, acetone, toluene, ether, citrus cleaner, hot water and detergent, dry cleaning solvent.....

Monty primary drive gear is on a taper which can be very difficult to separate. There are previous postings on the ways people have got the gear off. Yes the seal holder is just like a Bultaco.

If something snags the cable without safety wiring, the outer can pull out of position and may return on its own or with some assistance, but the outer does not see a tension loading (the inner takes the tension load). If the outer is safety wired and then is snagged, the outer may be stretched which would probably leave no options for continuing riding as outers cannot be recompressed once stretched.

I don't care which side, but I do care how ergonomic they are. A few years ago my wife wanted a modern bike and the choice was down to a Beta Rev 3 (left side kick)or a 2 stroke Yamaha motor Scorpa (right side kick). We bought the Beta because the end of the Scorpa kickstart was so high she couldn't get her foot high enough to reach it. She had no probs starting the Beta.