feetupfun

320mm to 340mm will work. Top mount position depends on what shock length you want to use and what the shaft travel of the shock is and how high you want the rear end

Very common mod to increase rear wheel travel and if you use the same shocks, raise the rear. Was standard on Yamaha frame Majesty 175/200. Lots of TY175s modified that way by owners in the 1970s and still commonly seen being done. There was a kit available for it a few years ago.

Measuring the peak pressure on a two stroke will always give variable results. Without changing anything else, the amount of oil at the ring/bore interface at the time of the test will make a significant difference to the result. Fitting new rings without honing the bore will make the results of that sort of test even more variable.

Somewhere within a very wide range depending how well the restoration was done

I wouldn't assume anything about a fuel tap

What's a side casting?

Hidrol was/is a brand of fork oil and the 7 means 7 WT

It's a fairly normal thing to shift to neutral before you come to rest if you want to then sit in neutral with the motor running. It will shift easily to neutral if you shift when there is no torque on the gearbox (ie motor not driving bike or bike not driving motor). It is quite normal for shifting to neutral to be difficult with bike stopped and motor running (clutch drag puts torque on the gearbox).

I suspect that the single piece WES design may have been improved at some point in time regarding how noisy it is, because mine does not sound like how you have described yours. I bought mine maybe three years ago through TYoffroad

Yes. I suggest you put your new thread in the forum for the brand of bike you are working on. If it's a Beta there's a few novels worth of reading about clutch drag there already.

The sealant is usually low viscosity epoxy resin. There is a commercial kit that also contains cleaning agents that is popular (Caswells)

Yes that is standard on Shercos. Initially (in 1999) Shercos came with open main bearings and they didn't have a long service life. It was found that sealed bearings lasted longer. Main bearing ventilation/lubrication ports that existed on many two strokes previously, was lost in pursuit of a narrower engine when the Sherco trials motor was designed.

The original 348 seat is made of a foam core with a sewn black vinyl cover and has a very similar shape to the odd-coloured seat in baz348's photo. After the 348, Montesa went to using naked black rubber seats on their trials bikes.

As well as the suspension linkages, I can recommend you also service the swingarm pivot and steering head bearings. I hope you get to them in time before corrosion occurs. I bought a one year old bike a while back and found that all the rolling element chassis bearings had corrosion happening due to lack of any protective lubricant being applied when originally assembled. It seems that very little attention is given to lubing those parts during assembly on some new bikes. Sealed wheel bearings at least are usually greased by the bearing manufacturer and last longer before failing.

Nice 348 What is it about the seat that causes you to say it looks original?

Probably repairable, but welding up hubs is something that is an (expensive) last resort on rare bikes that you can't get replacement parts for. Front TY hubs are still being made and if you have a decent look on eBay you will probably find new hubs available ex Thailand. Apart from that there will be plenty of second hand TY front hubs available. I'll sell you one if you are stuck. It's quite rare to break a front hub. I'm interested in how it was achieved.

Sounds like you are asking about the WES TY175 systems. If you have a standard front section and replace the end muffler of the competition model TY175 (525) exhaust with a WES end muffler, the exhaust note will be a lot quieter and will no longer sound raspy, irritating and machine-gunnish. If your TY175 end muffler is the type fitted to the TY175 and TY125 that had to meet strict noise limits for road use (TY175JE and TY175JC and TY125), the WES end muffler will make it slightly more noisy. The one piece WES TY175 system is slightly more noisy than the WES two piece TY175 system. Both would be considered acceptable at a competition event as far as noise level goes. Yes the WES systems are different internally to the original designs and in my experience, enhance engine performance compared to the standard exhaust. The biggest bang-for-the-buck for performance and noise reduction comes from fitting a WES end muffler. Replacing the main section with a WES provides a good bang-for-the-buck handling improvement (weight reduction). Be warned that if you are planning to use a WES exhaust system and ride your TY175 at sustained high speed, you are at risk of the exhaust system catching on fire. They are intended for use in trials competition.

It is fairly common for premix fuel to form a gum on the surfaces it is in contact with, if the bike is left with fuel in the float bowl between rides. 4 weeks is long enough for this to happen. If you are going to leave the bike for more than a few days between rides, it is prudent to run the float bowl to empty at the end of your riding session or to drain it after your riding session. The main two places where this gum tends to cause problems are at the float needle valve and inside the pilot jet orifice. Your problem may be from this, or it could be one of a few other causes for a float needle valve not sealing off properly. To diagnose the cause of the problem, the carby will need to be taken off the bike and the float bowl taken off to allow internal inspection. It's good that you have learned that owners manuals are to be treated with suspicion.

The difference you are feeling is not an illusion. It is real and yes it is due to differences in geometry and mass distribution. The small differences in total mass between bikes are not a good guide to how a bike will feel to ride. People's bodies also have different geometry and mass distribution. Which bike feels best to each different person is why people should try before they buy. There is no single "best handling" or "lightest handling" modern trials bike for all riders.

The main reason why the 250 handling feels heavier is the increased gyroscopic effect of the heavier crankshaft/flywheel assembly.

Model 27/49/80 brake hubs are cast aluminium with a cast iron friction surface. The front 27/49/80 hub also has equal diameter spoke flanges. There are many things you can do to make the brakes work very well. As you already have mentioned that some of them have been done, here are other things that are important. Wheel bearings have no play. Drum friction surface is smooth (not grooved), parallel (not tapered) and round. Bikes that old usually are woeful in these areas unless the drum has been re-machined or resleeved. The friction material is of a modern high friction type, available from specialist shoe relining people. A thinker lining than off-the-shelf shoes have is usually required to achieve a big enough diameter for the lining OD on the backing plate to match a worn or remachined drum. I'd be surprised if the radius of off-the-shelf new brake shoes matches the drum curvature of such an old bike, unless the drum has been releeved back to original diameter. If you can pull the lever all the way in, then a likely cause of this is that the radius of the linings does not match the curvature of the drum. The springiness that allows you to pull the lever in is probably the shoes being flexed away from their relaxed shape. This flexing may well provide a contact witness on the shoes but makes the brakes useless. So my suggestion is to have the drum machined smooth and parallel and the shoes relined thicker and machined back to match them to the new drum diameter.

Impressive. Are you saying that motorbike fork tubes are manufactured to that degree of accuracy on diameter?

What stage are you at in the rebuild? I'm trying to work out why you need to compress the springs

There is variability in tube diameter when tubes are made new and when they are re-chromed. The process of finishing them (centreless grinding) is not a precision process, nor does it need to be.

Welcome Martin