feetupfun

yes

10/48 (4.8:1) is 12% slower than 11/46 (4.2:1). 9 teeth front sprockets are available. Bigger (custom made) rear sprockets are available.

Changing the reeds to boyesen makes a small difference to the way the motor runs. The evidence of this is that the air screw and sometimes the pilot jet requires a change made when boyesen reeds replace the standard reeds. The difference is because there is less pressure drop through boyesen reeds because they are lighter. Maybe it is too small an effect to be noticed on your bike. When you fit boyesen reeds, the reed stops need to be removed for them to work properly. Maybe you are asking about bending the reed stops when using the steel reeds? That's not a good idea because it reduces the time-to-failure of the steel reeds and might allow the reeds to bend instead of flexing within their elastic limit. Low budget gains in bottom end response on a TY175 which is in good mechanical condition include increasing the compression ratio, lightening the flywheel and experimenting with the ignition timing.

Depends on what the seal is made from. The one in your photo is likely to be Viton. Here is a chart that gives operating temperature ranges for the various materials that seals are made from http://www.pspglobal.com/low-temperature-seals.html

I'm sure you can work out what's going on when you pull the clutch cover off, without a manual. They are very simple machines. From the sound of what you have said the shifter may have had a bit of a hit. My friend's 1981 349 had similar symptoms after the shift lever was bumped on a rock just before a trial and he laid the bike over on its side, took the cover off and fixed the problem and then rode the trial. If you can't work it out yourself, pull the cover off and post some photos up here and you will get lots of help

Left crank seal leaking is one thing of many things that could cause that. First thing to check is if the idle air screw has been adjusted correctly. Other possible causes include pilot jet restricted or pilot circuit restricted.

Does the piston really go that far down at BDC? What does it look like with the piston crown edge level with the bottom of the exhaust port? Also the piston should be in there the other way around, with the short skirt at the rear.

I can't see in there either

It's likely to have been stamped with the frame number of the donor TL125 for road registration/insurance purposes

The kicker in the movie photo is a standard top pivot kicker. It is the S shape design though which is standard for that era and different to later (straight) kickers

It wouldn't be the first Bultaco that has had the forks and front wheel fitted the other way around. Looks like the one from the movie has a different diameter front wheel. How long do you reckon it would run for with the fuel hose touching the cylinder fins and check out the brake pedal position. Pretty sure no-one rides this bike.

If you already have excellent throttle control you will manage fine riding trials on a 300 two stroke trials bike. If you want to jump up big stuff, a 300 is better than a 250.

Are you asking about the skirt protruding beyond the sleeve in the bottom photo? Or are you asking about the transfer port sleeve hole being partly obscured by the piston?

Logically how could those pegs ever fit to the standard Bultaco footpeg mounts? And they look amazingly similar to these which are listed as needing brackets welded on https://www.inmotiontrials.com/product/footrest-kit-alloy-universal-with-weld-on-brackets-wide/

To test the fan switch, an easy way to do it is to unplug the fan switch and fit an electrical bridge across the connector. To test the fan and the bike's power supply, unplug the fan and connect 12V DC from an external source to the fan motor.

If the clutch pushrod has any load on it at all, the clutch will slip. The adjusting screw in the clutch pressure plate has to be backed away from contacting the pushrod to avoid clutch slip. Do this adjustment of the screw with the cable disconnected then fit the cable and make sure the cable has some slack too. As for the kickstart lever not returning by itself, does it have a return spring fitted? Have you fitted both dowels in the gasket face of the magneto cover? Is the kickstart shaft straight? Are you trying to use the kickstart without the magneto cover fitted?

If you can accelerate the the bike hard in top gear without the clutch slipping then the problem is with the kickstart mechanism. If you find the clutch slips when you try and accelerate hard in top gear, then it is the clutch slipping that is causing the kickstart to "not engage". If you can't start the motor because you can't get the motor to turn over with the kick start or by push-starting in 4th or 5th gear, then the clutch is slipping or the primary chain is broken. If you provide more information about the recent history and condition of the bike, you will get more relevant help.

What bike?

Yes that's the standard setup on the MAR. It is an odd and quirky design. I did think about doing something for a while and was considering using a rear wheel hub with the brake plate on the same side as the sprocket so a simple brake rod could be used. If I was going to do that I would choose a wheel hub that also had a bigger diameter axle than the problematic 12mm standard axle. If you do change it to a more conventional design please post up photos.

There's lots you can do to avoid the clutch-sticking-when-cold thing happening but lots of people don't bother doing them and just get good at freeing it up. Are you willing to do a bit of work on the Sherco to stop it happening?

The properties of different oils do make a difference to the issue of the plates not wanting to separate but the underlying cause is that the friction plate pads can act like suction cups against the steel plates. The proper fix is to roughen the finish of the steel plates to help break the vacuum and to remove any grooves in the basket fingers. Grooves there can hold the plates together when the clutch has load on it. For a temporary improvement, use the lowest viscosity oil that will also provide enough lubrication for the chain. If it was a racing motor, the primary chain would not enjoy it if you use ATF but for trials use, ATF is usually OK for lubrication and is very good at breaking the vacuum seal. There are also name brand low viscosity gearbox oils that will work well for this issue and questions like yours usually get lots of suggestions for which oil is the best. OSSA primary drive casings retain some of the old oil during an oil change so if you want to get rid of all the old oil, an easy way is to loosen off or remove the clutch cover to drain the remainder out.

Two thicknesses of that plastic coating and that washer being new would not be helping the situation

Fan running backwards. Air trapped somewhere in motor coolant space. Radiator coolant passages blocked. Radiator finning blocked. Blockage/restriction inside coolant hose. Coolant lost into gearbox through water pump shaft seal. Coolant blown out through overpressure valve by head gasket leak.

I'm not going to yell at you about learning to ride with rubbish brakes. I hate rubbish brakes. Great brakes make riding trials or avoiding your older ladies on the road so much easier. I am going to yell at you because you can make your brakes amazing without fitting bigger (heavier) hubs. Get the brake drum surface re-machined. Get modern high friction linings fitted to your brake shoes. Get those high friction linings ground or machined to match the drum ID. Enjoy your amazing drum brakes.

Corrosion inside the rims. Not usually visible from the outside but very common on this bike and if the spokes also need to be replaced, they are a very unusual Z design which can be a problem to find replacements for.