feetupfun

The reason you got all that 2T related feedback is because in your post you said you were interested in the Beta 300. Beta make a 2T 300 and a 4T 300 and some people assumed you meant the 2T 300 and were trying to steer you away from it

Wasn't the original poster asking about a 300 Beta 4 stroke?

It would help with reading the stamping of your engine number if you scraped the paint out of the stamping

Here's a photo I lifted from the North Atlantic Trials Assn webpage that shows a 349 motor up close and it has the engine number where yours is stamped

I've just noticed that the frame is also Cota 349 not Cota 348

It doesn't look like a 348 motor. Clues being the shape of the clutch cover and the bolt-on inlet connector. I'm thinking Cota 349 motor. That doesn't solve your mystery though because 349 is 51M too. Here is a photo of the engine number on one of my 348 motors and it is located on the top of the front engine mount lug. I don't know where 349 numbers are stamped but it might be worth a look at the front engine mount lug

Yep, should be M. How about a photo?

Spec sheets are not a good source of geometry information because they don't specify if the bike is laden or unladen or if there is any static sag or what pressure is in the tyres or what tyres. You will get real numbers by reassembling the important bits and measuring it yourself. If you are trying to copy a proper trials bike, measure the proper trials bike yourself so you are comparing apples with apples. Be aware that modern front and rear trials tyres are a fair bit taller than front and rear trials tyres of the past and the heights of the axles are very much affected by pressure.

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Motor parts are very expensive so make sure there is nothing wrong with the motor

It would help with diagnosis if you were to either say what carby it is or post some photos. They were and are a very rare bike so not many people would know what carby they came with. Also there is a Montesa (Cota) forum. This is the Sherco forum

Those sliders are quite readily available second hand. If you want to retain the original slider, the slider can be restored for service by a machinist fitting a sleeve near the top. You've got Buckley's chance of getting second hand aluminium original guards that are in good condition. If there was such a thing available, they would be more expensive than the modern replica aluminium guards because of their originality (the replicas are not exact replicas).

What is worn on the fork bottom/slider that needs fixing?

Excellent

I've seen people fit XR600 (alloy) kickstart lever to the Yamaha knuckle, but they shorten them a lot to get them about the length of that one in the photo. I bought an alloy Honda lever for this purpose but haven't gotten around to playing with it yet. I've got an MX250B kickstart lever assembly and a DT250 something kickstart lever assembly as emergency spares for my TY250s and they fit all right but are pretty long and the MX one is very heavy and has a bigger diameter knuckle pin. The standard TY250 kickstart lever is quite a short and light thing compared with the other Yamaha dirt bikes of the era that use the same spline. I think the TY lever is so short because the kickstart shaft is at the top of the motor while the others are only halfway up the motor.

and how did it go?

Could also be a leaking crank seal on the primary drive side of the crankshaft. The test for this is if the gearbox oil is disappearing.

The popular two strokes all depreciate due to age for about the first 3 years then their value becomes almost entirely dependent on condition. The desirability of particular brands on the second hand market waxes and wanes depending on what is happening with spare parts availability, or the expectations of spare parts availability in the future.

I would reuse that piston and if the ring end gap is still OK, reuse the rings. For the little end bearing, do a visual inspection on the rollers, the pin and inside the conrod hole with a magnifying glass. If it all looks OK, reuse them too. You will find that trials two strokes running modern oils gum up the rings at an alarming rate, especially if there is too much oil in the fuel.

Maybe they are good storage container for bike parts?

Yes the cable may have been damaged, making the return action (throttle being closed by the spring above the slide) sticky. Another common crash damage thing is the twistgrip tube being pushed against the end of the bars, also making the cable return sticky

My 348 is 195 mm centres

There is normally only a tiny clearance between the torque link and the pivot arm

Post up a photo if you want to know what's up

could also be cable damage