feetupfun

I also have used silent blocks for swingarm pivots and in Australia, individual silent blocks can be bought from Blackwoods

Yes they have been a good thing on every old two stroke trial-type motor I've put one on so far

For GasGas and Sherco, their 4 stroke development was driven by pressure from the FIM who were planning to ban 2 strokes. GasGas and Sherco development was dropped when the 2 stroke ban was dropped.

Left side. Nothing else fits. Right side. DT125/175 1974 to 1977 (ABCD models) fits. YZ125/175 C/X fits. Later DT175, IT175 and YZ125 don't fit without mods.

Quick answer. Yes

GasGas manual downloads available here https://www.thehellteam.com/technical-support/gas-gas-parts-and-service-manuals.html

Another interesting thing about the bike is that the swingarm looks non-standard. Maybe it's a stronger design to cope better with sidecar use

Hmmm that stamping looks a bit dodgy for a few reasons. They didn't make a bike model called a 51H. The number after the 51H is too low to be on a 349 motor. 51M series started in 1976 and there were many thousands of M51 Cota 348s made before your series 1 Cota 349 was made but yours is stamped number 00221. The appearance of the metal surface after the 51 is smoother than at the 51 meaning the original letter and number after the 51 could have been ground away and re-stamped with a different letter and number. Another possibility is that Montesa made trials sidecar-specific motors that had a different engine number series and this has not become widespread knowledge yet. What does the frame number stamping look like?

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.