The frames with tubular loops welded under the front of the engine correspond to the "second series" 348. Those bikes also had another improvements like an optimised shifter for the gearbox, the air assisted fork, and many other details.
No Bings on Montesa trials. You need an Amal 2627/411 with 150 main jet, 20 idle jet, 106 needle jet, 2B1 needle, and 3 cutaway throttle slide. But the symptoms you describe are more likely to be electrical. I'd check the ignition point.
I don't know if the 247 will be suitable for your swingarm. It's not only a matter of chain side, but also of physical measures. The 123 swingarm can be narrow for the tyre, be careful.
That's the easiest way to do it, allyboy. I'm not sure if you also could buy a complete 21 wheel from another trials Montesa. The hub must be the same, but maybe you'll need some spacer.
The rear tyre is not hard to find here in Spain, but the front one is almost impossible. If you want to ride the bike, you'd better change it for a 21.
There were seven different swingarms for the 247 along the years, but only two chainguards: the fiberglass one from 21M0001 to 21M4381, and the plastic one from 21M4382 till the end of its production.
Montesa Cota 248 (1981)
in Montesa
Posted
You'll need SAE 10W30 for the clutch and SAE 90 for the gearbox (API 4 if possible). 300 cc in both cases. I think that is 10,5 GB fl oz.
Mix can be 1,5% with modern synthetic oils.