‘74 Montesa won’t run!

[Plymouth]

Hi all! 

I just joined recently to see if anyone might have a potential solution to my problems with my Montesa Cota 172… hopefully I’m in the right place.

The bike has been in my family since new, my older brother was the last to use it 20 odd years ago. 

I got it with the intent to get her running, here’s a list of what I’ve tried to do to get it going;

1.Replaced crank seals

2.Checked spark (bright blue and consistent with a brand new spark plug, verified coil is working correctly) 

2.Verified timing (hasn’t changed since new)

3.Rebuilt the Amal 620 carburetor (tickler works as intended now) 

4.Cylinder leakdown test passed

I’ve tried various fuel mixes ranging from 32:1 to 50:1 like my Yamaha Ty250, but no luck.
 

ALL of this and she will only sometimes give me a bang once and a while! I’m completely at a loss, I’ve used up all of my mechanical knowledge here. Any advice would be appreciated, thanks! 

[greychapel]

Check that displaced wadding hasn't  blocked the exhaust