‘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

[woody]

Does the plug get wet? If not, as it has been left for 20 years, a blocked pilot circuit in the carb is a possibility, if a pilot jet or aiway is blocked it can be vitually impossible to start a bike. Put fuel down the plug hole or spray easy start into the carb, if the bike runs but quickly dies, chances are it's a blocked pilot circuit, try having the carb ultrasonic cleaned

You mentioned the ignition timing, check that the woodruff key hasn't sheared when you've been kicking it and the flywheel moved out of position

[Plymouth]

On 3/1/2026 at 8:40 AM, woody said:

Does the plug get wet? If not, as it has been left for 20 years, a blocked pilot circuit in the carb is a possibility, if a pilot jet or aiway is blocked it can be vitually impossible to start a bike. Put fuel down the plug hole or spray easy start into the carb, if the bike runs but quickly dies, chances are it's a blocked pilot circuit, try having the carb ultrasonic cleaned

You mentioned the ignition timing, check that the woodruff key hasn't sheared when you've been kicking it and the flywheel moved out of position

Yup plug gets wet. I have tried starting fluid many times but no luck. Woodruff key is still in place, I’ve had the flywheel off to confirm that. I can see light through the pilot jet and main jet, I’ve had the carb apart and cleaned a couple times thinking a stuck float or blocked jet was the problem but no luck.