Hello, I've recently purchased a 348/349 montesa cota! The frame number is 51m20333 and the engine number is 51m00221. Can anybody help identify what model and what engine it has? I want to buy the correct sized carburetor for it  as it runs very poor! Cheers.

I would do a leak down test, before buying a new carb for that beast..

2 hours ago, lineaway said:

I would do a leak down test, before buying a new carb for that beast..

Hello, it's a great bike. What is a leak down test? The bike ticks over but as soon  as you want more revs the bike backfires and fluffs big time. Not rideable. Tried a 24mm carb from a fantic and ran slightly better, but still not rideable . The carb on it is unrecognisable. No numbers. Cheers.

Doesn't matter if it is a 348 or a 349, a 26, 27 or 28mm carby would be ideal.

If you post up a photo of the carby someone will be able to ID it. Same for the bike and motor.

The engine number is definitely a 348 and frame is likely to be a 349 but 349 frames change over time, hence the photo.

1 hour ago, tried said:

Hello, it's a great bike. What is a leak down test? The bike ticks over but as soon  as you want more revs the bike backfires and fluffs big time. Not rideable. Tried a 24mm carb from a fantic and ran slightly better, but still not rideable . The carb on it is unrecognisable. No numbers. Cheers.

Back firing usually is timing. A leak down test is to check the main seals. It is well known that the 348 can have leaking main seals.Put your money where the problem is.

Timing could be sheared woodruff key.      I have seen these bikes with the piston installed backwards as well. Good luck.

Thankyou for everybody's help. I measured the carburettor and it's a 28mm Mikuni. So i dn't think its that. Going to to address tha timming, points and condensor, also the leak down test next. Cheers 

 

 

I assume we have a brand new plug in it at this point?

56 minutes ago, bultobill said:

I assume we have a brand new plug in it at this point?

Yes, brand new plug, which oiled up quickly.

Fresh fuel mixed at the recommended ratio. If you are unsure try 33:1 semi synthetic two stroke. Ignition timing on the 348 is 17.5 degrees before top dead centre. When you remove the flywheel to check the timing check the crankshaft is not damaged and lap the flywheel to the crank for a perfect fit as the woodruff key is only to locate the flywheel in the correct position. The flywheel nut is left hand thread and torqued to 72 lb ft. When you re-install the nut it looks like it is not on far enough but don,t worry they are all like this. If the crankshaft oil seals are worn the bike will either be very difficult to start as it sucks in too much air and makes the mixture very lean. They can also suck in gearbox oil and oil the plug up.(you will get a lot of white smoke) A Mikuni 28mm carb is a common retrofit for bikes with worn Mk2 Amal carbs. 26/27/28mm carbs should be ok for these bikes. Check the carb for worn slide,worn jet needle and needle jet etc and the float height is correct. Even if you think the spark plug is ok put in a new one as they can look ok when checking for a spark outside the head but break down under compression. It is a case of checking everything methodically to rule it out and you should get there in the end. I don,t know the recommended jetting for a Mikuni carb but if it is a direct conversion kit the jetting should be reasonably close. Also check for air leaks on the inlet side and check the exhaust ring is tight . These bikes can also run very poor if the exhaust is completely coked up and the baffles are blocked. Good luck with getting it sorted.