When a 2 stroke is run but not under load the mixture in the crankcase dosent get fully forced into the combustion chamber via the transfer port and some remains in the crankcase. The air fuel mixture then gets very rich and droplets stay in the crankcase causing the motor to 4 stroke, also happens on the overrun if you dont occaisionally blip the throttle, this then often olis the plug or at least causes the motor to be very hesitant when opened up again.Two strokes like being used on 1/4 plus throttle openings and benifit from occaisionally being revved out. Dont expect a 2 stroke to behave like a 4 stroke. Well that is what i've found over the years.
Re the carb if yours is an Amal concentric and has a choke lever then it is probably a Mk 1 1/2 , i have one fitted to my James, which came without a tickler but instead had a choke or more correctly an enrichening lever. If it is a mikuni as Woody says they are very prone to flooding especially if the bike is facing downhill. I have one fitted to an Ossa and it regularly floods when going downhill or off drop offs.
BTW be careful you dont break anything on the Montesa as spares are quite hard to come by.