Well the strange thing is that even with a sticking choke... if everything else is alright... it will still start... but will foul a plug within 5 minurtes of running.
On a typical mikuni.. the plunger goes in from the top.. so try to pull off the 'triangle shaped' thing and see what is under it... if I am guessing correctly... there will be a litle brass shaft with a barrel on the end down inside the carb... and it gets pulled up and down by the diaphram.. if you get to it.. take the whole shaft/barrel piece out and make sure it goes all the way to the bottom of its travel.. then wire it so it can't come up.
OK... more thoughts... some of what is happening here is just not right... maybe your flywheel has sheared its key and the spark is arriving at the totally wrong time.
You will prob have to remove the flywheel to find out.
(I say this because if this carb is new then the choke shouldn't be the prob)
You aren't running diesel fuel innit are you ? :)