My Suffolk Punch wont run without the choke open?

I have been told that if I continue to run it like this it will run (rich) which is no good for the engine.

It runs fine with the choke a 1/4 open but as soon as I try and close it to cut the lawn it slows, then the blades stop spinning and eventually cuts out. This is a new problem and I have been using the mower for a year without any problems at all.

What action do I need to take to fix this problem?

Your help is greatly appreciated as I love this little mower.

Thanks.