The carburettor has 2 tuning screws.
These should be tightened (not too tight!) and then slackened 1 turn
(ie 360 degrees) to give the correct tuning.
If this doesn't work, judging by your symptoms, you can try opening the
low speed screw (the one nearest the engine block) a little (1/4 a turn
or so)

I don't think it is the gauze in the exhaust (spark arrester) as that
would prevent it revving as the previous poser says. A clogged fuel
filter would have the same symptoms.
Are you sure the fuel is OK? - stale fuel is a common problem with
similar symptoms. Have you tried emptying the fuel tank and using
freshly bought fuel - run it for a minute or so to clear out any old
fuel?

As another poster says, crankcase pressure can cause problems but IME
this prevents running completely and is a very rare occurrence.

I don't know which plug this machine takes. I would pop into your local
friendly independent garden machinery repair place with the old one and
they will give you a new one to match.

I'm not convinced about the plug being the problem either. I think that
if the tuning or fuel doesn't sort it then I would have it
professionally looked at in your local repair place. My money is on a
carburettor problem. Please let me know what the problem is. I am
intrigued.

Alan.