lawnmower not generating power

[Griffin]

Briggs & Stratton - model 10T502 0457

Hello,
My lawnmower is 3-4 years old at most and has been working fine up until 2 weeks ago when I had to cut some really long grass. It would start fine and then slow to a stop in the long grass. Then it started surging.
I have done the following.
1) cleaned the air filter
2) cleaned the carburator
3) replaced the carburator diaphragm
4) checked the spark plug
5) cleaned out the undercarriage of grass / debris
6) raised up the deck to increase the clearance between mower and grass.
7) changed the oil and gas
I have not yet sharpened the blade.

It now doesn't surge but it still doesn't sound like its spinning at full rpms and doesn't seem to cut effectively. It starts easily.
I've tried raising the deck and this helps somewhat but its still not spinning cutting like it used to, especially in longer grass.
Any thoughts about what might be causing this?

Thank you

 

[Boudreaux In Eunice La.]

Well I can relate a story to that might help.

I went to a buddy of mine that owns a outboard shop today. He works on chainsaws and weedeaters also. He needed some fuel line. He was cutting grass and hit a prop on a outboard motor and it knocked his governer plate out of whack. He had to take the filter box off and readjust it.

It might that on your mower ??