Had my Craftsman push mower (6.75HP mulcher) for about 8 years and while recently mowing under a tree the mower coughed and died. Restarted fine, but now the engine runs like a 2.5 HP mower and does not have the same power it had. Checked the plug - clean. Oil good. Gas fine. Under the mower there is no obstruction around the blade or shaft. It starts up and runs fine, but seems to have alot less rpm's. Checked the throttle - maxed out. Air filter good. Any thoughts?