why is my mac2816 string trimmer not revving up to speed?

After sitting up over the winter my Mcculloch 2816 string trimmer restarted rather easily (I did use fresh gas and the 40to1 gas-to-oil mix), but now, compared to my end of season use last year, the trimmer seems severely handicapped with less power, less high speed rpm. (When I finish using
the trimmer, I have always drained out the gas tank, tho I rarely ran it till it's out of gas; perhaps this is the problem?? ...after almost
10 years of use like this...?) The law here in AR doesn't require me to have a spark arrester, so there is none; I've cleaned the air filter, but haven't done anything with the fuel filter -- ever. The spark plug is gapped correctly: .025" . What's wrong?

winnerarkansas
April 2006