Before departure i did the run up checklist in a cessna 172, testing the left magneto i noticed a drop in rpm if more than 125 rpm, y cleaned the magneto as i was tough (rpm to 2000 and leaning the mixture for a minute). Then testing it again it was within the limits.
I departed an climbed to 8000 msl and did cruise checklist. On 2300 rpm and a light tailwind i noticed IAS didnt go higher than 80knots neither groundspeed. I tried leaning the mixture but it remained the same. Y ask for deviation to the departure airport and during descent at 1900 rpm engine had roughtness. Landed a tested the magneto and it was rought.
Now i am thinking about it, if i did well, or what was the procedure, i said ATC what was happening but i am not sure if it is an emergency or a single incident if It needed to be notify.