Marner 200 HP 2.5 missing a cylinder- running rough WOT

[SC Bullet]

1995 Mariner HP serial# OD935921

Last weekend motor ran good no issues then started making strange noise like plug fouled or missing a cylinder half throttle then would sound fine WOT. Toward end day motor sounded rough when coming out hole like it was missing a cylinder then sound fine WOT. Motor idles fine. Could I have gotten some bad gas? Where to start looking first?

Thanks

[JOE54]

check for spark on all 6 , replace plugs , mabe you did foul one

[lpugh]

compression test first thing, got to make sure the motor is sound before making other repairs.

[billnorman1]

compression test first thing, got to make sure the motor is sound before making other repairs.

While you're taking the plugs out put them in a piece of cardboard with the cylinder number don't mix then up. If there is any damage to a plug or metal transferred onto them, the story was already told.