I don't own that keyboard, so I cannot say for certain, but "1+8" sounds like it could be something to do with playing the root note of a chord, plus the note one octave above it. So, as an example, perhaps a C-major chord would be sounding the notes C, E, G, and then the C one octave above the first C, instead of just C, E, and G. But I'm not sure why that would sound bad. Or, maybe "1+8" indicates that it is only playing an octave -- the root note, plus the note one octave up, with no other notes. That would certainly affect the sound, because you wouldn't be getting full chords.
How are you playing your chords? Single finger? Or are you fingering the chords? Beyond that, I imagine there is a selection of auto-accompaniment chord/fingering types to choose from -- I'd check that setting.
EDIT: I checked the manual and reference guides online -- It seems there is a "1+8" type of "chord" recognized when you are in one of the "fingered" modes of auto-accompaniment, and it does just play the octave -- the root note, and the note one octave above it. My guess is that you want a full chord background (like a full major, minor, or seventh chord) but are just playing an octave with your left hand in auto-accompaniment. Based on what I saw in the manual (again I don't own the keyboard, and have no direct experience with it), that will not work. In a fingered mode, playing an octave will get you just that -- two notes in an octave. If you want a full chord, but without playing all the notes of the chord, then it looks like you have to use "single finger mode", and then play the notes as described in the manual to get the desired chord -- such as just playing one note to get the major chord for that note, or playing one note and a black key to the left of that note to get a minor chord.
Again, I'm just speculating here -- like I said above, let us know how you're playing the chords or notes on your left hand, as that may help more to determine what's going on.
