1997 merc 200efi...gauge issue???

[blue wonder]

hi im new to this whole outboard thing...just bought my first one...its a 1997 champion 19' elite bass boat...it has a 1997 merc 200efi on it...the problem i have is the tach, fuel gauge and trim gauge all bounce around like crazy...sometimes the tach reads 4k idling and sometimes it reads 0...also at speed it is all over the place...ive looked under the dash for a ground wire loose or off and cant find anything...where does the ground hook up to the motor at???...or does it go to the battery???or is this even my problem???...and suggestions???...im trying to do some prop testing to get more top end out of the boat but with a tach i cant trust idk where to start

[Capt.Insane-o]

You probably have a power supply or ground issue. Not knowing what wiring harness, or if champion used their own, you'll have to look further at your wiring and all connections. The gauges should be powered and grounded in series. You can try hooking up a direct power/ground to the battery and see if that corrects it.