Up until just a couple weeks ago my 96' Wellcraft Scarab with a 454 Mag MPI has been running flawless; then one Sat we went out and I noticed as I started to throttle up to get onto plane the motor started to run really rough. What I mean by rough is, that it sounds like its running on 7 cylinders. No coughing or backfiring, just not like it usually sounds. I backed out of the throttle and it went back to idle and sounded fine. Again as I tried to get up on plane it started running rough, but I continued to throttle and at about 3000 rpm it cleared up, so I just assumed maybe I had got some bad gas. All day the same scenario happened, so I ran quite a bit and tried to run out as much of the tank as I could. Other than when coming up on plane, the boat ran great. I took it to my mechanic and had him run the scanner on it, and no codes came up. We went ahead and changed the fuel/water filter, plugs, plug wires, and dizzy cap, and had him to check the timing just to eliminate the cheap stuff first. After filling it up with gas from a reputable station at our lake, we took it back out this weekend and I'll be damned it's still doing it. It seems to run rough between 1800 and 3000 rpms, but that's it. If anyone can help, I would really appreciate it. Thanks