89 120hp rough at low and mid range throttle

I just put this motor, J120TXCEM on a boat I just bought. Motor starts on first revolution, idles ok and sounds like it's missing up to about 3800-4000 rpm then takes off. Once on plane and at 4200 rpm the rpms and speed will increase, usually to 5000 rpm. I have to be attentive to the throttle because the motor will gradually rev up and down. The motor sat for over a year and maybe two. Prior to starting motor I removed all old gas from tank and cleaned the tank, drained the car bowls and ran clean gas through them, replaced all fuel lines and filters and had to replace a powerpack. The plugs had a black coating on them, compression is 115-118 on all cylinders. I have not adjusted timing etc. I bought the motor will controls that were set up on another boat and just left everything as it was.

After the first run and having the missing I replaced the plugs with QL77JC4 which is what the manual calls for, but with no change, these plugs come out a little wet. The plugs that were in there were L82YC. Should I go back to the original plugs or does this sound like a fuel problem. Thanks, John

I bought four carb kits and installed them yesterday, there was some debris in carbs. Motor runs better but still breaking down at mid throttle. I made one of Joe's spark testers and the spark would not jump the 7/8's of an inch and trie with a phillips screwdriver with a weak spark about 1/2 in or so. Any ideas. John

7/16" spark is good for those.---------Timing is fixed at the factory and should not require adjusting.----You can however run with a timing light to see if there is an ignition miss at mid throttle.

Racerone, I meant to type 7/16 instead of 7/8. I'll try the timing light. John

Tried the timing light today and all cylinders seems to be firing through the throttle range. I took the plugs out and three of them have black insulator and electrode, the fourth has half black and half whitish color on insulator. I have no idea if that means anything. Also tried choking the motor when it's breaking up and it just makes the motor bog down more. Any help is appreciated. John

Well my motor is running better. Cleaned the other wires that are attached to each coil on advise of a local mechanic, then went through the whole synchronizing and timing procedures outlined in Seloc and the WOT timing Joe Reeves describes. Motor still breaks up around 2600 to 3400 rpm but not as bad as before, then smoothes out and runs nice above 3500. I think I'll advance the timing 1 degree, then retard 1 degree from where it is now to see if that does anything. John