1997 Challenger running rough

[squad1]

Newly rebuilt single 787. Got it in the lake yesterday and after a little rough 1st 5 minutes of low speed/rpm figure eights near the launch with about 6 stall outs, I had the boat running well. No major hesitation, kept the rpms to 5500 or less to start the break-in process. Kept her out for about an hour of the same. Took a break and let everything cool down, went back for another round and started to get the dreaded won't go above 5k rpms. Boat just feels like it has no pull. Same thing today, starts and idles strong, tach showing right at 1500, but sputtering a bit with throttle and won't get above around 4.5k. Sputtering seems to be a bit more pronounced. Plugs look like a decent coffee color, PTO may be a slightly darker brown. Carbs had been rebuilt with genuine Mikuni kits just before the rebuild. The new engine only has the time mentioned above on it, except for a few minutes on the hose at home.

I've searched and read a bunch of threads trying to find one that describes something similar to my situation, but so far it has proven difficult on mobile. Any ideas where else to investigate? At the lake with limited tools and I left my compression gauge at home...

Thanks in advance.

 

[squad1]

Rechecked the plugs and noticed they were no longer coffee colored, now a darker carbon color. Tried the choke pull, and engine started to die out, so pretty confident it's not running lean.

Put in a new set of plugs, and aside from a little low end lag, the boat ran well on plane. Still kept the rpms down around 5.5-6k since the motor is still being broken in.

Is it normal to foul plugs while breaking in a rebuilt?

 

[squad1]

Forgot to add that I reset the low speed screws and opened to 1 full turn. High speed still fully in.