new motor 509/550hp big block chevy,1.50 grear bravo drive,2000 awsome 260fx 26 foot cat, no throttle response-climbs to rpm slow/ also rpms are all over 4100-5100, my msd box is set for 5600 im running a 24 pitch prop tops 71mph on gps, and everyone else is running a 28 pitch prop with 500 hp running 80's on gps, any ideas I changed the carb from a 950 to a 750 just to try no change so i put the 950 back on, no back fire at full throttle.

hows the ignition system?

Timing is off. And or, weak coil, Bad MSD box, Springs loose or worn out inside distributor. Probably should have that engine set around 32 degrees total.

Check engine grounding.

Ah Yes