Ok my brother has a ski boat (I know, I know).

It has a 351 windser Hi output, He bought it at the beginning of the season, and it needed a bunch of maintenance. It was backfiring, but not anymore.

It has new plugs, wires, cap, rotor, rebuilt carb.

Motor sounds strong, but has a miss while idiling, and when you take off its VERY sluggish out of hole, but top end is fine.

It does have a minor exhaust leak.

Where would you start???

Carb, what kind? Is the accelerator pump working and adjusted correctly. Vacuum secondaries or Mech.? Vacuum at idle? Vacuum needle boucing?

I'd check cylinder cranking compression, manifold vacuum, idle timing and 3000 rpm timing.

Fuel Filters !!!! especially in a marine application. Also a miss at low RPM can be anything..bad plug gap ..wires..how about what kind of plugs are you running.???

What kind of carb
What kind of ignition
What specs are the engine set at
What do the cap & rotor look like
When is the last time the fuel filter was replaced

If you have already replaced normal tune up parts make sure that you haven't crossed any ignition wires first off. If all OK then do a compression test. How many hours?

holley carb. the plugs wire cap, and rotor are all new and the correct ones. I know he bought them at the dealer. The wires arent crossed and it runs strong at wot, its just taking off.

Fix the exhaust leak. Then try again.