Re: 470 performance problems
You probably ought to do the whole check up & set up on the motor as you were starting to hint. Check or change plugs, wires, cap, rotor, and timing. Check & set choke. Verify that choke opens fully when warm. Set idle mixture screws. Check for air leaks around carburetor base and intake manifold with a squirt of carb cleaner.
I know I make it sound trivial, and I know parts and tools have a cost. If you don't have a timing light they probably have rentals or loaners at chain auto parts stores. It's good to have a shop tach too, old boat tachs can be very wrong-I have had this fool me more than once.
I don't think I'd do that high throttle shut down thing, there's a possibility that the engine could diesel, which is running on without spark. That can happen if the combustion chambers have enough stored heat to ignite the fuel without spark, dieseling can cause engine troubles. When you shut down at idle with the timing right there is so little fuel that dieseling is not supposed to occur.
You would not get the spark plug analysis that the other poster recommended though.
It still would be a help to photograph the used plugs and post pics, lots of folks on here know how to analyze them.
