Offshoreonly.com - 1990Bayliner CIERA 2556 Difficult to restart the engine

Please help us.
We’re working on repairing the following ship’s engine, but it doesn’t work.

•The engine is 7.4L BRAVO GM 454 V-8
•The ignition system is Thunder bolt5
•The engine serial number is OC869161

<The condition>
When the engine is cold or immediately after warm-up, it does work.
When over 30minutes have passed after the engine stopped, it doesn’t work even with the accelerator pomp.

The engine gradually starts up, when we start the starter motor and open the throttle slowly. It also does work, immediately after this, but after over 30minutes, it doesn’t.

<already replaced parts>
Fuel pump
Fuel filter
Ignition coil
Spark plugs
Picup sensor
Distributor cap
Rotor
Exhaust
carburetor
Thermostat
Rush adjuster
Intake manifold gasket
Starter motor