I’ve rebuilt the carbs, installed new #2 coil pack and ignition module unit was bad not firing #2 coil pack, replace regulator rectifier unit not charging, replace head gaskets and exhaust cover gaskets, new thermostats, new plugs. Engine has about equal compression on all cylinders about 100-110 psi. Engine runs great at idle and part throttle, at full throttle it starts to loose power/rpms until ign switch is pressed in and priming/choke circuit is active then you can hear engine come up to speed. Engine will run good for about 10 mins then rpms will droop again. Rpm readings at full throttle about 5000-5500 when engine operating correctly, when rpms dip its running at 4500-4600rpms. Any help would be greatly appreciated on this engine. Am I missing something simple. PS pistons and cylinders look good no scratches or gouges, pistons seem loose like there’s some play but great compression and engine starts great with the fuel system primed. It’s kind of cold natured.
A simple test would be to run it on a completely different fuel source.
I had an older boat where the pickup screen at the bottom of the fuel tank was clogged and I'd intermittantly lose RPMs @ WOT. I added a Racor f/w separator and removed that screen.
Or have someone pump the primer bulb when it starts to do that - if it recovers, it could be the fuel pump diaphragm.