Contrats on your purchase, and welcome to the forum. We look forward to pictures.
The Tuliped Valve problem is normally on the Intake valves, usually caused by being "over-propped" (too much pitch), especially on under-powered cruisers using high power at low rpm. If a cruiser does a lot of "plowing" for extended periods (not quite up on a plane), that could be a cause. And as you stated, your symptoms indicate an intake valve problem. A 328 with twin 7.4s does not sound underpowered, so hopefully you just have a sticking valve that would improve with some use. Hopefully you can tell with your borescope examination.
There is no reason that you cannot remove the head while the engine is in the boat, but it is not a horrible job to remove the whole engine either. 2 experienced people can remove an engine in 2 hours.
Your engine building experience will serve you well. For the most part, automotive parts can be used. There are only a few main differences with these marine engines:
1. Some marine parts are "ignition protected" (no open sparks) to prevent igniting possible fuel vapors in an engine compartment ("ka-boom"). These are the Starter, Alternator, and Distributor.
2. Carburetors must be marine to ensure fuel vents and overflows are directed into the intake manifold. Carbs and throttle bodies have spark arresters instead of air filters. Fuel filters must not have glass bowls. Crankcase ventilation is directed to the carb or intake.
3. Water circulating pump should be marine (brass or sst impeller), and the Thermostat should be marine (brass and SST contruction and 160deg) unless the engine is a closed cooling system with antifreeze. If one of your engines is reverse rotation, the circulating pump must be reversible (straight vane type).
4. These engine should not be built up like race engines to produce 500HP at 8000rpm, unless you are actually racing and prepared for a very short engine life. These boats require engines that can work with a heavy load at 3000-4500 rpm for extended periods, and still have long durability.
5. The water cooled exhaust manifolds and risers are a headache all their own, which you can look up lots of information about them on this forum.
6. Use corrosion protection every chance you get. Products like Boeshield T-9 or Corrosion-X for external surfaces, Permatex/Aviation form-a-gasket or Tef-Gel on threads and mating surfaces, OMC triple-guard grease on lubricated components.
http://www.volvopentastore.com is a good source for engine diagrams and part numbers, even if you are going to your local marine supplier.
Ray
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings