I have a boat with an inboard/outboard Volvo Penta 350, which is essentially a GM smallblock modified for marine use. It has a mechanical fuel pump, electronic ignition, and a Holley marine carburetor. I bought the boat used a couple years ago, so I don’t know the history.
The boat, when I first got it, seemed to operate perfectly. Started right up. Idled fine. Out on the water, it ran great.
About the third or fourth time we had the boat out, after an hour or so of running around the lake, the boat sputtered and stalled. I could barely keep it running by careful throttle control. The engine would run ok at low RPM, but if I tried to speed it up, it would sputter and quit.
The next time I brought the boat it, it did the same thing, but after only a short time on the lake. Again, I was able to limp it home by keeping the RPMs low, and pumping the throttle when it wanted to quit.
The problem has gotten progressively worse each time I’ve had the boat out, so that now, I can barely leave shore and as soon as I get on the gas, the engine stalls and will usually not restart, though it does sound like it wants to start.
If I come back the next day, the boat starts up and idles perfectly again, though the problem occurs every time I get the boat out and give it gas.
I felt sure it was a fuel problem. First I changed the two fuel filters (including a water-separating filter after the fuel pump) and checked the fuel pickup inside the tank. Then I tried running the boat off fresh gas. The problem did not go away. I even ran my car off gas from the boat’s fuel tank without trouble.
I rebuilt the carburetor. I had exactly the same problem. I replaced the fuel pump with a new one. The problem hasn’t changed.
A good friend suggested that the coil might be bad, so I swapped coils with one I had in the garage. No change, the boat still stalls as soon as I get going on the water.
I’m at a loss. Suggestions?
Jamie
I can sympathize with you. I’ve chased many boat engine problems that were very difficult to fix. Many times, the engine will run just fine unless you put a load on it so “driveway testing” does not produce the symptoms as it does on the water. Even outboards in a bucket do not adequately load the engine so an I/O is even tougher to load down. Here’s my suggestion- although there are many knowledgeable and well intentioned people on this site, you’d be better off posting this on a dedicated boating site to gain access to professional boat mechanics. I frequent iboats and there are many people there with direct knowledge of your exact set up. BOAT is an acronym for Break Out Another Thousand… 
I suspect something in the carb, sticking float perhaps? Can you see lots of black smoke from the back of the boat to tell you it is running way too rich? Could the choke be going on, or staying on improperly? Have you cleaned out the spark arrester? The arrester can look ok but still can be blocking free flow of air.
If all the carb issues check out is there excess water pressure in the exhaust system? Perhaps there is carbon somewhere? Where does the exhaust exit the boat? Through the hull exhaust? Through the prop? Or, does the exhaust exit elsewhere on the outdrive?
Just some items to check as you troubleshoot the problem.
If the engine has an HEI ignition system the problem is usually a bad ignition module. They are really intermittent when they go bad. It sounds like a bad ignition module.
HEI modules will sometimes fail in this manner. This is such a relatively simple and inexpensive swap I would recommend doing it. If it isn’t the problem you’ll have a spare module that you’ll likely need one day.
For the electronic wizzards here, HEI modules will sometimes fail to increase the dwell as RPMs increase. At Idle they are designed to run a dwell of about 10* but as RPMs increase the dwell should pull out to 30*+ at 4,000 RPM. Failure to increase dwell will limit the spark severely. It is not a common failure. I don’t think I have seen it more than 5 or 6 times.
If you haven’t done it so far, look at the rotor. I remember that a lot of the HEI distributors would burn a hole through the rotor to the advance mechanism effectively shorting out the high voltage.
If that isn’t the problem, I would just replace all the high voltage components, cap, rotor, and wires. One advantage of having the engine ‘inboard’ is that you can check its operation while under the failure conditions. While someone else operates and navigates the boat, you can check the spark timing and fuel flow (using a mirror).
Let us know what you find on this.
Is the coil built into the distributor cap?? That’s HEI. If it has a separate free-standing coil, that’s some sort of after-market system…
Well it’s gotta be fuel, spark or compression.
You need to test each one rather than guess at what part to change or rebuild.
My first guess would have been fuel since marine fuel tanks love to collect trash, but it sounds like you’ve got that covered.
Check the carb throat for the accelerator pump stream when it’s acting up.
There are several ways to check for spark strength using a tester, an extra plug or just a well insulated screwdriver.
I doubt it’s a compression problem, like burned valves.
I would however feel better if the used marine engine I bought had a compression test.
Salt water or fresh water? The heat exchanger elbow, where the “raw water” is dumped into the exhaust can become plugged up with corrosion choking off exhaust flow. The engine will idle, but that’s about it…
If you have vacuum advance, check for frayed wiring at the ignition pickup; it’s in the distributor. This isn’t a Chevy problem and is rarely the problem with any engine, but I have seen it before. When the breaker plate moves, the broken wire pulls away and opens the circuit. It would just be a shame to miss it, if it is the problem.
When it stalls and won’t start, can it be started by pouring some gas in the carb or using starting fluid? If so, that would pretty conclusively mean it’s starving for fuel. Have you tried disconnecting the fuel line at the carb and holding it over a bucket while cranking the engine? That’s a quick and dirty test for fuel volume. Do you smell gas when cranking it when it won’t start? Have you tried taking one of the plugs out to see if it’s saturated with fuel? You can also lay one of the plugs on the engine with the wire attached and see if you get spark when cranking.
I had a problem somewhat similar with a 7 horse 2 cycle engine, it ended up being a bad head gasket. A compression check would be a good thing to try.
We had a similar problem with our Volvo engine. The trouble turned out to be a slipping timing belt. I don’t know if your model has a chain or not.