'94 350 Aft Cabin Engine Conk-Out Problem

R

RHock

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Joined
May 3, 2013
RO Number
33242
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7
Our '94 350 Aft Cabin has a strange problem. At least 4 times out of the last 7-10 times we have returned from a cruise (of 1 to 6 hours) the following has happened. We get back to the entrance of the short channel to our marina. We cut our speed back to about 3-4 knots, then as we get into the marina, before we make our first turn, we start maneuvering with the engines rather than the wheel. This is done so we can maneuver the next two quick turns into our slip. The problem: At some point between the channel entrance and the first turn, the port engine conks out. It will usually briefly re-start a few times, we get the rpm up to perhaps 850 rpm and then the rpm goes down over a period of 4-10 seconds. During that time the throttle has no effect whatever. A re-start is attempted 3-4 times then it fails to catch at all. Now for the stranger part: The starboard engine does a similar thing while the port engine saga is taking place, but we usually get it restarted and it gets us to our slip. Meanwhile sometime during the first few minutes of the whole process the chartplotter has turned off. (Our chartplotter is brand new. The previous one did the same thing.)

A somewhat similar event has happened a couple times when out in the river. Two people have suggested a "ground wire" problem. So far I haven't found a problem in the ground wires.

Sorry this is so long. Any solutions will be immensely appreciated. (By the way, the carburetors were re-conditioned just 10 engine-hours ago.)
 
Definitely electrical. If there is enough length, unplug the engine harnesses and plug them back opposite each other. If that causes the problem to swap motors you can eliminate the problem at the motor and in the harness. If it doesn't swap motors it is in the harness on the motor.

Do you turn on your chartplotter before or after you switch your keys on and start the motors? The start relay may be cutting power to the plotter circuit when it cranks the motor. Similar to a car.

Last, if you have not checked the motor ground that is they first place I would start. You would be amazed how quickly a corroded one of those can muck things up.
 
Suggest you post your question in the engines forum as it is an engine problem, not a Carver problem.
 
I don't think this is an engine problem at all, I think this is an electrical issue with possibly the engine charging system or ground wire.

Are you sure your alternators are charging correctly? The lower rpm may be bringing the alternator down this the battery down enough to kill the engine. When trying to restart if your plotter goes out there isn't enough voltage to keep it live.

If they are 454 crusaders there isn't much of an engine harness to speak of.
 
Many thanks for the very helpful (and quick) replies!

John and 370 -- I'm really relieved to hear your opinion that it is an electrical problem and not an engine problem. An electrical problem may be a bit harder to track down, but (I hope) probably a lot less expensive to fix. I am going to follow up on all the things you suggest. They make a lot of sense. Regarding when I turn the plotter on, when I an shoving off, it is engines, then plotter. Once the engines have gone down I think I usually don't turn it back on after the engines have failed.

Charlie -- The engines are 7.4 L Mercruisers. 490 hours each.

Again, Thanks to all! It's great to have the forum and its time-generous contributors as a resource, especially for those of us who are low on experience (and some days not too hefty on smarts, either.)

Ran
 
The repeatable timing when coming off cruise may be significant. Don't have any knowledge of your boat but whenever you throttle down an engine that is at temp I worry about vapor lock. The engine is hot and the fuel flow slows down so the gas spends more time in the fuel line and may boil. If your burning fuel from last fall it may be winter fuel (lower vapor pressure) and more susceptible to vapor lock.
 
Coil or alt are possible culprit. Also check belt on alt. Alt increases resistance with increased demand. Might need someone watching belt while underway.

along the lines of fuel problem.. check that the choke is open. Also float could be too high...maybe sticking open. Gonna need more info about the way the engine acts when throttling and nothing happening. Got me a bit stumpted.
 
Sounds like the problem I had just before it was tracked down to a bad distributor on my port engine.
 
I would go along the same lines as Tommyfmu on this too. It could be heat related to your ignitions. Mine had that and I spent 4 seasons trying to figure it out. Ended up putting in electronic ignitions and distributors and fixed my issue. The heat and stress on the distributor over time would break down your spark and cause this. Over time, you will start to see it impact while you are under way, the engine will begin to stutter.

If you let the engine sit, turned off, for 30 minutes to let it cool off, are you able to fire it back up and idle without issue?

Do you have points and condensers still and have they been replaced? If you still have these, you may want ot have them replaced or go with electronic ignition.
 
I had that issue on my last boat for two years. Turned out the plug wire from the coil to the distributor not in all the way. The boot was over it but the metal was not was laying over the top of the post up in the boot, not even touching metal for a connection. Our theory was that the boat would run great at first but the coil would overheat and cause issues when docking.
Jim
 
Check timing advance, on the 84's the advance weights rusted and it depends where they get stuck where they run right..
 
I think you should put this into the engines forum. Likely to get more responses. This IS an engine problem. Engine problems can be separated into mechanical, cooling system and electrical/electronic. Besides, read the title of the engines forum.

The throttling with no response sounds like the ignition system just shut down. There are many possibilities along those lines alone. We're gonna need to know all the details of the type and components in the ignition system.
 
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