Atomic 4 troubleshooting an odd problem? - SailNet Community

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Old 08-27-2008
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Atomic 4 troubleshooting an odd problem?

The boat I regularly crew on is powered by an Atomic 4 motor and we have an odd problem with it.

The motor normally starts up easily at the dock. Powers us out to the race course easily. It starts up easily after racing and powers us into the dock easily. It seems to idle well, and run well during all of this.

A couple of times each year we do longer deliveries, and schedules dictate that as much as we might like to sail, we end up motoring or motor sailing. We do not let the boat heel much if we are motoring. We run the throttle at about 75% when using the motor.

On the longer deliveries, when we end up motoring for long periods of time, the motor works fine, until we try to slow it down. As soon as we try to slow the motor it will often stall, or won't run slower. We either have to rev it up, or it wants to quit. It still runs fine at 75% throttle. If it stalls it is very hard to get started, to the point at times it won't start. If it does start it does not want to run unless rev'ed up, and even then it no longer runs as smooth as it was running before.

The problem does not appear to be heat related. When this happens, we have sailed the boat into dock (fun, especially a strange dock at night) left it for 7 hours until morning, and still had it showing the same problems. We got a tow out of the marina for that race because the motor still wouldn't run. Then after the race it fired right up, and we had no more issues until after the next 8+ hour motor-sail.

The engine is well maintained. New points, condenser, spark plugs, coil, the carb was rebuilt totally, and the problem still exists.

What do we try next?
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It sounds like you have one of two problems: either a clogged fuel filter or a bad fuel pump. The diaphragm in the pump will get tired after its been run for extended periods, then as the engine idles down, can't pull enough fuel. Or possibly the built-in check valve is getting sick. I had the same problem with a Volvo diesel, but the mechanics are the same.

Personally, I'd remove the mechanical pump and switch to an electric fuel pump for a couple of reasons. The main one is that a mechanical fuel pump, which runs off a lobe on the cam, steals a large amount of horsepower. The actuator rod on the pump also rubs on the cam, and puts minute metal particles into the oil, which is never a good thing. Secondly, the electric pump makes its own particular noise, and that makes it easy to hear a problem in the offing.

Hope this helps.
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I was thinking about changing to the electric fuel pump. But to some extent I don't like making changes based on guessing.

I don't think it could be a clogged filter, or how could it run for 8+ hours at 75% thottle? And the filter has been changed, fuel tank replaced, and it still has the same problem.

(The tank was not replaced due to this issue. It was a 30 yr old tank, we were replacing the floors in the boat and replaced the tank because it was opportunistic.)
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I'd second he fuel filter/fuel pump solution.
change the fuel filter first, then, either rebuild the pump or remove it and install an electric.

It doesn't take much gunga-munga (technical term) to inhibit filter flow.

(geez, don't go hog-wild on those hi-performance race pumps, you need about 4lbs of pressure to run the motor.)
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If the filter was changed, it isn't the filter obviously... so the fuel pump has got to be the problem.

BTW, the reason the fuel filter might show similar symptoms is that as it clogs up running the engine at higher speeds provides enough vacuum to pull the fuel through a partially clogged filter....but as soon as you slow down, the engine doesn't generate enough vacuum to draw the fuel through the clogged filter media and starves, then dies. With an electric fuel pump, you aren't constrained by engine speed to generate sufficient fuel pump pressure.

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Originally Posted by KRCSWO View Post
I was thinking about changing to the electric fuel pump. But to some extent I don't like making changes based on guessing.

I don't think it could be a clogged filter, or how could it run for 8+ hours at 75% thottle? And the filter has been changed, fuel tank replaced, and it still has the same problem.

(The tank was not replaced due to this issue. It was a 30 yr old tank, we were replacing the floors in the boat and replaced the tank because it was opportunistic.)
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Old 08-27-2008
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are you sure you have the right plugs and the gap is set right?
they could be fouling and causing problems with hot restart if so.
a way to know if fuel is the issue is make sure the fuel line is primed then close the fuel line to the tank while its running and the engine should run good for a minute or so then stall.
during this you should have enough time to go thru the rpm range and see if its still acting up.
if not, then chances are a bad fuel pump,clogged vent,bad fuel etc..
also check the wiring from the coil to the distributor for continuity.
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Hmmm... Sounds like carb, to me, but I'm hard-put to imagine what failure mode would result in the symptoms you describe.

Try posting over in the Moyer Marine forums. Maybe Don Moyer will have an idea.

Jim
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First I would check all the clamps on the fuel line from the tank to the fuel pump. If that does not fix the problem replace the fuel line.

Rick
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This isn't a solution, but you might want to keep a can of starting fluid on board. This will keep your starter and your battery from getting unnecessarily taxed.
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Old 08-28-2008
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The problem existed before the carb rebuild, and after.

The fuel filter is changed annually. This problem has plagued us for the last few years. Changing the filter has not made a difference.

The plugs are also changed annually. They are gapped correctly before install. They don't seem to make a difference.

If we run the motor for 1-3 hours we don't seem to have an issue. It's only after longer running times. Say 6+ hours. We only motor that long a couple of times a year. Otherwise, the motor works beautifully.


I guess we prepare to change the fuel pump, and hope that cures it.
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