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Atomic 4 troubleshooting an odd problem?

9K views 26 replies 20 participants last post by  tommays 
#1 ·
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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#2 ·
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.
 
#3 ·
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.)
 
#5 ·
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.
 
#4 ·
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.)
 
#6 · (Edited)
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.
 
#10 ·
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.
 
#13 ·
KRC,
I'm thinking the problem is fuel-related. If this is the case, I think there's a reasonable chance that it has to do with either the fuel "stratifying" or that some crude is coming loose and mixing with your fuel after a long sail or motor sail. With the boat heeling, or just boucing through wave action, the stirred-up fuel could cause it to burn differently in the carb. The ethanol in the fuel could also be a contributing factor. I've had luck adding regular fuel stablizer. It takes a while for it to really improve the performance, as I've had to burn through all the old fuel first.
Good luck.
 
#14 ·
I will check the other forum for assistance, thanks.

The fuel line doesn't really run past anything that is really hot. Not to mention that allowing the motor to cool, overnight, doesn't always solve the issue.

I doubt the problem is crud in the tank related. Over this past winter we replaced the fuel tank with a brand new one. The problem existed before and after the change. I will admit, although we replaced the tank and the filter, we didn't replace the fuel line itself. It looked in good shape, and no gunk came out when we blew it out with compressed air.

If it was a fuel additive or such causing the problem, wouldn't it have issues all the time? And without changing fuels, what would then make the issue go away?

I'm not trying to discount anyone's ideas. We have been working at this problem for a long time and are quite frustrated. Thanks for the help.
 
#23 ·
As to the fuel line.

You are not looking for crud but an air leak. Fuel pumps hate air leaks!! Especially at an Idle!!

Try a couple of wrenches at the intake side of the pump or screwdriver and while you are at it run you hand along the fuel line and see if you can detect any fuel.

When you replaced the fuel tank did you use the same cover with with the old pick up and sending unit?

Does the problem seem to happen at about the same fuel level?? You motor to the race and then and then to the fuel dock and it starts fine after that. You motor to race and it runs ruff and you don't go to the fuel dock and it wont start?

Because this problem only happens after long runs (hours) at 75% throttle I am ruling out--

Vent--if it supplied air for that long should be good for idle.
Fuel pump--if it can supply fuel for hard long run it should provide fuel for idle, you can try running it at full throttle and see if the engine surges--a sure sign either the pump is going or the filter is not passing enuff gas.
Idle jet--clean fuel filter should keep it clean and if it was plugged it would be pluged.
Coil--usually shuts off like a timer and then works after cooling.
Condenser--when it goes it goes.

Rick:cool:

I will check the other forum for assistance, thanks.

The fuel line doesn't really run past anything that is really hot. Not to mention that allowing the motor to cool, overnight, doesn't always solve the issue.

I doubt the problem is crud in the tank related. Over this past winter we replaced the fuel tank with a brand new one. The problem existed before and after the change. I will admit, although we replaced the tank and the filter, we didn't replace the fuel line itself. It looked in good shape, and no gunk came out when we blew it out with compressed air.

If it was a fuel additive or such causing the problem, wouldn't it have issues all the time? And without changing fuels, what would then make the issue go away?

I'm not trying to discount anyone's ideas. We have been working at this problem for a long time and are quite frustrated. Thanks for the help.
 
#15 ·
If the tank is new, then it could be stratified fuel. The new ethanol gasoline has a shelf life of approximately two months, and then it begins to "separate" in tiers. I'm not an expert on this, but I've been investigating a similar problem on my A-4, and read some helpful articles in Boat U.S. about the ethanol problem. Fuel stabilizer helps, as it extends the life of your fuel to a full year. This is important for sailboat owners, as we often just use a gallon or two at a time. Old fuel that has no stabilizer additive will begin to degrade much soon. I know I'm not explaining this all that well, as I'm in a hurry, but I think you can get my drift... If you refill after every sail in order to lessen the effects of condensation, then you are barely dilluting the fuel in the tank. A motorboat, on the other hand, can blast through 3/4 of a tank in a single afternoon, and then refill with new fuel. Stale ethanol fuel can lead to varnish build up, and it can also lead to symptoms similar to what you're experiencing.
Good luck.
 
#16 ·
I would not want to venture a opnion on why the engine is quitting, but having had three A-4 two with elec fuel pumps I can say installing a elec fuel pump is easy. I purchased my elec fuel pumps from the local NAPA dealer, noting fancy needed just thier basic 12 pump, it was easy to install and wire in. If I remember right I just took a small piecie of mild steel (or maybe it was aluminum) made a cover with a paper gasket where the mech pump installed. In any case intalling a elec pump was a simple inexpensive matter. I know it has been mentioned before, but contacting Moyer Marine is a excellent idea. If anybody knows the fix they would. If you expect to hang onto the A-4 look at Moyers A-4 manual. The manual is well worth the money. Best of luck on finding the specfic fix
Ocean31
 
#17 ·
Only an intuition here, but you might wish to swap out the coil. It has been known to overheat when mounted on the block and to cause starting problems thereby.

The solution (beyond getting a new coil, checking the timing and replacing the wires) is to mount the coil on the bulkhead away from the block. This allows cooler air to keep the coil at a functional temperature.

That said, the layering or cruddy gas is a possibility, and you might wish to check out your tank vent for partial obstruction from insect activity. Spider goo killed my engine once, and it was very frustrating because the engine would die gradually over 10 minutes or so, wouldn't start for five minutes and then would start...only to die again. Switching up to a 5/8th inch vent hose and replacing the screens did the trick.

Good luck.
 
#18 ·
Another reason to replace the fuel pump is the failure mode of the diaphram. If it's perforated it will fill the oil pan with gas. Had it happen on our A4 and it was a disconcerting, to say the least, to have gas and oil squirting out the oil breather and into the bilge.
 
#19 ·
You night want to

Check the fuel tank vent. If the vent gets clogged, it will cause a vaccuum in the tank stopping the fuel flow. Take the fuel filler cap off when it happens again and see if that works. WOnt cost you snything.
I am more inclined to go with the fuel pump.

Fair WInds
Cap'n Dave
 
#20 ·
I don't have any experience with an A4, but had similar problem with my previous boat. I had two 350 Chevrolet engines. One would never idle after a long, offshore run. Filters, electric fuel pumps, points, wires, everything that everyone is mentioning. Turned out to be a defective (or perhaps overlooked on tune up) condenser. It was fine cold or on eight or ten mile trips down the river. For what its worth, when it is difficult to start, have you checked to see if you are getting a strong and CONSISTENT spark? Coils and condensers sometimes break down with heat, not stopping altogether, but making a weak, inconsistent spark. Clearly - the fuel problem is getting the most votes. The condenser still may be worth another look.
 
#21 ·
It sounds like Capn_Dave has a good approach, the engine sounds like it it not getting fuel. I would also suspect the ignition coil. They can fail under warm conditions and cool off and run normally. Even a coil that is newer can act this way.

The fuel tank vent could certainly be your culprit.

Good luck and please post your progress.
 
#22 ·
Sorry but I'm not buying the fuel pump theories. Logically if it will run at 3/4 throttle wouldn't it be getting enough fuel to run at idle?

A-4 carbs have a main and idle fuel circuit. It sounds like you have junk in the idle jet. IF you have good compression (have you done a comp.test?) it should idle if the jet isn't clogged. No compression guage? Pull the plugs and ground the coil to Diz cap wire. Crank the motor and put your thumb over each plug hole. It should POP your thumb off each one.

Hold the coil wire off the block 1/4". You should see a blue spark if alls well.
If the ignition system parts are all new, and the mechanical advance isn't stuck (with the diz cap off, will the rotor rotate and return about 20 degrees?) then the problem is likely elsewhere than ignition.

I'd clean the carb if you have POP and spark.:eek:

Dave
 
#25 ·
Atomic 4 problem

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

You didn't say where you keep your boat, but yearly may not be frequent enough for a filter change. I'm in hot country and pulled my fuel filter after 9 months and found lots of crud that made it to the carb and caused poor performance. You say the problem is not heat related, but how old is the coil? After prolonged running it gets hot and can malfuntion especially if it is an old beast.
Moyer Marine recomments a priming pump in the fuel line before the main filter to help in diagonosing these kinds of problems and to prime the filter after it is changed.
I second the idea of an electric fuel pump, they aren't expensive.
 
#26 ·
Good Day,
None of the filter/pump arguments are convincing to me. I had a similar problem and the root of the problem was fuel tank airvent line was blocked/impeded. The fuel pump at cruise power could pump hard enough to keep the engine running but when throttle was reduced the partial vacuum in the fuel tank caused the engine to kill.

Temporary fix is to open the fuel filler cap once in a while to allow air into the tank. The permanent fix is to remove debris, insects, etc from the fuel tank air vent hose. Make sue the debris does not get into the tank and remember that a fuel tank is a potential bomb, particularly when vapor is present.

Best wishes,
soarbooks
 
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