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Old 08-29-2008
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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.
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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.

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Old 08-29-2008
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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.


Originally Posted by KRCSWO View Post
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.
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Old 08-29-2008
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Where the points gapped correctly? I would suspect points or condenser.
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Old 08-29-2008
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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.
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Old 03-18-2009
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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,
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Old 03-18-2009
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HOW TO: Testing your boats fuel system. - iboats Boating Forums

There are a few simple test that will take the guess work out of this problem
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