Engine dies after it warms

[toscaindub]

First things first -- what I did last week:

Adjusted engine idle. At the dock, I put the engine under load and rotated the distributor (which is using electronic ignition, by the way) until I had max RPMs. Engine ran great after this, it was off a touch before. I did re-tighten the distributor.
Changed the oil.
Installed water temperature and oil pressure buzzer/alarm (the gauges don't work)

Saturday I tried to take the boat out. I started the engine to let it warm while I took off sail covers, etc. The engine slowed and stalled. I re-started it, and it ran for a much shorter time and it stalled again. After a couple rounds of this, it got to the point that I could let the engine sit for 3 or 4 minutes, and it'd turn over and fire up and run for two seconds, refusing to re-start unless it had 3 or 4 minutes to sit and think about it.

Things I tried:

Compression seems fine
Plugs are clean
I cleaned the carb, twice -- seems alright
Fuel pressure seems okay. When the engine dies, I tried using the manual lever on the gas pump to see if there was back pressure, and there was.
I took out 1/3 quart of oil -- I overfilled the engine slightly during the oil change
Tried running off of a spare coil: no improvement

After letting the engine sit for a couple hours, I again fired it up and it happily ran for 15 minutes or so. I just left it idling, and after 5 minutes it sounded a little slower and less rhythmic, then after 10 minutes it was worse. Then at 15, it just petered out.

I've no guesses. Then again, I'm not much of a mechanic. I figure these symptoms might mean something to somebody. Hopefully I've added all the detail anyone would want, but just ask if there's anything I've left out. Thanks!

[Don Moyer]

Hi,

The best chance you have to determine what's causing your engine to shut down is to do a few quick checks while it's not starting. The assumption being that the thing that's preventing it from starting is the same thing that caused it to shut down in the first place. Here's a short check list of things to check while the engine is not starting:

Close raw water through hull as soon as it's determined that the engine is not starting within the normal time.

2) FUEL: Remove the flame arrestor and check for the presence of raw fuel. If the choke is closed completely, there should be raw fuel puddled in the bottom of the intake throat within 15 to 20 seconds of cranking (3 or 4 five-second attempts). If the carburetor intake throat is "bone dry", after this amount of cranking, the reason for the non-starting is either a problem in the fuel supply which prevents fuel from getting to the carburetor, or some problem within the carburetor that prevents the fuel from getting through the carburetor (most likely a blocked main jet).

3) Ignition: Remove the secondary lead from the center of the distributor cap and hold it approximately 1/4" from the cylinder head while someone cranks the starter. You should see a good arc between the end of the coil lead and the head that can be stretched to 1/2" or even 3/4". If you see no spark, the reason for non-starting is clearly within the ignition system, most likely a break down within the primary ignition circuit.

This quick check of the ignition system does not rule out timing issues. While the Atomic 4 has virtually no history of slipping out of time once the timing has been properly set, if the non-starting follows work that was accomplished on the ignition system, the timing should be rechecked.

4) COMPRESSION: With all spark plugs removed, hold your thumb over each spark plug hole to check compression as someone cranks the engine for a second or two on each cylinder with the starter. An Atomic 4 will usually start if any two cylinders have normal compression as indicated below.

a. Compression sufficient to force past your thumb no matter how hard you press it against the plug hole would confirm normal compression of approximately 85 psi or above.

b. If you can hold your thumb against the compression, but not easily, a compression value of approximately 40 to 50 psi would be indicated, and starting could be problematic.

c. If you feel virtually no compression on any one of the cylinders, the problem is likely a stuck valve.

5) If all of the above checks prove to be satisfactory and the engine still won't start, the problem is probably somewhere within the secondary ignition system, down stream of the coil. The components within this part of the secondary ignition system are very difficult to inspect, but they are fortunately not very expensive so I recommend replacing them in the following sequence: plugs, distributor cap, plug wires, and rotor.

Don

[Bob Wieber]

Have you checked your fuel tank vent? If plugged, you may have a vacuum in your tank which slowly relaxes.