Engine quit after an hour + running

[Luckybag]

So it happened two weekends in a row. Engine runs great in the slip, take it out sail and motor (about an hour + motoring), running great the whole time. Heading back into the channel to our slip and engine begins running a little rough (maybe missing on a cylinder?) then quits and won’t restart. Both times I was able to coast (thankfully) into our slip. Engine restarted after 10 mins to an hour. Ran fine and couldn’t reproduce the rough running in the slip. Anyone have any ideas?

[ndutton]

Basics first, determine if the problem is spark or fuel. The tests must be done as soon after shutdown as possible during the 10 minutes it won't start. Remove a spark plug, ground it with the wire still attached and crank to observe whether or not it's sparking. No spark = problem.

Spraying starting fluid into the carburetor and trying to start is a good fuel test. If it pops, fuel is the problem.

Once that is determined you can proceed logically.

[joe_db]

Is there any way we can have a sticky thread for this?
This seems to be about 1/3 or more of forum traffic.

I'll start off with the very basics:
An engine needs fuel, air, spark, compression, and timing to run.
Compression and timing are very unlikely to come and go at random and unless a rag falls on the flame arrestor, you have air.
Cheap-and-quick: If the engine quits and you spray a little ether in the carb and it starts right up, it was lacking fuel. If it does not run, it was lacking spark.

[Luckybag]

Thanks, checked spark at the plugs (ok), engine started easily after a short cool down (which I am thinking suggests it’s not the fuel). Leaning towards coil right now. I understand when they go bad…they heat up and can cause what I describe. BTW, I have the old ignition system (distributor cap and points).

[ndutton]

Thanks, checked spark at the plugs (ok)

Did you do this check at the time the engine would not start?

[scratchee]

Thanks, checked spark at the plugs (ok), engine started easily after a short cool down (which I am thinking suggests it’s not the fuel). Leaning towards coil right now. I understand when they go bad…they heat up and can cause what I describe. BTW, I have the old ignition system (distributor cap and points).

I had a problem like this and it did turn out to be the coil. When I replaced the coil, not only did it fix the engine problem but it unexpectedly fixed another problem that I had been ignoring and didn't realize was related: my tach needle had been bouncing around and was useless. With the new coil, the tach worked again. Do you have any problems with your tachometer?

[Sam]

No way am I suggesting to skip any analysis but I am strongly suggesting you always have a spare coil AND a spare condenser - both can go bad over time [ usually a long time] and each can give you the symptoms that you have experienced. If the ignition switch was left on accidently while engine was not running [engine quit] the coil failure time can be measured in minutes. Check for spark, first with coil lead at the time of the engine quitting per Neal's comment. Most folks have upgraded to electronic ignition with different coil and the condenser is not a factor. I still have points/condenser after 45 yrs because I am old and that is what I am comfortable with. BTW - if you need to change the condenser make sure you maintain the same wiring sequence on the distributor/points post otherwise you will short out the circuit and the engine will not start.

[Surcouf]

could also be a tank running under vacuum (vacuum line plug), which gets into the "lack of fuel" category. The easiest way to see is to install a fuel pressure gage right before the carburetor. You will capture all issues regarding filters/pumps/negative pressure in tank.

The only time I had similar issue (engine shutting down after some time running), was after my partial overhaul, I had moved the coil closer to the exhaust, and it was overheating (some oil could be seen at the top bolt). My fault 100%. Some exhaust wrap, a heat shield, never happened again

[TimBSmith]

Plenty of great advice here already, just a brief point on bracketing the problem.

I have two open threads that are due for a separate update. One on a fuel pressure issue, one on a oil pressure sender issue.

As I have literally grown up as an A4 engine mechanic exclusively on this forum..

I cannot be reminded enough to sit down and slowly bracket trouble shooting. Have a plan and stick too it, adjust based on evidence, and reaching terminal points on the diagnostic. Not wishful thinking or untested supposition.

Owners here have ways of collecting most evidence you can collect on this engine's operation.

The longtime contributors will come back to fuel, compression, spark. And diagnose stepwise where these systems stand alone and where they intersect. Outlier conditions sometimes present...though not often.

So I would enjoy hearing how you are thinking about your bracketing of the diagnosis and the evidence you need at each step in the bracketing. Take the fuel system first. Then the spark. And as close to the moment of system failure as possible. What have you done? What do you plan to do?

Look forward to supporting. Best with resolution. Fair winds. Tim

[Luckybag]

Sorry, I can’t figure out how or if I can respond to individual comments (they are much appreciated). So I will just try to respond to all in this response:
I checked the spark within 10-15 minutes after coasting into to slip, got spark on all four plugs. But the engine did restart.
I do not have a tachometer…will install one from Moyer
Thanks for the suggestion about spares…
After cool down the engine runs great, and I haven’t been able to reproduce it in the slip (but I haven’t run it that long). The strange thing is it happened after about the same time duration of motoring…seemingly to suggest it is thermal related. Not sure how to detect or evaluate coil performance via traditional troubleshooting methods but on the Atomic 4 FB group, almost all the commentors said they had almost the exact same thing happen and after replacing the coil, the problem was gone. Will replace as soon as I get one and will report back 😉 Thanks for the warning about leaving the ignition switch on..

[joe_db]

I have had two different thermal issues:
1. Coils. Before they just die for good, they can go through a phase of going bad, cooling off, and then working again. There are numerous threads on here about this.
2. Vapor lock. After running for some time in hot weather, I would lose fuel pressure. This would resolve when everything cooled down. I would have never figured this out without a fuel pressure gauge.

I haven't had it happen that I know of, but fuel pumps themselves can overheat and quit working.