Atomic 4 compression and plug fowling issues


Jan 26, 2009
2 323 Great Kills Harbor
I hope you are all well, or reasonably well.

I am having two issues with my A4 that are causing me to scratch my head.

Compression, warm, wet.

1: 125
2: 100
4: 125.
All four hold compression for more than five minutes.

I am having spark plug carbon fouling on 1 and 2 while 3 and 4 show no soot or carbon and appear to be burning correctly. I have tried adjusting the carb lean settings with no difference, too rich and 3 and 4 show soot.

Has anyone experienced any similar issues?

Best regards,

Feb 10, 2004
Hunter 40.5 Warwick, RI
I had an A4 in my 30' Morgan 25 years ago, but I will throw out some ideas.
I would put in new plugs, with correct gap.
Check that the observed spark on each plug is blue-white and not yellow.
Run the engine 5-10 hours and check the new plugs.
While running and fully warm, measure the engine temperature with a IR thermometer. I presume that you have the raw water cooled model- the temp should be about 165-170 measured at the thermostat housing.
If the ignition wires, spark quality, and temperature are all good, but you still have #1 & 2 fouling with 3 & 4 clean, then I would check one last thing- This is a long shot.....

The cooling water on this engine splits into two paths. One path goes to the thermostat housing bypass and the other path goes to the plate on the side of the engine to cool the block & cylinders. When the engine is cold, the closed thermostat blocks the water from the cylinder block (except for a trickle) but allows full flow of water to the exhaust manifold through the thermostat bypass and then out of the engine.
As the engine comes up to the thermostat temperature, the T-stat opens and as it opens the water flow from the cylinder block increases and the flow through the thermostat bypass decreases, but all water still goes to the exhaust manifold.
Now when the water enters the block through the side plate, cylinders 1 & 2 are nearest the thermostat where the water exits from the block. So the water will tend to cool cylinders 1 & 2 more than 3 & 4. Although it doesn't show on the drawings of the engine or in the parts list, my engine had an elbow on the inside of the water inlet plate that directed the water toward the bottom of cylinder 4. This was done to cool all of the cylinders more evenly.

So you could check to see if you have such an elbow and that it is pointed in the correct direction.
If you don't have the internal elbow or if it is not pointed toward the bottom of #4, that could cause cylinders 1 & 2 to run cold and 3 & 4 to run hotter. That could explain the difference in the sooting of the plugs.

I realize that this explanation is complicated, but if you reread it a couple of times and look at your engine documentation, I think it will make sense.

Good luck.
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Aug 18, 2015
West Marine Kayak Tampa Bay
The level of knowledge on this site never ceases to amaze me. Great post Rich.
Jan 18, 2016
Catalina 387 Dana Point
As Rich said, uneven cooling can contribute. His suggestion of an IR thermometer is spot on.

Your compression readings are fine, for an A4.

Hot spark and proper gap can go a ways to help - I always ran electronic ignition on my A4: hotter, more reliable, and I hate adjusting points. (I had a Pertronix distributor as modified by Moyer. There's others out there too)

And how bad is it? Could be a case of don't worry about it and change the plugs every x hours. With those compression numbers I doubt you have excessive blow-by and likely are just running a bit cold.


Feb 11, 2017
Islander Freeport 36 Ottawa
I had plug fowling on the A4 in my C&C 27. I was replacing plugs annually. After a while I also had problems with starting. It turned out to be the ignition. I replaced the distributer, plug wires, coil, points. After that it was a new engine. No more plug fowling and it was very reliable. If I were doing it now I would get the solid state ignition kit available from Moyer Marine.
Check your distributor and plug wires.