Pearson Boats - Common Systems > Engine and Drive Train

Atomic 4 compression test results

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T_schlueter:
I removed all spark plugs to perform test and drissled a little mystery oil in each cylinder. Here are results:
#1 110
#2 80
#3 70
#4 110

Should I be concerned about 2 & 3?

Dolce_Vita:
70 is at the bottom edge of acceptable, although the engine can run even with all four down around 65.  (it just wouldn't have as much power)

When you do the compression test, it is important that ALL 4 plugs be removed, choke is fully open, and throttle is fully open. (this last item is often overlooked) Failure to observe these three "standard" conditions will produce lower readings that can't be directly compared to other engines.

Assuming you've done all that, low pressure in two adjacent cyls could indicate a leak in the head gasket between those two cyls.

T_schlueter:
FYI, a p.o. had the head gasket replaced 20 years ago.  Is there a way to test those two cylinders for possible crossover leakage?

Alma:
If the engine starts and runs well don't worry about it.

Is the engine raw water cooled or do you have a heat exchanger?
What do the spark plugs look like?
Why did you test the compression?

I just finished overhauling my atomic four with new pistons, valves, guides and bearings. It cost around 2 grand and that included boring and honing the cylinders, and measuring and polishing the crankshaft and camshaft and the parts. I tested it and it runs well.  It goes back in the boat Saturday if all goes well.

The reason I needed to act was the engine used a quart of oil in about 3 or four hours!

Checking the oil every three hours at sea was tedious and I worried someone on watch would forget- Otherwise I'd still be adding oil every two hours. It ran that well.
 
What I found was worn piston ring lands likely from salt water intrusion when I had a bad cylinder head gasket and they were a real mess.

If your spark plugs look alike and oil consumption is low I'd roll with it. I wouldn't worry about pulling the head unless you have other symptoms.

Removing the cylinder head can be easy or brutal depending on corrosion. Since it is a side-valve flathead, the head is just a lid- no valve train to worry about.

T_schlueter:
It is raw water cooled. It runs well and doesn't burn through oil. I put in new plugs and just got it started while still on the hard. Runs have been limited to time to pump through a five gallon bucket so I haven't pulled plugs for inspection.  I ran compression test to establish baseline as boat is relatively new to me.  I think I'll go with it for now since that's your inclination as well.

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