Author Topic: Atomic 4 compression test results  (Read 145 times)

T_schlueter

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Atomic 4 compression test results
« on: April 15, 2020, 10:17:59 AM »
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?
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Dolce_Vita

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Re: Atomic 4 compression test results
« Reply #1 on: April 15, 2020, 01:30:10 PM »
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.
@(^.^)@  Ed
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with rebuilt Atomic-4

T_schlueter

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Re: Atomic 4 compression test results
« Reply #2 on: April 15, 2020, 05:31:12 PM »
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?
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Alma

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Re: Atomic 4 compression test results
« Reply #3 on: April 15, 2020, 09:20:26 PM »
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.



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T_schlueter

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Re: Atomic 4 compression test results
« Reply #4 on: April 15, 2020, 10:47:37 PM »
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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Dolce_Vita

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Re: Atomic 4 compression test results
« Reply #5 on: April 16, 2020, 01:52:41 PM »
Another possibility for low compression is sticky valves.  The A4 is prone to sticking valves, especially if it has been allowed to sit unused for extended periods, or has gotten water in the cyls.

One way to combat this is to include some Marvel Mystery Oil in the crankcase oil.  This will help lubricate the lower part of the valve stems over time.  I generally include about 8 oz every time I change the oil.

For more severe cases, you can also include a little MMO in the gas, which will help lubricate the top ends of the valve stems.  It does produce a little smoke, and causes some additional fouling of the plugs, so I wouldn't do this as a long-term fix. 

If you do try the MMO, recheck the compression at the following oil change.
@(^.^)@  Ed
1977 P-323 #42 "Dolce Vita"
with rebuilt Atomic-4

T_schlueter

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Re: Atomic 4 compression test results
« Reply #6 on: April 16, 2020, 02:41:22 PM »
I keep MMO on hand but tend to forget about it. This compression issue will help me to use it.
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Alma

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Re: Atomic 4 compression test results
« Reply #7 on: April 16, 2020, 04:24:48 PM »
Raw water cooled is safe in the Lakes. Only advantage there to closed system is ability to run higher temp and prevent sticky valves and get very slight increase in efficiency. MMO is your best friend. Use it in gasoline only. Change the oil every 30 hours or when you run through a 30 gallon tank of gas.

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« Last Edit: April 16, 2020, 04:27:43 PM by Alma »
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T_schlueter

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Re: Atomic 4 compression test results
« Reply #8 on: April 16, 2020, 09:46:03 PM »
Looks and sounds great!
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Alma

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Re: Atomic 4 compression test results
« Reply #9 on: April 17, 2020, 10:32:58 AM »
Thanks. We'll see Saturday when we put it back in.
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T_schlueter

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Re: Atomic 4 compression test results
« Reply #10 on: April 17, 2020, 10:38:31 AM »
You're apparently not totally hobbled by covid restrictions!
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