Compression Testing for a Newb

[JSR]

...Never done one. How is it done on an A4?

Looking to buy a Catalina 30 and the engine is acting up. Want to test compression among other things, but I figure if the compression is bad I'll just move on.

Best compression tester for A4?
Is it as simple as pulling the spark plug wire off the top of a cylinder and plugging the tester in then cranking?

Any and all help would be much appreciated!

[Administrator]

This old post may help you along the way:

Here are a few things that you can look for:

1) It's obviously very important to check on the maintenance history of the engine, if one is available. The maintenance history, along with the ability to talk with the prior owner, can add insight to the findings of the following checks:

2) Check the exterior of the engine for signs of deep rust or scaling. Be especially critical of the alternator side of the block. In some cases, seals around water jacket side plates develop leaks, and given the limited access to that side of the engine on many boats, these leaks can go undetected for long periods of time, and deep scaling can result. In a few cases (fortunately very few), blocks have had to be scraped due to this scaling extending all the way through the block and into the crankcase.

3) The engine should be started and brought to a normal operating temperature of 160 to 170 degrees for raw water cooled engines and approximately 180 to 190 degrees for fresh water cooled engines. It is best to run the engine in "forward" and under load. If you are checking the engine in the slip, it's OK to simply pull against the dock lines. Check for any unusual noises. Atomic 4's are known for running rather quietly and smoothly.

4) Check for fluid leaks. Be especially critical of any evidence of gasoline around the carburetor or fuel lines.

5) Check for normal oil pressure of 30 to 40 psi by around 1500 RPM, and 20 psi or above at idle.

6) Run the engine for a short time at maximum power to check for any unusual sounds, excessive smoke out of the exhaust, or fumes from blow-by in the cabin.

7) In neutral, the engine should accelerate quickly as you flick the throttle from idle to full throttle without hesitation. It's important that you do not allow the engine to actually reach full RPM during this check. This is simply an acceleration check. The RPM never needs to go much above 2000.

8) Shut the engine down, and check the compression as soon as you can remove the spark plugs without burning your fingers. The combustion chamber volume in the heads of in-service Atomic 4s varies considerably, and compression can therefore vary from as low as 85 psi to as high as 120 psi, with the average being around 100 psi. Compression readings should have a maximum spread within 5 or 10 psi.

9) While the spark plugs are removed, check for excessive carbon buildup or oiliness. Do not be too critical of a black velvet sootiness as long as the plugs are dry.

If you are buying a boat without the opportunity to actually run the engine, perform as many of the above checks as possible, plus the following:

1) With the spark plugs removed, run the engine on the starter, and check for oil pressure. Oil pressure at starter RPM will usually be around 20 psi.

2) Check the compression. With a cold engine, compression readings can be expected to vary much more than in a warm engine, and if the engine has not been run in quite a while, valves might be a bit sticky, which will affect compression. These conditions are frequently not serious and will clear up after the engine is started and given a Marvel Mystery Oil treatment (5 or 6 squirts of oil in each spark plug hole).

Best regards,

Don Moyer

[ndutton]

If the boat is in the water it's absolutely critical the raw water intake remain closed during any cranking episodes without the engine running such as compression testing.

[joe_db]

IIRC, the spread from low to high is more important than the raw number.

[Dave Neptune]

Take yur' time

JSR, first welcome the the Afourian MMI Forum. Pay good attention to the list Bill supplied and ask of what you do not understand!

First be sure the battery is full and all plugs are out for inspection and a pic would be nice too! Have the choke off and the throttle at wide open during testing!!!!

Doing the compression check and anytime you cranking for anything keep the water closed as per Shawn. Now the numbers are not as important as being somewhat close to each other Do a compression test DRY first and record the numbers. After that squirt some oil (WD-40 OK) into the cylinders and check again. If the second set of numbers is higher that is normal, now we look for a consistent change in the numbers wet with oil. The split will tell us about the rings and the valves.

If the engine is dead on one or more cylinder it is probably a stuck valve which in most cases is no biggie! BTW I bought my boat with the engine seized in 1984! It took 2 days to get it broke free and one exhaust valve replaced and she has run flawlessly since, that's over another 25 years. I was planning on dropping in a diesel with the deal I got but had to get the boat moved in a hurry so I tried to get it running "temporarily" and I spent the diesel money on liquid bread a far better choice!

Dave Neptune

[Carl-T705]

JSR Welcome, what do you mean when you say A4 is acting up? Hard starting? smoking? missing?
Dave, My experience is test the compression dry, if it test low squirt motor oil, not WD40 in the cylinders through the spark plug hole. Retest, if compression comes up it indicates a ring sealing problem. If the compression remains the same it indicates a valve leakage problem.

[TomG]

JSR,

What exactly is the engine doing? A compression check is a great idea on any boat you might buy. But if you can tell how the engine is "acting up", the group here can probably give you other things to check and think about. Some issues can be readily resolved while others might be more work than they appear. Many times, a poorly running engine is returned to a well-running engine with something as simple as a new $11 condenser or a cleaning of the idle jet on the carb. The A-4 is a wonderful engine and depending on your access (which is pretty good in a C-30) is quite easy and forgiving to work on. Shoot some pictures and tell us a little about what the engine is doing. There are many extremely knowledgeable A-4 owners here that have helped me learn how to work on my A-4. I'm certain there are answers to all your questions and probably a lot of wisdom from which you can benefit as well.

To answer your question:

Best compression tester for A4?
Is it as simple as pulling the spark plug wire off the top of a cylinder and plugging the tester in then cranking?

Compression testers are rather simple tools. You can pay a lot for a high-end one or not so much for a Harbor Freight one. I got the HF compression tester like this one for $25:

HF Compression Test Kit

To check compression, simply remove the spark plugs and screw in the adapter and crank (As Neil says, remember to close the Raw Water Intake seacock while doing this. This is important because most marine engines use a water lift exhaust system. The Water Lift system uses exhaust pressure to push cooling water out of the water lift up and through the exhaust system. Without the engine running, there is no exhaust pressure to push the water out. The water will eventually fill up the exhaust manifold and then the engine itself. Although not fatal to the engine, it is a real hassle to correct.) The gauge will hold the pressure until you release it. Bill's and Dave's advice about hot and cold compression tests, with and without oil, gives you more information. Also, a stuck valve on an A-4 that hasn't been run in while is not uncommon. These stuck valves are typically caused by humidity causing mischief on the valve stem. A stuck valve is very often fixed by adding some lubricant to the fuel to send oil through the combustion side of the engine along with soaking the affected cylinder in a good lubricant (most of us use Marvel Mystery Oil, i.e. "MMO").

Good luck and keep the forum updated on your progress and please ask questions!

[sastanley]

Mmmmmmmm...liquid bread!

Except it was Neil that commented about the raw water being closed.

However, I will add further that the reason to do so is that the starter spinning the motor also spins the water pump..this pumps raw water in the exhaust, but if the engine is not running the exhaust gases cannot expel the accumulating water..so the water backs up the hot exhaust stack and dumps into the cylinders. Hence the reason to always close the raw water thru-hull if you can't get the motor running and are compression testing or otherwise troubleshooting.

Once the engine is running, it is safe to open, as the exhaust gases can take the water up thru the wet exhaust system to the transom. On a C-30 the raw water intake should be directly adjacent to the engine, near the carb & fuel pump.

JSR, several of us here have C-30's..great boats, I bet we can salvage the engine and you'll be fine. We'll be happy to help with whatever you decide.

[Whippet]

check oil too

another lesson i learned is to look for water in oil. oil w water will have a grey-brown milkshake appearance. i would think you can see by pulling dipstick, but the bigger the sample, the better. doesnt mean engine is toast, but could be a long series of headaches to find where water came from

[JSR]

I belong to quite a few other forums, flying, hunting, fishing, etc and I have to say, I have never had such a kind/informative/helpful response to anything. To that I offer you all a virtual beer. And if you ever find yourself around San Pedro, Ca, you are more than welcome to trade that in for a real one.

The problem is this, the engine started, idled and ran fine. Up until around 50% throttle the thing seemed great. After the 50%, it would sputter and lose power. To which bringing it back down would revive it.

I was thinking fuel starvation, perhaps in part by a dirty carb? Perhaps old fuel?

Thoughts?

I plan to take note of all the advice here and apply it on my next buyer's visit this weekend.

Side note: Seller said engine was overhauled in the last 4 years and isn't sure what the problem is. More support to fuel issue?

Again, thank you for the help gentlemen. I feel much more confident in pulling the trigger on this boat because of the support here!

[ndutton]

San Pedro????

I'm in Cabrillo Marina, Berth 31, dock 'C'.

[sastanley]

dirty main jet?

The problem is this, the engine started, idled and ran fine. Up until around 50% throttle the thing seemed great. After the 50%, it would sputter and lose power. To which bringing it back down would revive it.

I was thinking fuel starvation, perhaps in part by a dirty carb? Perhaps old fuel?

JSR, that sounds like it could be a clogged main jet. The idle jet controls fuel mixture until appoximately 1,200 RPM, then the main jet takes over. Those throttle up symptoms could be some junk in the main jet. Is there any change in the behavior if you use the choke?

There should be a horizontal bolt on the bottom of the carb bowl that provides access to the main jet. (be careful, don't lose the fiber washer..it is there because it is NOT a tapered thread!) - try removing that bolt (1/2" wrench) and flushing some fuel out of the fuel bowl into a clean container to see the condition of the fuel after it settles a bit. Ventilate very well until the fuel smell is gone and see how it goes. I am hoping a fuel bowl flush will dislodge any debris that may be clogging the jet. You may end up taking the carb apart, but this is a quick & dirty cross-your-fingers remedy I'd try first.

[thatch]

Personal story

JSR, About five years ago when I acquired my A4 powered Catalina 30, I bought it despite the "ugly duckling" engine, but now I wouldn't have it any other way. Mine was purchased in San Pedro and then "Motored" to Marina Del Rey and then on to it's present home in Oxnard. During that journey, thank's to our new "alcohol laced" fuel, I dislodged over 30 years of fuel system gunk. Since arriving at Oxnard I have changed out the fuel tank, fuel lines and filter and have had "zero" fuel system problems since. The Catalina 30 is an extremely "engine access" friendly design that makes engine service and repair far easier than in most other brands of the same general size. Before passing on this particular boat, my advice would be to talk to Neil. I'm betting that you will soon realize that this engine is the right choice for this size of sailboat.
Tom

[TomG]

that sounds like it could be a clogged main jet

Sure sounds like a main jet clog. Easy enough to fix.

Seller said engine was overhauled in the last 4 years and isn't sure what the problem is. More support to fuel issue?

When was it last running well? Are the fuel lines ethanol rated? How old is the fuel?

Ethanol wreaks havoc on non-ethanol rated fuel systems. Since boats last a lot longer than cars, and we use them less frequently, the problems are magnified. E-85 fuel contains 15% ethanol. Ethanol (ethyl alcohol) is a poor fuel for our situations, but it is a very good solvent. As fuel varnish builds up in our old gasoline systems, ethanol, being the solvent that it is, breaks up the varnish and attacks non-alcohol rated fuel lines and gaskets in our fuel systems. This creates a lot of trash in the fuel system. Most of this is caught by filter systems, but occasionally, some varnish in the carb or from the fuel lines finds its way to the idle or main jets.

The symptoms fit this very possible scenario. I'm also wondering about distributor advance, but if it were my boat, I'd clean the carb first (it probably needs it anyway) and maybe just pop the top on the distributor and check to see if the advance mechanism advances smoothly.

Good luck and keep us posted.

[JOHN COOKSON]

I Wouldn't Be So Presumptuous As To Speak For Neil......

But I have the feeling that for the price of a good lunch he would go look at the boat\engine with you.
Send him a PM and see where it goes.

TRUE GRIT