Cylinder Pressure Test - Is this a problem

[mikemoyer]

Do I need to do anything about this?

#1 – 120

#2 – 95

#3 – 80

#4 - 110

[Dave Neptune]

I've seen far worse numbers run for a long long time. Are you experiencing any problems. Did you do a wet test too?

My ole A-4 had worse numbers and ran for another 34 years just fine.

Dave Neptune

[joe_db]

I would be tempted to give the low cylinder a MMO treatment and see if maybe the rings are stuck.
Speaking of rings, is it possible for the gaps to line up temporarily and then move again?

[mikemoyer]

Originally Posted by Dave Neptune

I've seen far worse numbers run for a long long time. Are you experiencing any problems. Did you do a wet test too?

My ole A-4 had worse numbers and ran for another 34 years just fine.

Dave Neptune

Thanks! What is a wet test?

The engine cuts off when under power, but starts back up. Runs for 10 min, cuts off....

[mikemoyer]

Originally Posted by joe_db

I would be tempted to give the low cylinder a MMO treatment and see if maybe the rings are stuck.
Speaking of rings, is it possible for the gaps to line up temporarily and then move again?

Thanks!

I'm not sure what you mean by MMO or gaps lining up...

[joe_db]

Originally Posted by mikemoyer

Thanks!

I'm not sure what you mean by MMO or gaps lining up...

MMO = Marvel Mystery Oil
Piston rings all have little gaps. I think they slowly move around and can line up every now and again.

[Dave Neptune]

The numbers from your compression test are not enough to cause cutting out. The general "tune condition" would though.

Cutting out could be electrical (ignition) or fuel related though. When it shuts down is it like you turned off the key or does it stumble to a stop?

What type of ignition do you have points or electronic? And a mechanical or electric fuel pump?

The "wet" test can be put off at this time as it will not show anything to do with cutting out or stalling at this point.

Please describe your shut down, what you did when it shut down and how long it took to get restarted? Did you check for spark or anything else?

Dave Neptune

[Dave Neptune]

Joe, if the rings moved around and lined up it would not make any change to compression as the gaps are always there.

Dave Neptune

[mikemoyer]

When I put the boat away last year, on the way to the storage yard the engine cut off about an hour into the trip. I could feel the power loss and then it shut off. It starts right back up, but cut off every 10 minutes or so. If I took it out of gear it would stay on, but shut off again when put back in gear.

This year I had a tune up and a carburetor cleaning. The problem started out again when I left the storage yard. I took it back to the yard and they adjusted the transmission (not sure what this means). And did the compression test. I posted the results.

Based on my research the compression levels are acceptable. But there is a wide variance which I am concerned about. I do a lot of my own work, but I'm not an engine mechanic....

I think this issue must somehow be related to the fuel. The pump is mechanical (1979). Some have suggested that it might be related to "crud" getting sucked up from the tank and settling off after the engine dies. Or maybe it's not venting properly...still working on it!

[ndutton]

On this forum research Fuel Pressure Gauge and Auxiliary Tank Test.

[joe_db]

Originally Posted by mikemoyer

When I put the boat away last year, on the way to the storage yard the engine cut off about an hour into the trip. I could feel the power loss and then it shut off. It starts right back up, but cut off every 10 minutes or so. If I took it out of gear it would stay on, but shut off again when put back in gear.

This year I had a tune up and a carburetor cleaning. The problem started out again when I left the storage yard. I took it back to the yard and they adjusted the transmission (not sure what this means). And did the compression test. I posted the results.

Based on my research the compression levels are acceptable. But there is a wide variance which I am concerned about. I do a lot of my own work, but I'm not an engine mechanic....

I think this issue must somehow be related to the fuel. The pump is mechanical (1979). Some have suggested that it might be related to "crud" getting sucked up from the tank and settling off after the engine dies. Or maybe it's not venting properly...still working on it!

Low compression = low power and hard to start.
It is next to impossible for it to be related to sudden stops and starts. Atomic 4s with MUCH worse numbers than that still run. Heck, a high school buddy had a boat with 0 compression on #3 and used it that way for years.

[Dave Neptune]

I suggest a fuel pressure gage for diagnosing. Were all of the filters changed out with the carb rebuild?
Check all of the hose clamps from the tank to the carb. If one is loose it will allow for air to be sucked in and not the fuel.

Did you try pumping the "bail" on the pump immediately or during a shut down?

Sounds like a fuel delivery problem that can't keep up with the load.

Dave Neptune

[mikemoyer]

what is the "bail" and how do I pump it?

[Dave Neptune]

On the mechanical pumps there is a "U shaped" wire sticking out of the pump housing, it is to operated the pump manually for priming. I know of a few that have got home by using it as the pump fails.

Dave Neptune

[capnward]

Dave is right as usual. My mechanical pump has always been able to handle the load unless there was a restriction in the fuel line upstream of the pump. If you get a fuel pressure gauge, you will see the needle go up as you pressurize the line with the bail. The bail will get looser until it stops pumping and has no resistance, then the gauge will be as high as it can go, maybe 3 psi., and fuel is pushing against the float valve in the carburetor. Change filters when you clean the carb. If you have a water separator filter, drain the bowl at the bottom and see if there is a lot of water in the gas. I wonder what your 'mechanics' did to the carb. Sounds like they did not make sure the idle jet was unclogged. This would lead to the engine stalling under load. That tiny jet needs to be removed and a thin wire run around in it. Spraying carb cleaner through it is not enough to dislodge the speck of crud.
Although you say you are not an engine mechanic, I suggest you learn about cleaning your carburetor anyway. This is not rocket science. Unless your fuel is clean, this can be a regular occurrence. You won't always have access to a mechanic. The Moyer Atomic Four manual and this forum will benefit you more than some mechanics.
Sometimes a piece of crud will lodge in the adjustable main jet, if you have one. Turning the needle in until it seats, then turning it back out for 1.5 turns can open it up again.
Use non-ethanol gasoline, and 2 oz. of Marvel Oil per 5 gallons of fuel, and your mechanical pump diaphragm will not dry out and crack.