No compression on 1 and 2

[Crazer]

Finally got a compression gauge on the Atomic 4 in my Grampian and the results are not encouraging. Cylinders one and two have zero compression. Three and four have 90 and 105 respectively. A friend and I are going to try and pull the head this weekend to see what's going on. I have had MMO soaking in the cylinders for a couple weeks. Cylinder one has a little compression because it blows air out when I spin it over with the starter. Two has nothing, just some burbling MMO. First thought is, of course stuck valves. I also have a new head gasket in case that's the issue. Unfortunately, there does appear to be some water in the oil. I don't know how it got there, it's not a lot. The previous owner winterized the engine but I'll also pull one of the drain plugs to make sure there's AF in the engine block. My thought was maybe condensation but who knows. I'll change the oil this weekend too. The engine spins freely both by hand and with the starter. Anything I'm missing/should look for?

[tenders]

Have you been able to see the valves with a little flashlight, and maybe a dentist-style mirror? It’s been a while since I had to do this (knock on wood) so perhaps one of the cheap Chinese USB cable cameras they sell on eBay for $10 could make this easier.

If you align everything just so, you can see the valves moving (or not) when the engine cranks.

Assuming the valves are stuck, the one thing I’d do before pulling the head is to see if you can work the valves manually, by coaxing them down with a gentle tap or two with an Allen wrench or other suitably angled jig. Crank the engine a bit, if the valves are still sticking, squirt a bit more MMO directly over the valves (don’t just dump it into the cylinder, it won’t get to the valves), and tap them down again.

If there’s a bit of rust on the valves, this working can knock that free, and once the engine starts and they operate a lot the problem can go away completely.

Personally, I’d be willing to cycle stuck valves at least 50 times before I took the head off.

[CajunSpike]

I just had a motor with zero compression on #3, but semi decent compression on the rest.
Soaked in mmo for a few days. Found the points were so far adjusted the motor would not start. When I did get it started, #3 unstuck itself from the vibration of the engine. No disassembly was done. Don't give up yet on the mmo and try to start.

Somebody will warn you that cranking the motor for a long time, without the thru hull water pump line closed off, can result in water backing up into the exhaust ports of the motor, since there is no pressure to push it out the exhaust.

[Crazer]

Many thanks, hadn't heard of the allen key trick. I'll give that a go this week and have a look with a mirror. The boat is on the hard so no danger of flooding the motor.

[tenders]

I should mention that, unlike at the dentist, the mirror stays outside the orifice (the spark plug hole), and is used to bounce the light from the flashlight at a shallow angle into the far corner of the cylinder where the valves live.

Without the mirror, the flashlight either blocks the view of the valve or doesn’t shine anywhere near the valves. While I would like to say it took me less than fifteen minutes to figure this out, I cannot.

Even so, the view isn’t great, which is why the little USB camera might be nifty. But it’s enough.

[Crazer]

Understood. All I'm looking for is movement of the valve. Knowing about the allen key trick and having to squirt MMO directly on the valve makes me more optimistic. My big fear is a cracked block but we'll know more once I pull the drain plug and look around with the mirror.

[Sam]

I had a similar problem with my 1966 early model A4 back a number of years ago. I injured my shoulder and could not row out to the mooring can nor be a responsible skipper. Boat sat almost 5 months on the can. Your compression numbers on two cylinders are very good and my vote is for stuck valves. The engine will fire and possible run erratically on two cylinders if primary and secondary systems are all well. [I drained the carb bowl prior to the boat going into suspension] MMO did not work for me so I squirted into the open valves a little engine flush fluid. Got the engine to fire, then run a little and after a few more attempts the two stuck valves became free. After 20 minutes I changed the oil and all was well. Just a thought.

[Dave Neptune]

Avery, you have nothing to loose by trying to get it to run on the 2 good cylinders and those two shaking and vibrating will often shake the others loose.

Do have some MMO or 2 stroke oil in the fuel. Mix about 1 1/4 oz + to a gallon. That is about 100 : 1 mix and the oil in the fuel gets to the valves while there shaking, a good thing!

A bit of "motor crack" may help to get it running especially with all the oil in the cylinders. Try spinning the beastie with the plugs out at wide open throttle with a towel on top of the engine to catch the oil flying out of the plug holes. Then try starting.

Dave Neptune

[Crazer]

Okay, so using the allen key trick and some creative lighting I discovered that one and two both had stuck valves. I managed to work both free and they now operate correctly. I couldn’t get a good compression reading with the hand crank (is that normal?) but it feels much better turning it over by hand.

[JOHN COOKSON]

It's normal if you do a compression test that way. But that is the wrong way to do a compression test.
Be sure the battery is fully charged and the raw cooling water valve is closed.
Remove all the spark plugs. Open the throttle wide open. Crank the engine with the battery.

TRUE GRIT

[Crazer]

Okay, as I thought. I didn’t have a battery onboard so I couldn’t do anything conclusive. The boat is on the hard.

[Crazer]

Did a proper compression test today. Dry, cylinders one and two came out at 65 and 75 psi respectively. After adding a little MMO to each cylinder, got 120(!) and 95 respectively. I was pretty surprised by that first reading! Anyone seen any similar reading on an old A4? It's all within range at this point which is thrilling. Anyways, onwards and upwards! Time to get the old beast running.

[Al Schober]

No, not surprising. I suspect you were expecting something more like 6.3x14.7=93 psi - but that would be for compression at constant temperature. The actual compression takes place quickly and the air doesn't have time to cool off, giving more of an adiabatic process. A fully adiabatic compression would give more like 300 psi. The actual pressures seen during the compression test reflect some cooling and some leakage.

[Dave Neptune]

Al, good point and don't leave out "cylinder fill" at cranking speeds and the overlap which all lets some of the "air" to bounce back and forth.

Crazer, with those numbers she'll run. And once running the rings just may shake loose hopefully with in an hour or so of running.

Dave Neptune

[TomG]

Would the amount of MMI in the cylinder also effect the PSI? It seems that the volume of the cylinder would decrease with the addition of relatively incompressible MMI and cause the pressure to increase. At any rate, this is good news!

[ndutton]

Absolutely correct Tom. As fluids are non compressible they will affect the compressed volume by displacement. Enough fluid and a hydraulic event can happen damaging the crank, especially our no-center-main-bearing crank. Four rod journals is a long distance between bearing points.

[Crazer]

Originally Posted by Dave Neptune

Al, good point and don't leave out "cylinder fill" at cranking speeds and the overlap which all lets some of the "air" to bounce back and forth.

Crazer, with those numbers she'll run. And once running the rings just may shake loose hopefully with in an hour or so of running.

Dave Neptune

So if I understand correctly, the difference in dry and wet compression is that the oil creates a better seal between the piston rings and the cylinder wall. So the idea is that, while running, the rings will shake loose from the piston and on their own seal better against the cylinder wall?

I'm quite pleased with this turn of events The engine needs a lot of other work but now I can proceed confidently!

[Dave Neptune]

Crazer, that's what were looking for. She may be tough to start from all of the oil in the cylinders and that will soak into the rings as you crank.

Do run some MMO or my preference in the fuel a good TCW-3 rated 2 stroke oil mixed in the fuel at around 100:1 or even a bit richer. This "top lube" getting into the combustion chambers will also oil up the rings while lubing the cylinder walls and the valve stems (that part that sometimes sticks) all at the same time~a good thing.

Patience, I have seen many get "restarted" with poor numbers that after a bit of running the numbers were very acceptable.

Dave Neptune