Low Compression

[jaadler]

Hello,

I have now had a A4 with a new to me 1970 Coronado 30 for about 1 year. The A4 has been easy to start and received a new carb in 2011. Recently it felt sluggish. I checked compression with the engine warm and here are the results:

Dry
#1 120
#2 120
#3 100
#4 70

I rechecked #3 and #4 after adding MMO to the spark plug opening:

#3 100 (no change)
#4 100 (increased)

My question has to do with the next step. If the specs for the engine put compression at 100, why would I get 120 on the first 2 cylinders? Also, with the compression rising in #4 with MMO, does this implicate rings or could this be a valve that is partially stuck? My understanding is if the valve was completely stuck the compression would be 0.

From what I read the #4 exhaust valve often gets stuck. People seem to use MMO and an allen wrench to work it free, but I am not sure this my problem.

Do I need to be thinking a bad head gasket? There is no water in the cylinder or oil.

Thanks for any direction you can point me.

Jeremy

[Mo]

Jeremy,
I'd add some MMO into the plug holes and see if it changes over the next little while. I generally don't run an engine hard with valve issues. "Generally" the addition of oil improving compression could mean a ring issue, however, could also be a slow non-seating valve. A few treatments of MMO in the plug area and subsequent gentle running should give you a good indication if it will resolve and free-up.

Don't worry about 120 on 1 and 2 or any cylinders...that's really not uncommon.

[tenders]

Sluggishness at this time of year is very often related to barnacle growth on the prop. If you haven't cleaned it in two months or more, I'd suggest ruling that out before you go looking for engine trouble. A little growth hinders the prop a LOT.

Your compression is fine. Maybe add a little Margel to the fuel and see if that last valve frees up, but there is no other need for action.

[Al Schober]

Jeremy,
70 psi isn't really that bad. The engine will run with one cylinder at 70.
Keep in mind that rings do wear on these engines. This is a good thing, as the blocks don't seem to. I have yet to do an engine that I've had to ream a ridge! Rings are easier to replace than pistons (after boring the block oversize). The ring that generally shows the most wear is the lower oil control ring. Spec end gap on this ring is less than .010", but I've seen worn engines with a gap over .10"! That ring was just along for the ride - no wonder the owner was complaining of smoke in the cabin.
So, how do your plugs look? Any smoke from the oil filler?
What's the history of use of Marvel Mystery Oil in the fuel? IMHO, I don't like it in the fuel. I feel it tends to build up on the backs of the inlet valves leading to restriction (and perhaps poor seating). A bit in the crankcase is fine.
Don't get too excited about the absolute compression numbers. When new, these engines had a head gasket that measured .060" compressed - asbestos between copper sheets. The 'SeaCraft' manual lists the original compression as 6.3:1. Shave a bit off the block or head and install a thinner gasket and compression will go up. The thing to look at is the variation between cylinders, and your numbers indicate something going on with #4. The improvement with addition of oil indicates something going on with the rings.
How do you feel about your ability to pull the engine (during the off season - assuming you have one..) and doing a ring job? I'm an advocate of the 'spare engine' school - have a rebuild in the basement ready to go. Last swap I did the old engine (cracked block) came out one afternoon, and the replacement was in and running the next day.
"Discarded' engines are not expensive - free to a few hundred. The rebuild will cost at least $100 for the rings and $100 for the gasket set - plus other stuff that you decide to buy for your new 'baby'. My last rebuild (still in the basement) got the stainless cross shaft for the tramsmission. Also needed ignition cables, but happened to have a set of those (Volvo) in the garage.
The A4 is a great engine - keep it reliable. You want it as reliable as gravity - drop the rock & it will hit the ground. For the A4, turn the key and it will start.

[romantic comedy]

I have 120 to 125 in all of the cylinders, in my 1974 Atomic Four.

[Dave Neptune]

Ah #4

Jeremy, the #4 cylinder is almost always the one that takes a beating. It does not always get the cooling as the other 3 and it is the one most likely to experience water coming back up the exhaust. It also runs richer and is more likely to have a bit of carbon in the ring grooves. Having a bit of compression trouble with it is not that uncommon. Mine ran low for many years after seizing and is now fairly good and I have no worries about it.

I'm with Al as far as the MMO in the fuel but I do highly recommend that you do put some good 2 stroke oil in. A mix of 80~100:1 will get oil that is DESIGNED to lubricate and burn with fuel through the engine. I'd run at least a tank or two of it and then check the compression again. I would also BE SURE to do a compression check after warming the engine up to operating temp. Pull the plugs with gloves or something creative and get the numbers ASAP. Then after the engine completely cools like the next day do another check first dry and then with a bit of oil in the cylinders.

I have been a mechanic most of my life and have found many marine engines as well as our A-4's with sticky rings. These rings often stick when the piston is cold and free up when warm. This is something you can fix if it drives you NUTZ or just live with it and be happy!

Dave Neptune

[Minnesota_Slick]

I do highly recommend that you do put some good 2 stroke oil in. A mix of 80~100:1 will get oil that is DESIGNED to lubricate and burn with fuel through the engine.

Whoa, is this a general recommendation for everyone, or only for folks with possible compression problems?

[Dave Neptune]

Confused

Hey Slick, it's just for loosening rings or valves! Otherwise I personally only use and recommend regular gas from a gas station and absolutely no additives. I have also not had any fuel problems for 28 years~29 in October! My tank is now 43 years old, just like the engine.

I do burn up my old 2-strokle and yard gas to start the season though, it will have some 2 stroke oil in it. That oil will be gone by the second trip. I motor about a 150 hours a season.

Dave Neptune

[Minnesota_Slick]

Thanks for the clarification, Dave.

[Mo]

Hey Slick, it's just for loosening rings or valves! Otherwise I personally only use and recommend regular gas from a gas station and absolutely no additives. I have also not had any fuel problems for 28 years~29 in October! My tank is now 43 years old, just like the engine.

I do burn up my old 2-strokle and yard gas to start the season though, it will have some 2 stroke oil in it. That oil will be gone by the second trip. I motor about a 150 hours a season.

Dave Neptune

Pretty much my take on fuel additives as well Dave. Stabilizer in last tank for the season before she comes out for winter...that's it.

[jaadler]

Thanks so much for all the replies. I haven't been down to the boat, but it looks like I am in for a whole lot less than I thought. I did just have the bottom scraped so that may help. I did add some MMO to the gas and I'll see what that does as well. This forum is amazing!

Take care,
Jeremy