Disappointing Compression Results

Tried before to get a compression test on the motor and came to the conclusion that my old compression tester was 'kaput'.
Called in at a couple of auto places the other day and looked at a few new ones. Decided to get a mid-range priced one - $40.
I drained the remaining oil and refilled with 20w-50w.
With David's help, I was able to get some readings. Using the starter from a battery pack and about 5 turns of the motor.
These are the results:
1 =less that 50 psi, 2 = 95 psi, 3 = 70 psi, 4 = less than 50 psi.
Very disappointing. Not sure the best way to go from here, I'll still try and get it to run and I may take the head off to see if I can get any further information. A 'wet' test did not make any appreciable difference to # 1 cylinder and I had little motivation to continue with the others.
Not a good day for the MGB. :(

Chris

You should test with the plugs out and give it 10-12 spins/cylinder.
Try resetting the valves.
If they are set properly and oil doesn't bring up the compression, you are likely looking at bad valves. If it were burned pistons, the head would have to come off anyway so it really doesn't matter at that point.
You can test the head while off by pouring solvent into the manifold inlets at the side and seeing if the fluid then runs out the valve.

Being that low and wet not making a diff would indicate valve problems. If you care to diagnose any further, you could put air pressure to the plug hole and listen for it comeing out the exh. Common for burnt exh valves especially the early engines. Stellite valves and hardened seats on the exh is the way to go.

Also make sure that your throttle is open

Sounds as if your problems are in your head.

Do you have hardened valve seats?

Wow, where to start??? First, three cranks is usually sufficient. Second, if your compression is that low and your gauge is good, it could be valves, rings, pistons, anything. A leak-down test would help you determine which but you know enough to know that at least the top end is coming apart. Pull the rocker cover and unless you see a lot of valves stuck open, pull the head. Sorry. Basil

Is this a running engine? If so, I'd put a new set of plugs in and warm it up, then remove them and redo the test.

Often a bit of crud falls off the plug threads when you pull old plugs, and this can mess with your test. Not saying you will then have 160lbs in all cylinders, but the readings should be higher giving a more accurate picture.

$40 for a mid-range compression gauge...wow they have really gone up since I bought my last one 30 years ago!

Chris,
How long's it been since this engine was running?

that low of compression doesnt matter new plugs or what it is not going to run at all i say needs at least 90 lbs compression to at least run

You know that the head will need to be done anyway, even if you do a valve job now it won't be money you wouldn't have spent doing a complete engine rebuild. I'd try pulling the head and doing it first. The only real thing you will lose is your time and some gaskets.

I never put alot of faith in a compression check, at most it's a check procedure to see if things are really bad, and in your case I fear they are. I've seen good compression reading form cylinder with broken rings, the last time I saw a low reading (20 pounds) there was a hole in the piston when I got the head off. A leak down test would tell you one thing for sure, is it just the head or it is the rings as well, this would save you time of going through a installing afresh head on a worn bottom end to only need to pull it again. Here's the big question I would like to know, are you getting blue smoke out of the tailpipe, if so, it's more than head. You might also get your hands on a pressure checker, one that gos on the radiator cap, to make sure the cooling system is holding pressure, this can let rule out a blown head gasket.

Thanks for the responses.
To try to answer all the questions:
This motor is out of the car, bought out of the car. PO said it was running when he took it out in 1990. Took it out because of a g/box issue.
It's on a stand I built with new clutch and gearbox attached. I'm running it with the starter motor I rehabed off a 12v battery pack.
Oil was the blackest I've ever seen and very sludgy. Changed to new oil.
Motor spins freely and I did the checks with all plugs out.
I think I know the answer here, I just don't want to admit it.
I think the only thing that was running when I bought it was the PO after he got my money.

Chris

Since its on the engine stand already you may as well pull the head. De-carbonize the combustion chambers and check for scoring of the cylinder walls. Do a leak down test on the head. The valves are probably leaking like sieves. If the valves aren't burnt you can probably get away with a shade tree valve lap by taking the springs off, put a cordless drill chuck onto each valve (one at a time of coarse) rub a small amount of valve grinding compound around the valve seat, then run the drill while pulling and pushing the drill to seat and unseat the valve until you have a clean seal....Hey, it's cheap, but effective. As soon as the valves pass a leak down test you can slap the head and new head gasket on and Voila!..Compression!

I had a similar problem on a rebuilt motor that I bought with no information. I turned out to be the cam timing was way off. Retarded by 4 to 6 degrees. Read the topic I put out today "1977 - Can't set timing - update".

It may be worth it to take a look at the timing chain and gears. Maybe the timing chain needs to be adjusted being that you have a worn in motor. Best of luck.

Well, that's tough. Like I said, the head has t ocome off regardless of your plans, and you can test whether it is the valves by pouring solvent into the manifold openings and looking for leakage. If there is, you know that valves is part or all of your problem.
By the by, I have had a VW engine running on 25-50Lbs compression, all 4 cylinders. And it moved the bus.

Any chance that an engine sitting that long would have sticking rings?

Did you do the test with the throttle wide open? That can make a great difference.

A thought came via some of your responses - since it's not been run for over decade, could it be that some of the valves are sticking and with the rocker cover off how can I tell if the valves are returning to their seats correctly?
I feel a rebuild is in my future but I would like to get a result with minimal intrusion into the motor if at all possible.

Question about carburettors. Twin SU's with out the air filters on and I can see the cylinder looking thingy's looking into each inlet. Should these be free to move up and down? Mine are stuck solid in the down position and won't move up with a far bit of pressure applied with my fingers.

Chris

Yes they should move freely. Take the three screws out on top and remove the air chambers. Then the pistons will lift right out. Clean everything real good with some carb cleaner and reasemble. Don't use any sandpaper on anything cause those pieces are machined to exacting tolerances. Hopefully, they are just varnished up & not corroded

sweep Wrote:

Or you could unbolt the carbs and test again. Definitely wide open throttle that way!

So the engine hasn't run in some time, and is already out and on a stand? If it were mine I'd just tear the whole thing down. That would be the best way to diagnose it's true condition at this point.

The carbs also!

Good plan Rod. At the least it would cost time and a gasket set but the peace of mind would be priceless; to me anyways.
This compression testing with open throttles; I've never worried about that. I want to know what the cranking compression is just like a car that is waiting to start. In 10-12 revolutions you will have a compression number that will not climb anymore.