Compression Test Results

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Model T Ford Forum: Forum 2016: Compression Test Results
Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry Saville on Friday, August 26, 2016 - 11:06 am:

Cylinder 1 50 LBS
Cylinder 2 50
Cylinder 3 50
Cylinder 4 40

I took all spark plugs out and used the starter with a full charge (6 Volt battery).

We started it the other day and it backfired most of the time we drove it. VERY low power. The exhaust manifold is warped (going to replace it).

I will be right there when I replace exhaust manifold, so I may as well take a look at the valves. Any idea what I may find, if anything?

Any takes on this?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Friday, August 26, 2016 - 11:10 am:

The compression test eliminates any question about the valves. Those are good numbers. No reason to look at the valves. Your head gasket is good. Your rings are good.

You likely have a fuel or ignition problem. I would start by spraying some WD-40 around the intake ports with the car running. If any change is detected then you know you have an intake leak that needs to be fixed.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Friday, August 26, 2016 - 11:17 am:

Are you running on the coils and magneto or do you have some other ignition system. A backfire is usually caused by fuel which does not burn and goes into the exhaust system. Then later hot exhaust or a late spark causes it to explode. This can be caused by retarded timing. You will notice it on a good running engine if you go downhill with the spark retarded. I think the source of your problem might be intermittent spark on one or more cylinders.
Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Harold Schwendeman - Sumner,WA on Friday, August 26, 2016 - 04:11 pm:

Royce - I agree that a cylinder with 40 lbs will fire just fine and run that way for a long time. However, if you're saying that it's not possible that #4 has 10 lb less compression that the other three cylinders because of a leaking valve, I'm not sure I agree with that. It's a known fact that a stock "T" engine will run great with 40 lbs compression or less, however, it's also a known fact that all four cylinders being reasonably EQUAL in compression, even an engine with considerably less than a healthy 50 lbs or so, is better than having all four cylinders with 50+ lbs. For the little effort it would take for Jerry to give #4 cylinder a wet and dry compression test (after all, he was all set up for compression testing anyway) I'd sure do the wet/dry compression test on #4. Obviously, if some oil in the cylinder does not raise the 40 lb number considerably, a valve is leaking, and I certainly would not rule out that possibility. Actually, a leaking valve seems more feasible than one cylinder with less compression due to "blow-by" from worn or damaged rings. When you think about it, a burned exhaust valve does not usually happen instantly; it usually happens due to a slight leak in an exhaust valve, maybe just from a little piece of carbon that does not let the valve close tightly, and that leak is leaking FIRE, and as such, will gradually become a burned valve if it is allowed to leak long enough. And personally, I'd rather pull the head and attend to one leaking valve before it gets worse and ends up as a burned valve. Anyway, just "thinking out loud", ....FWIW,....harold


Top of pagePrevious messageNext messageBottom of page Link to this message  By Harold Schwendeman - Sumner,WA on Friday, August 26, 2016 - 04:17 pm:

Actually, I've babbled on and on here too much already, but having done that, I should add one more "thought". I don't think I'd pull the head and do anything with the valves due to 3 cylinders with 50# and one with 40#, but I'd sure keep an eye on that one cylinder for awhile! It's quick and easy to pull that one spark plug, add a couple squirts of oil and check compression in that cylinder. Maybe every 10 or 15 miles or so. And if it stays at 40 lbs,.....great! If it goes down in the 30's or lower, THEN,.....pull the head and take care of the leaking (and burning) valve. Again, just my opinion,.......harold


Top of pagePrevious messageNext messageBottom of page Link to this message  By Harold Schwendeman - Sumner,WA on Friday, August 26, 2016 - 04:23 pm:

Jerry - Actually, I forgot to consider something you said in your original post,.....the backfiring! You didn't say, but I'm assuming you meant backfiring they' the exhaust, not thru' the carburetor. If backfiring thru' carb, might be that besides the warped exhaust manifold, you might have an intake manifold leak. If backfiring thru' exhaust, you might just have a sticking valve,.....FWIW,.....harold.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Harold Schwendeman - Sumner,WA on Friday, August 26, 2016 - 04:58 pm:

Oops! Just re-read this whole thread, and you already addressed the possibility of an intake manifold leak Royce,....sorry Royce,......harold


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