Compression Test

[William Kelchner]

Trying to isolate a intermittent miss on my 30. The miss is sort of like a 'gasp' from the carb (Zenith).
Fuel is clean, Zenith filter clean, good fuel flow, the 'gasp' (for lack of a better description) is intermittent and only happens under load like driving up slight grades and hills. Of course I am unable to duplicate this problem idling in the driveway.
Took compression readings today and they were in the mid 60's 63-68, which according to the Model A Mechanics guide is not all that good. Did the test with hot engine, plugs out, throttle wide open. Plugs all looked the same, some carbon and tips slightly white.
Guess I am looking at a tired engine perhaps?

[700rpm]

If you have a stock head your compression sounds OK to me. But you may have a sticky valve. Try some MMO in your gas.

[Tom Wesenberg]

Have you held the coil wire 1/4" from a head nut to check for a hot blue spark?

Also do a google search for "coil polarity" and read the entry by "Chicagoland MG Club".

The wrong polarity makes the plugs fire a bit harder, and this would only show up when you need the most spark, like hill climbing or passing.

[Tom Wesenberg]

Coil Polarity

I was converting my older British car over from positive to negative ground when I came across the question of coil polarity. I discovered coil polarity is very much misunderstood. In researching it, I was very confused until I found out there are two definitions of coil polarity. I talked to three or four knowledgeable people on the subject and read several technical books and articles. Everything made sense in itself but didn't jive together until I found out they were talking apples and oranges.

Definition #l Coil Polarity (In relation to battery)

The polarity of the coil should match that of the battery by connecting it so (+) goes to (+) and (-) connects to (-). But don't worry about which way you install the battery (positive or negative ground) or which way you install the coil (regardless of coil markings) it will automatically adjust itself. The coil will work efficiently and put out the same voltage either way it is hooked up, but the spark plugs are more sensitive when it comes to polarity, hence our second and more important definition.

Definition #2 Coil Polarity (In relation to spark plugs)

Coil polarity should be such so as to provide negative polarity to the spark plugs center electrode.

It has been found that it takes approximately 15% less voltage to form an arc at the plugs, if the hotter center electrode is negative, and the cooler (by comparison) ground electrode is positive. The center electrode is hotter since heat transfer from the tip must make its way through the porcelain insulator past the sealing gaskets to the shell block and then to the water jackets.

If your center electrode is positive, your car will probably still run fine until, with its 15% handicap, it exceeds the coil output. If you live where temperatures dip down to 0° you may not get your car started. Driving with a full load and accelerating hard up a hill may cause an ignition miss. If your ignition system is well worn to where you have various voltage losses, you could get a miss.

Correct coil polarity won't eliminate these problems, just put them off by 15%.

If your coil has - & + markings by the primary terminals, you will be pretty safe by hooking it up by those marks, but test it for correct polarity anyway, using one of the tests listed further on. If your coil has CB & SW or BAT & DIST markings, there is no way of telling if the coil was marked in relation to a positive or negative ground car, and the only sure way to tell if the coil is installed right is to test it out.


You test for correct polarity by hooking up a voltmeter with the negative lead to the plug terminal (which should be of negative polarity) and the positive lead to the block (which should be of positive polarity). Set the meter on the highest volt range. These connections remain the same whether you have a positive ground or negative ground electrical system. The secondary winding's polarity which we are testing is determined by the combined hookup of the battery and primary windings, so it may or may not match the battery's ground.

Cranking the engine over (you don't have to start it) should show an upward swing of the voltmeter needle (don't be concerned with taking a reading). If the needle swings down off the scale, your coil is hooked up wrong. To correct, reverse coil primary leads. Do not worry about the coil markings (refer to definition #1).

If you don't have a voltmeter, test by removing a plug wire from a plug and hold a plain lead pencil point in the path of the arc. A flair (hard to see) towards the plug shows correct polarity while a flair towards the coil shows reversed polarity.

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[Patrick L.]

Compression readings seem fine especially if the needle 'bounce' was uniform from start to ending number. If that happened then you probably don't have any mechanical issues, but, some Sea-foam or MMO sure won't hurt.
Have you tried opening the GAV a bit to see what happens ?
Checking for a good spark and correct polarity is a good thing.
Let us know what happens, if these don't work, we'll go to work again.

[Fred K-OR]

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Have you checked your point setting lately? I had same problem and my point setting was way off.

[William Kelchner]

Thanks for all the suggestions - looks like a Monday project. I'll report back.

[Rex_A_Lott]

Lower than expected compression readings arent necessarily bad, as long as they are all consistent.
Intermittent breaking down under load is most likely ignition related, as has been mentioned.
Good Luck on your hunt!

[Paul from Maine]

I tried the pencil test and it worked! Showed I had correct polarity! I would advise anyone who tries it to remove the metal eraser cap and then to wrap electrical tape over the now exposed graphite core! If one doesn't, he will quickly learn the reasoning behind the conductor used in modern spark plug wiring!

[Mike V. Florida]

Also look at using a vacuum gauge it see what is happening.

[William Kelchner]

Tried the Voltmeter test last night and did not see any noticeable movement of the needle. Despite having the gas shut off it still starts immediately on fumes I guess. Pencil test is next. I have a vacuum gauge but no vacuum port on the intake.

[newshirt]

The gasp sounds ignition related to me. As if you are temporarily losing spark because of a loose connection or intermittent short. E.g. comes and goes as things vibrate.

These things could cause it (among many others):
1. Points arm occasionally touching lower part of points block
2. Frayed wire from the upper distributor plate to lower plate
3. Modern distributor plate
4. Loose coil wires
5. Lose ignition switch wires
6. Failing ignition switch

[Tom Wesenberg]

In addition to what Ray just posted, I have seen a distributor loose it's ground. The head and shaft of the dist. body should be clean of rust and paint. I like to coat it with grease or antisieze.

[hardtimes]

Originally Posted by William Kelchner

Tried the Voltmeter test last night and did not see any noticeable movement of the needle. Despite having the gas shut off it still starts immediately on fumes I guess. Pencil test is next. I have a vacuum gauge but no vacuum port on the intake.

Hey William,
Thanks for feed back ! It looks like you do not have a fuel problem , if it starts on fumes ! I agree with those guessing on elect problem. Good news is that that can be eliminated given thorough system check. Do NOT take anything for granted,eh ! For an example, I had a similar elect problem. Turned out to be a 'new' spark plug..go figure ! The bad plug had absolutely no evidence 'showing' that it had a factory defect.
Check/clean/retighten each every connection...especially in dist. Ground to dist, ground dist to block, ign switch. You will find it. I advise to make a written list so that you do not over look/forget/repeat.. anything in this process.

[William Kelchner]

Don't we just love these intermittent problems.. Tried the voltmeter again with same results. Tried the pencil test taking all the recommended precautions of course and promptly had the curl taken out of my hair and all my appendages retracted, I have abandoned this test. Gheeze.

I'll rig something up and try the coil lead and see if the spark is nice and blue later. Thanks again for all the advice...