Ford Model AA Trucks

By Wade L. Ratzlaff, President San Fernando Valley Model A Club

Compression, Spark, Carburetion—and Knowing Which One to Blame

There is an old rule in Model A mechanics: before you start adjusting things, figure out what is actually wrong.

That sounds obvious, but anyone who has owned a Model A longer than about three weeks knows how this usually works. The engine starts running rough, so we adjust the carburetor. That doesn't fix it, so we change the points. Then we advance the timing, retard the timing, clean the plugs, adjust the carburetor again, replace the condenser, and eventually discover that cylinder number three had a burned exhaust valve the entire time.

By then the Model A may still run badly, but at least we've disturbed nearly every adjustment Henry Ford carefully specified.

A proper Model A tune-up should be approached in three steps, and the order matters: (See Andrews, Model A Ford Mechanics Handbook; MAFCA, “Tune-up tests – compression, vacuum, ignition, carb.”)

1. Compression and mechanical condition
2. Ignition
3. Carburetion

Think of it this way: the engine needs something to squeeze, something to spark, and something to burn. If it can't squeeze properly, there isn't much sense blaming the carburetor.

The Shade Tree Toolbox

You don't need a computerized diagnostic machine, an oscilloscope, or a fellow wearing a shirt with his name embroidered over the pocket. A surprisingly small collection of tools will tell you quite a bit about the health of a Model A engine.

You should have:

• Compression gauge
• Vacuum gauge
• Battery hydrometer
• Voltmeter
• Feeler gauges
• Spark plug gap gauge
• Basic hand tools
• Oil can
• Hand crank
• A reasonable amount of patience

The last item is often the hardest to locate.

Modern compression gauges sometimes come with rubber adapters that are too small for the Model A spark plug opening. A larger rubber adapter can be fabricated from something such as a shock absorber rubber cushion. Model A mechanics have been manufacturing perfectly functional tools out of miscellaneous objects found around the garage since approximately five minutes after the first Model A warranty expired.

Part One: Compression—Before You Blame Everything Else

Compression testing tells you about the mechanical condition of the engine.

Before testing, run the engine until it reaches normal operating temperature. The pistons, rings, valves, and cylinders will then be approximately the dimensions they normally see while the engine is operating. (Andrews, Model A Ford Troubleshooting & Diagnostics; MAFCA, “Compression testing.”)

Shut the engine off and remove all four spark plugs. Open the throttle completely. Install your compression gauge into the first cylinder and crank the engine with the starter through approximately four or five revolutions, or until the gauge stops climbing. Record the reading and repeat the test on the remaining cylinders. (MAFCA, “Compression testing”; Page, The Ford Model A Car.)

On a healthy engine, the exact number is somewhat less important than how close the four cylinders are to one another. Ideally, the readings should be within approximately 5 to 10 pounds of each other.

Example of an encouraging set of readings:

• Cylinder 1 — 72 psi
• Cylinder 2 — 74 psi
• Cylinder 3 — 71 psi
• Cylinder 4 — 73 psi

Something like 72, 71, 48, and 73 psi means cylinder number three would like to have a conversation.

Two Low Cylinders Next to Each Other

If two adjacent cylinders have unusually low compression, suspect a possible head gasket failure between the cylinders. (Andrews, Model A Ford Troubleshooting & Diagnostics.)

Before dismantling anything, check the cylinder head nuts and torque them properly. A commonly used Model A specification is approximately 50 ft-lb, although head type, gasket manufacturer, and engine configuration should always be considered.

Do not simply keep tightening head nuts in the hope that compression will eventually return. There is a fine line between properly torqued and 'Well, now I need a new stud.'

One Low Cylinder

If one cylinder is considerably lower than the others, perform a simple wet compression test. Squirt a small amount of engine oil into the cylinder through the spark plug hole. Try to direct it toward the cylinder wall instead of flooding the valves. Repeat the compression test. (Andrews, Model A Ford Troubleshooting & Diagnostics; MAFCA, “Compression testing.”)

If the compression increases substantially, the oil temporarily helped seal the piston rings. That points toward:

• Worn rings
• Stuck rings
• Worn cylinder walls
• Scored cylinders
• Piston problems

If the compression remains low even after adding oil, suspect:

• Burned valve
• Leaking valve
• Valve not seating properly
• Valve clearance problem

This little test can save you an enormous amount of unnecessary tinkering.

The Vacuum Gauge: The Model A Lie Detector

A vacuum gauge may be one of the most useful diagnostic tools you can attach to an old engine. It does not tell you exactly what is wrong, but the needle’s behavior gives you clues about what is happening inside the engine. (MAFCA, “Tune-up tests – compression, vacuum, ignition, carb.”)

Before connecting the gauge, make certain the intake manifold connections are tight. Check the manifold nuts and gaskets. A vacuum leak will give you misleading readings. (MAFCA, “Vacuum leaks (thorough).”)

With the engine warmed up and running properly, you may see approximately 17 to 21 inches of manifold vacuum, depending on engine condition, elevation, camshaft, and other factors. Do not become obsessed with one exact number. What matters greatly is whether the needle is steady and how it reacts when the throttle changes. (Andrews, Model A Ford Troubleshooting & Diagnostics.)

The Quick Throttle Test

With the engine idling steadily, quickly open the throttle and then let it close. A healthy engine should generally:

• Drop vacuum sharply when the throttle opens
• Rise considerably above normal when the throttle snaps closed
• Settle back to a steady idle reading

A typical example might be 18–20 inches at idle, about 2 inches with the throttle opened, roughly 25 inches when it closes suddenly, and then a return to normal idle vacuum.

What the Vacuum Needle Is Trying to Tell You

Low but Steady Vacuum

A steady reading around 15 inches may indicate worn piston rings, scored cylinder walls, damaged pistons, or incorrect ignition timing. A steady reading closer to 10 inches can suggest incorrect valve timing. Use other symptoms and tests to narrow things down.

Slowly Wandering Needle

If the needle slowly wanders between roughly 14 and 16 inches, look at ignition point adjustment, spark plug gap, and carburetor adjustment. The engine may simply be unevenly burning the mixture.

Sticking Valve

A sticking valve may cause the vacuum needle to occasionally drop several inches and then return to normal. This is particularly useful because an intermittent sticking valve can otherwise make you question everything you thought you understood about engines.

Burned or Leaking Valve

A burned or leaking valve can cause a repeated sharp drop in vacuum every time that cylinder comes around. If the needle regularly drops at the same rhythm, pay attention. Your Model A may be telling you exactly which cylinder is unhappy.

Weak Valve Springs

Raise the engine speed. If the needle begins bouncing significantly at higher RPM, weak valve springs may be allowing the valves to float. Weak springs might behave acceptably around town but become troublesome when engine speed increases. Of course, “high RPM” in a Model A is a relative term. Nobody is preparing for the Indianapolis 500 here.

Intake Leak

A serious leak at the intake manifold, manifold gasket, or carburetor mounting gasket can cause unusually low vacuum. Before rebuilding the engine, make certain you are not dealing with something as simple as a loose manifold.

Compression Leak Between Cylinders

Large fluctuations, sometimes ranging between roughly 5 and 19 inches, may indicate compression leaking between adjacent cylinders. That brings the head gasket back onto the suspect list.

Restricted Exhaust

If vacuum starts normally but then slowly drops toward zero while the engine continues running, suspect a restricted exhaust system. A crushed exhaust pipe, internally collapsed muffler, or significant restriction can prevent the engine from breathing. An engine is basically an air pump. If the exhaust cannot get out, eventually the intake cannot get in.

Don't Tune Around Mechanical Problems

If the compression and vacuum tests uncover burned valves, bad valve seats, worn cylinders, broken valve springs, severely worn rings, or damaged pistons, stop the tune-up and fix the mechanical problem first.

You cannot adjust the carburetor enough to repair a burned exhaust valve, although generations of shade tree mechanics have certainly attempted it.

A Word About Model A Valves

Proper valve clearance is essential. Valve tappet clearance affects when the valve actually opens and closes. If the clearance is incorrect, engine performance suffers. (McRee and McRee, The Model A Engine, Distributor and Oil Pump; Ford Motor Company, Model A Ford Service Bulletins.)

Too little clearance can prevent a valve from fully seating as the engine warms up. A valve that does not completely seat cannot properly transfer heat into the block. Eventually that valve may overheat and burn.

Symptoms can include:

• Poor idle
• Low compression
• Loss of power
• Overheating
• Difficult starting

Loose valve guides should be replaced, and weak or broken valve springs should also be corrected. When performing valve work, do not grind the valve face excessively. Removing too much material changes how the valve contacts its seat and can create overheating problems. Sometimes a new valve is cheaper than proving how talented you are with a grinder.

Part Two: Ignition—Where Electricity Becomes Motion

Once you know the engine is mechanically healthy, move to the ignition system. The Model A ignition system is wonderfully simple, which unfortunately means every single connection matters. Before touching the distributor, start with the battery.

Check the Battery First

A weak battery affects cranking speed, starting, ignition voltage, and electrical testing. Make certain the battery terminals are clean and tight. (Ford Motor Company, Ford Model A Instruction Book; McRee and McRee, The Model A Generator, Starter, Cutout and Starter Switch.)

Corrosion can be cleaned using warm water and baking soda. Once everything is clean and dry, a light coating of petroleum jelly or appropriate terminal protectant helps slow future corrosion.

A hydrometer can be used on a conventional serviceable lead-acid battery to check specific gravity. A fully charged battery cell might read roughly 1.270–1.280. A reading below approximately 1.225 indicates the battery should be charged before further electrical testing. Many Model A owners today use different battery technologies, so follow the battery manufacturer’s recommendations when appropriate.

Check the Starter Circuit

On a Model A, voltage drop matters. The original electrical system operates on only six volts. Losing half a volt through a dirty connection is much more significant on a six-volt system than on a modern twelve-volt system. (McRee and McRee, The Model A Generator, Starter, Cutout and Starter Switch.)

That is why Model A battery cables need to be heavy, connections need to be clean, and grounds need to be excellent. While cranking the engine, battery voltage should remain reasonably strong. A substantial voltage collapse could indicate a weak battery, excessive starter draw, poor cables, dirty connections, or a poor ground.

Grounds: The Invisible Trouble Maker

One of the best rules for troubleshooting an old Ford electrical system is: When something electrical behaves strangely, clean the grounds before purchasing anything.

Check the battery ground connection carefully. Also remember that electricity has to travel through the engine, frame, cables, switches, and various connection points. Paint, rust, grease, corrosion, and ninety-something years of dirt are not particularly good conductors. A shiny connection is your friend.

Primary and Secondary Ignition

Primary Circuit

The primary side includes the battery supply, terminal box connections, coil primary wiring, ignition switch, armored distributor cable, distributor points, condenser, and associated wiring and connections. (MAFCA, “How to test the ignition circuit”; MARC, “How the Model A distributor works.”)

Loose or corroded connections create resistance. Resistance creates voltage loss. Voltage loss creates weak spark. Weak spark creates poor starting and rough running. And poor running creates a Model A owner standing alongside the road explaining to passing motorists that everything was working perfectly yesterday.

Secondary Circuit

The secondary ignition system includes the coil high-voltage winding, coil wire, distributor rotor, distributor cap, spark plug connectors, and spark plugs. Inspect all of it for carbon tracks, cracks, corrosion, loose terminals, damaged wires, and burned contacts.

Distributor Points

Remove the distributor cap and rotor and inspect the points. They should be clean and flat. Replace points that are badly burned, pitted, or cratered.

The generally accepted Model A point gap is approximately .018 to .022 inch. Check that the point arm moves freely on its pivot. Point spring tension also matters. Too little spring tension can allow the points to float at higher engine speeds, while incorrect excessive tension can create its own problems. (Ford Motor Company, Model A Ford Service Bulletins; MARC, “How the Model A distributor works.”)

If the points are badly burned or pitted, also consider replacing the condenser. A condenser can be properly tested with specialized equipment, but because condensers are relatively inexpensive, replacement is often the practical roadside or garage diagnosis. That said, modern replacement condensers can vary greatly in quality. Do not automatically throw away a known good condenser merely because the new one came in a shiny box.

Reading Your Spark Plugs

Spark plugs are tiny windows into the combustion chamber. Remove them and look carefully. (MAFCA, “Spark Plugs: Reading – an engine analysis.”)

• Light tan, brown, or grayish: Generally healthy combustion.
• Black and sooty: May indicate a rich mixture, too cold a plug, excessive idling, or weak ignition.
• Oily: Can suggest oil-control problems, worn rings, worn guides, or other internal wear.
• Blistered or unusually white: May indicate excessive temperature, an overly lean condition, or a plug that is too hot.

Spark plug heat range matters. A hot plug holds heat longer. A cold plug transfers heat to the cylinder head more quickly. Cars driven slowly for short distances sometimes prefer a different heat range than cars operated continuously at highway speed.

Spark Plug Gap

Use a wire-style spark plug gauge. A traditional Model A spark plug gap is approximately .027 inch. (Ford Motor Company, Model A Ford Service Bulletins; Andrews, Model A Ford Mechanics Handbook.)

A gap that is too wide can contribute to hard starting, misfiring, and poor high-speed performance. A gap that is too narrow can result in weak ignition, rough idle, and poor low-speed performance. Set all four plugs consistently. An engine likes equality.

Timing the Model A

Timing a Model A is not difficult once you understand what you are doing. The goal is to establish the distributor position with number one cylinder at top dead center on its compression stroke. (Ford Motor Company, Model A Ford Service Bulletins; MAFCA, “Ignition Timing,” Parts 1-3.)

1. Move the spark lever to the fully retarded position.
2. Remove the timing pin from the front timing gear cover and insert the opposite end into the hole.
3. Slowly rotate the engine with the hand crank.
4. As number one piston approaches top dead center, the timing pin will drop into the recess in the timing gear. Stop there.
5. Loosen the distributor cam screw enough to rotate the cam.
6. Rotate the cam counterclockwise so the points open, then slowly rotate it clockwise until the points are just beginning to close.
7. Lock the cam in that position.
8. Reinstall the rotor, distributor cap, and timing pin.

Do not forget that timing pin. Leaving it out produces an impressive oil leak, usually discovered five miles later when the underside of the car has successfully rust-proofed itself.

Part Three: Carburetion—Last Means Last

Now we finally reach the carburetor. This is where many Model A owners begin their tune-up. It is where they should finish it.

Carburetor operation depends on the engine having adequate compression, correct valve action, strong ignition, and proper timing. If those things are wrong, no carburetor adjustment is going to make the engine truly happy. (Moller, The Model A Ford Carburetors; MAFCA, “Carburetor: Principles of operation.”)

Carburetor Inspection

If the engine still refuses to idle properly after compression and ignition have been confirmed, inspect the carburetor. If necessary:

1. Remove the carburetor.
2. Disassemble it carefully.
3. Clean the passages with proper carburetor cleaner.
4. Blow passages clear with compressed air.
5. Inspect jets and passages.
6. Replace damaged gaskets.
7. Replace worn components as necessary.
8. Reassemble carefully.

Do not run drills through precision jets unless you know exactly what you are doing. The jet opening determines fuel flow. Enlarging one with a drill bit changes the carburetor calibration. Sometimes cleaning quickly becomes engineering. (Pargeter, Zenith Model A Carburetor Restoration Guidelines; Biggar, Restoring the Model A and B Carburetors.)

Setting the Idle

Warm the engine completely before adjusting the carburetor. Place the spark lever in the retarded position. Use the throttle stop screw to keep the engine running at a slow, stable idle. Then adjust the idle mixture screw. (Moller, The Model A Ford Carburetors; Pargeter, Zenith Model A Carburetor Restoration Guidelines.)

Turn it slowly until the engine begins running unevenly, then gradually adjust in the opposite direction until you find the smoothest idle. Make small changes and give the engine a few seconds to respond between adjustments.

A Model A carburetor is not a combination lock. Spinning adjustment screws wildly back and forth seldom improves anything. Once the engine idles smoothly, recheck the idle speed and make the final adjustment.

The Shade Tree Tune-Up Order

When troubleshooting a Model A that is not running correctly, remember this sequence:

Compression → Ignition → Carburetion

Not: Carburetor → Carburetor → Distributor → Carburetor → Buy new carburetor → Discover burned valve.

Compression tells you whether the engine is mechanically capable of running correctly. Ignition determines whether the mixture is being lit correctly. Carburetion determines whether the proper fuel-and-air mixture is being delivered. Each system depends upon the one before it. (Andrews, Model A Ford Mechanics Handbook; MAFCA, “Tune-up tests – compression, vacuum, ignition, carb.”)

Final Thoughts From Under the Shade Tree

One of the wonderful things about the Model A Ford is that the car will usually tell you what is wrong if you are willing to listen.

The compression gauge tells you about cylinders and valves. The vacuum gauge tells you how the engine is breathing. The spark plugs tell you how it is burning fuel. The voltmeter tells you whether the electricity is actually getting where Henry intended it to go. And your ears will eventually become another diagnostic instrument.

A properly tuned Model A should start easily, idle smoothly, accelerate without hesitation, pull strongly, and give reasonable fuel economy.

Do not adjust three things at once. Do not replace parts simply because they are old. And above all, do not start turning carburetor screws because the engine coughed once at a stop sign.

Test first. Adjust second. Drive third.

Because the entire purpose of tuning a Model A is not to spend Saturday afternoon staring into the engine compartment. It is to get the hood closed before your buddies arrive and make it look like you knew what you were doing all along.

Reference note: Parenthetical references throughout the article point to the full entries in the bibliography below.

Bibliography

Selected factory literature, technical books, restoration manuals, and periodical articles relevant to Model A Ford engine service, diagnosis, ignition, carburetion, and maintenance:

Books, Manuals, and Factory References

Andrews, Les. Model A Ford Mechanics Handbook. Cottage Hill Publishing, 1997.

Andrews, Les. Model A Ford Mechanics Handbook II. Cottage Hill Publishing, 2003.

Andrews, Les. Model A Ford Troubleshooting & Diagnostics. Cottage Hill Publishing, 1997.

Biggar, Gordon. Restoring the Model A and B Carburetors – The "Rex" Reheis Way. 2004.

Clymer, Floyd, ed. How to Restore the Model A Ford. Clymer Publications, 1961.

DeAngelis, George, Edward P. Francis, and Leslie R. Henry. The Ford Model A As Henry Built It. 1971.

Fahnestock, Murray. Know the Ford. 1930.

Fahnestock, Murray. Questions and Answers on Ford Service. n.d.

Ford Motor Company. Ford Model A Instruction Book. Dearborn, MI: Ford Motor Company, 1931.

Ford Motor Company. Model A Ford Service Bulletins, 1928-1931. Dearborn, MI: Ford Motor Company, 1928-1931.

Hopper, Gordon E. Model A Ford Restoration Handbook. Floyd Clymer, 1966.

McCoul, Marith. Practical Information About 1930 and 1931 Coupes. Revised ed., 2005.

McRee, Paul H., and William H. McRee. The Model A Engine, Distributor and Oil Pump. 2002.

McRee, Paul H., and William H. McRee. The Model A Generator, Starter, Cutout and Starter Switch. 2002.

Model A Ford Club of America and Model A Restorers Club. Model A Restoration Guidelines and Judging Standards. Jointly published by MAFCA and MARC.

Moller, Paul. Model A Ford Restoration and Maintenance Handbook, Vol. 1. 1981.

Moller, Paul. Model A Ford Restoration and Maintenance Handbook, Vol. 2. 1984.

Moller, Paul. The Model A Ford Carburetors. 1982.

Page, Victor W. The Ford Model A Car: Construction, Operation, and Repair. 1929. Later reprinted as Model A Ford: Construction, Operation, Repair for the Restorer.

Pargeter, Steve. Zenith Model A Carburetor Restoration Guidelines. Version 3, n.d.

Periodical and Newsletter Articles

Model A Ford Club of America. "Tune-up tests – compression, vacuum, ignition, carb." The Restorer, Nov./Dec. 1979, p. 20.

Model A Ford Club of America. "Compression testing." The Restorer, Mar./Apr. 2014, p. 12.

Model A Ford Club of America. "Vacuum leaks (thorough)." The Restorer, Mar./Apr. 2016, p. 8.

Model A Ford Club of America. "How to test the ignition circuit." The Restorer, Mar./Apr. 2009, p. 23.

Model A Restorers Club. "How the Model A distributor works." Model A News, Nov./Dec. 2017, p. 20.

Model A Ford Club of America. "Ignition Timing, Part 1 – Terms." The Restorer, Mar./Apr. 2009, p. 13.

Model A Ford Club of America. "Ignition Timing, Part 2 – Common problems & specs." The Restorer, May/June 2009, p. 12.

Model A Ford Club of America. "Ignition Timing, Part 3 – Physical timing." The Restorer, July/Aug. 2009, p. 17.

Model A Ford Club of America. "Spark Plugs: Reading – an engine analysis." The Restorer, Jan./Feb. 2000, p. 6.

Model A Ford Club of America. "Carburetor: Principles of operation." The Restorer, Mar./Apr. 2024, p. 15.