Poly 318 Ignition Timing, Points Adjustment, Distributor Conversion

(applicable to 277, 301, 303, 313, 318, 326, 390, 402 and LA 273, 318, 340 and 360 engines)


Poly 318 and LA Ignition timing, distributor points cleaning and adjustment, and electronic ignition conversion can seem confusing and overwhelming due to the multiple variables, so I wrote this article to discuss my recommendations and the methods I use.

Timing is highly dependent on the specific engine build, the air-fuel mixture, distributor specs, and other variables and should be considered. Manual recommendations are baselines for economy commuting and are not necessarily the “correct” timing for optimal performance. I break the timing procedure into the camps of “basic” and “optimal.” I recommend using the “optimal” method on all engines since it will give the best timing for the specific engine, but I provide both methods since some people may not want to use a quicker “typical” method if optimal timing isn’t desired. Even if you are interested in the “optimal” method, read through the “basic” method first since I provide necessary details that apply to the “optimal” procedure.

Basic Timing Procedure (good for stock engine street cars):

This is the type of tuning the majority of people come across in repair manuals, which doesn’t make it incorrect, but it doesn’t make it optimal. For the majority of stock and mildly hopped-up street engines, it will do the trick to keep the engine running.

The following specifications are a good starting point for tuning, and I go into further details to explain what these numbers and terms mean:

  • Hot Idle Speed: 500 – 850 rpm as the cam requires in gear for an automatic and in neutral for a standard transmission.
    • Note: set hot idle with the A/C compressor running.
  • Initial Timing set at hot idle without vacuum advance attached: 10° BTDC
  • Total Advance at 4,000 rpm without vacuum advance attached: 34° – 36° BTDC
  • Total Advance at 4,000 rpm with vacuum advance attached: 42° – 52° BTDC

If I wanted to perform a basic ignition tune-up for a stock street engine with vacuum advance, this is how I would proceed using a timing gun equipped with an integrated tachometer and dwell meter (if running points).

  • Note: Performance mechanical-advance distributors—such as a Mallory YL dual-point mechanical—are a different beast and designed for wide-open-throttle racing, so follow the distributor instructions for adjusting the total mechanical advance and the advance curve.
  1. With the engine off, remove the air cleaner.
  2. Loosen the distributor clamp just enough to rotate the distributor with some resistance so it will not vibrate around on its own.
  3. Unplug the vacuum line from the distributor and plug the hose. Tip: A golf-ball tee makes an excellent hose plug (I still use the one my father used in the 1960s).
  4. Setup the timing gun with 12V power, a confirmed ground, the spark lead connected to cylinder 1 (Figure 1), and the dwell lead hooked to the coil negative (if running points). Ensure the wires/pickup are clear of exhaust, fan blades, and belts.
  5. Start the engine and bring it up to running temperature ensuring visually the carburetor choke plate is fully open. Resting a hand atop the radiator at the intake port will tell when the thermostat opens when hot coolant rushes in.
  6. Set proper idle speed to 500 – 850 depending on the cam’s lope by adjusting the screw on the carburetor with the transmission in gear (automatic) or neutral (standard).
  7. Set the carburetor’s idle air-fuel mixture (see my carburetor tuning article for this adjustment).
  8. For points, check the dwell and adjust as necessary (see the section on points adjustment below).
  9. With air-fuel mixture, hot idle speed, and dwell adjusted, set the timing gun to 10° BTDC and strobe the timing mark (Figure 2). Slowly retard or advance the timing by rotating the distributor holding the distributor body until the harmonic damper and pointer timing marks align on 0°, which means the timing is set at 10° BTDC. The distributor should rotate with little force necessary but with some drag. Tighten or loosen the clamp bolt as necessary.
  10. Raise the gun timing setting to 34° BTDC and switch the gun to tachometer (the vehicle’s tachometer or a portable tachometer can be used too, although using a timing gun with built-in tach is easiest). Smoothly open the throttle by hand until the engine reaches 3,500 rpm. Do not simply flick the throttle but smoothly increase the rpm. Hold the rpm at 3,500 and switch the gun to the strobing mode. Adjust the timing up or down on the gun until the damper and pointer timing marks align. Advance the engine rpm up to 4,000 to see if the mark moves at all. If it moves, keep increasing engine RPM until the mark stops moving. Holding at this new rpm, adjust the gun timing until the timing marks align on 0°. Note the gun timing number and bring the engine back to idle. The measurement I just took includes the initial timing of 10° plus the distributor’s total mechanical advance or what I’ll call “total advance with mechanical.” Most V8 engines with iron heads will run best if this number is between 34° and 36°.
  11. Adjust initial timing to where total mechanical advance will be 34°, reset the hot idle speed, and recheck the total advance with mechanical following the steps above to confirm 34°. As an example, let’s say my timing gun read 32° when I first checked total advance with mechanical. I would raise my initial timing from 10° to 12° to bring my total advance with mechanical to 34°.
  12. Once total advance with mechanical is confirmed at 34° BTDC and hot idle is set, plug the vacuum hose into the vacuum advance can on the distributor.
    • Note on vacuum ports: Ensure that the vacuum line is attached to ported vacuum on the carburetor and not manifold vacuum. There is misinformation online with some people claiming vacuum advance should be attached to manifold vacuum and that ported vacuum was used because of smog regulations. This assumption completely misunderstands the mechanics of vacuum advance. Looking at Figure 3, ported vacuum is piped into the carburetor venturis just above the throttle plates where the vacuum signal starts and increases as the throttle plates open. Manifold vacuum is piped into the carburetor baseplate far lower than the throttle plate and is highest with the throttle plate closed. As soon as the throttle plate begins opening, the manifold vacuum decreases, whereas the ported vacuum increases. The purpose of vacuum advance is to advance the timing as the throttle is opened to assist with combustion. Contrary to what some might say online, if vacuum advance is connected to manifold vacuum the vacuum advance will be highest at idle with the throttle closed and will retard as soon as the throttle is opened and the manifold vacuum decreases, which defeats the purpose of vacuum advance and will result in poor engine performance.
  13. Repeat the steps I used to check total advance with mechanical, only this time set the timing gun at 44° as a baseline before revving up the engine. Increase engine rpm until the timing mark stops moving, increase or decrease the timing on the gun until the damper and timing tab marks are aligned on 0°, and note the timing number on the gun. This number is “total all-in advance” and, ideally, will fall between 42° and 52° on a n iron-head V8. If you subtract total advance with mechanical from this number, you get the amount of advance coming from vacuum advance. For example, if total all-in advance with vacuum is 44° and total advance with mechanical is 34°, the vacuum advance is giving me 10° (52 – 42 = 10). If the total advance with vacuum falls within the 42° – 52° range, I am ready to take a test drive and see how the engine reacts. If it falls below 42°, I either need to raise the initial timing or physically modify the slots in the mechanical and/or vacuum advance mechanisms to increase the limits. In this example, since my initial timing is set at 12° to get a total advance with mechanical of 34°, I could raise initial advance by 2° to get 36° total advance with vacuum, which still falls within my desired 34° – 36°. If my total advance with vacuum is higher than 52°, the engine risks detonation at higher rpms while under load, especially on a hot day or with cargo. In this example, I can either retard the initial or modify the mechanical and/or vacuum mechanisms to lock out advance. With my initial at 12° in this example and total mechanical at 34°, I would not want to retard initial much more than 2°. Keep in mind that initial advance above 16° is getting up there for an iron-head V8, and anything below 6° likely won’t perform optimally.
  14. Once my total advance with mechanical and my total all-in with vacuum are set, I quickly rev up the engine by hand using a smooth but relatively quick motion to about 4,000 rpm. I am looking to hear how the engine responds without load. If it backfires, bogs down, or acts otherwise problematically, I know I need to diagnose the issue since the engine will not run well on a test drive under load.
  15. If I am comfortable with how the engine responds to throttle without load, I use a marker to mark a line across the distributor base and the block or intake manifold to note my initial timing location for ease of resetting timing.
  16. Snug down (do not overtighten) the distributor clamp to where the distributor will not turn with reasonable one-handed force, and confirm the marker lines align. Replace the air cleaner.
  17. I then take the vehicle out to test drive in normal around-town driving with traffic and stoplights/signs paying attention to low-rpm and mid-rpm throttle response and any signs of bogging down or detonation. I take my tools with me for ease of tuning. If I hear or feel detonation at low rpms, either initial timing is too advanced, or the vacuum advance curve is coming in too early. For vacuum cans with adjustment, I use an Allen key through the vacuum port to either slow down or speed up the curve (counterclockwise delays vacuum, clockwise advances it). If throttle response lags or the engine stumbles, initial timing might be too retarded, vacuum might be coming in too late, or I might need to adjust the accelerator pump.
  18. Once things are working well around town at low- and mid-rpm, I find my preferred boondocks road or freeway on-ramp where I can test with wide-open throttle passes from a rolling start and during hard kick-down shifting, again paying attention to throttle response, how the engine revs up, and sounds of detonation. If the throttle response is slow or the engine stumbles above 2,000 rpm, I might need more vacuum advance or to adjust the vacuum pod to adjust the vacuum curve to come in earlier. Fuel and spark quality may also be culprits to consider if I have difficulty resolving the issue with timing. If I hear/feel detonation, I have too much all-in advance and need to either lock out mechanical vacuum by modifying the mechanism’s slot or bring in the vacuum later by adjusting the vacuum curve with the Allen wrench.

The key to dialing in timing is patience, experimenting making single and small changes at a time, and recording everything I change. Using marker lines on the distributor/intake as I make adjustments to initial timing lets me track changes and quickly move back to earlier settings on the fly without using the timing gun while on test drives. If I am having issues diagnosing a problem, unplugging the vacuum advance, plugging the vacuum hose, and driving without vacuum advance is a basic step in troubleshooting to remove that variable. A healthy engine should run just fine with no vacuum advance and 34° – 36° total with mechanical for the purpose of diagnostics. It will not be optimal for street driving long term, but it will tell me if the vacuum advance is the issue if the problems go away.

Optimal Timing Procedure (for stock and performance engines):

With performance comes a more complex method of timing, and even a stock engine will benefit from this method. The first thing we need to do is “baseline” the distributor. Having a note-taker in this procedure really speeds up the following process.

  1. With vacuum advance plugged into ported vacuum and the timing gun hooked up as described above, start the engine.
  2. Adjust the gun until the damper and pointer timing marks align on 0°. Record this initial number, for example, 10° BTDC.
  3. Slowly and steadily, increase the throttle by hand until the timing mark just begins to move. Adjust the timing light advance until the damper and pointer marks align on 0° and record this number and the rpm. Continue this procedure in 500 rpm increments until the timing mark stops advancing, recording the numbers at every 500 rpm. The last recording will be total all-in advance with vacuum.
  4. With the engine back at idle, unplug the vacuum advance and plug the hose.
  5. Slowly and steadily increase the throttle by hand until the timing mark just begins to move. Adjust the timing light until the damper and pointer marks align on 0° and record this number and the rpm. Continue this procedure in 500 rpm increments the same as above until the timing mark stops advancing. This is total advance with mechanical. Subtract this number from the total all-in with vacuum and I have advance from vacuum. Subtract initial timing from the total advance with mechanical and I have my advance from mechanical. For example, initial is 10°, total all-in is 46°, and total with mechanical is 36°. Subtract 10 initial from 36, and I know mechanical is giving me 26°. Subtract 36 from 46 and I know vacuum is giving me 10°.
  6. Take graph paper with “rpm” on the Y axis in 500 increments and “degrees advance” on the X and plot all the timing numbers recorded at 500 rpm increments to form a graph. Mark the different rpms that advance comes in and mechanical advance stops. I now have the advance curve for this exact distributor.

Now that I have the distributor baselined, I can focus on timing. Good idle quality is key to performance engines, especially those with a lot of cam overlap.

  1. Disconnect vacuum advance and plug the hose.
  2. Set initial timing to 6°.
  3. Adjust hot idle speed to where the engine will idle well in gear for an automatic transmission and in neutral for a standard. Record this rpm in in both park and neutral.
  4. Adjust the idle air-fuel mixture (see my carburetor tuning article for this procedure).
  5. For automatics, place the car in gear and record the idle rpm and manifold vacuum.
  6. With the engine in park, increase initial advance by 2° to 8° and record the rpm and vacuum, which should both be higher than the baseline measurments.
  7. Reset idle speed and record the new vacuum reading both in park and in gear, which should be higher than the baseline.
  8. Continue this process until vacuum just begins to decrease.
  9. I shut off the engine and sit down to analyze all the data I have recorded. Out of all these recorded results, I choose the highest vacuum at the lowest initial timing when the engine was in gear. This initial advance will likely be between 8° and 16°. Err on the side of lower timing than eking out every last bit of vacuum. For example, if the best vacuum is 12 inHg at 14° advance, but going to 12° advance only decreases vacuum to 11.5 inHg, I go with the lower initial advance and sacrifice the vacuum. This will be the ideal initial timing for the engine, so all timing adjustments should take place in the mechanical and vacuum advance mechanisms thereafter.
  10. If I luck out, the distributor will give the perfect amount of mechanical to put total advance with mechanical between 34° and 36° and total all-in with vacuum at 42° – 52°.
  11. If mechanical advance puts too much in and puts advance over 36°, I have to remove the mechanical plate and weld the stop to limit movement.
  12. To adjust vacuum advance limits, they make different vacuum cans. Double the can number to give you engine advance, so a #7 can will give 14° of total vacuum advance. To lock out vacuum advance, I use epoxy and a piece of rebar tie wire as a stop since welding like I do on the mechanical plate will melt the can’s rubber diaphragm. If the engine detonates at low rpms, I try turning the Allen screw through the vacuum port counterclockwise to delay vacuum. Clockwise will bring in vacuum advance at a lower vacuum if the engine is sluggish at low rpm throttle.
  13. At this point, I am ready to test drive the car as described above.
  14. For even more precise tuning of the advance curve, I can either cough up the money to pay a dyno tuner or go to the drag strip. Ideally, the advance curve will be set by a dyno tuner (kill two birds with one dyno tuner by having carburetor jetting tuned too), but I can get in the ballpark at the track. The general practice for performance street cars is to have the mechanical advance all-in by about 2,000 rpm, and I use different gauge springs to adjust the curve. In comparison, most stock mechanical advance does not come all-in until 4,000 rpm, which is way too high for performance builds. The rules to follow when tuning the mechanical curve are (1) speed up the advance curve as much as possible without causing running issues until reaching the best trap speed and (2) if denotation is an issue at full throttle, delay the advance curve.

Mopar A-block firing order
Figure 1: Firing Order and Cylinder Numbers

Mopar A-block timing mark
Figure 2: A-block Timing Mark
Figure 3: Ported vs. Manifold Vacuum for Vacuum Advance

Electronic Conversion

While points ignition can be perfectly reliable and last a very long time on our classic vehicles that are not driven daily like they used to be, LA-design electronic ignition is a superior design and easy and affordable to convert onto an A-block. If you plan on driving your vehicle more than 3,000 miles annually or if you are looking for the best performance, I recommend upgrading to electronic ignition; if you have a period-correct build with a stock distributor or Mallory dual-point or if you drive less than 3,000 miles per year, I detail points adjustment below.

Since the distributor is a crucial, delicate component of the ignition system, I recommend staying away from cheaply made imports, including Pertronix and Proform. Pertronix is all over the internet largely because they are inexpensive and have strong marketing strategies to the point one might confuse them for a high-quality brand. Do your own research online to see if their parts are actually made in the USA or China regardless of what they advertise since they have been proven to claim they are made in the USA but in fact use Chinese parts. For my personal experience, I have purchased two Pertronix distributors in the last ten years a few years apart for two different engines. With the first distributor, the timing mark jumped all over the place. I removed the distributor and setup a dial indicator on the distributor cam and found it was machined out of round creating the fluctuating timing. About five years passed where I thought maybe the company had improved, but the next distributor I purchased had a similar issue with the timing fluctuating, only I found the shaft was bent.

Some people also recommend the 2000 – 2013 Mopar Performance distributor, which uses miniaturized Accel GM HEI internals, but these units are known to have electronic issues including erratic timing that is the root cause for a host of issues. I stay away from them.

Many Mopar owners and racers, myself included, have had excellent results with Richard Ehrenberg’s trademarked HiRev 7500 distributor (P3690430), HiRev 7500 ignition control unit, coil, 0.5 ohms ballast resistor, and wiring harness (Figure 4). His kits can be purchased through him on ebay, user name rehrenberg. They are high-quality USA-made components with a reputation in performance circles who want the stock look since they look nearly identical to a factory electronic setup and are based off the 440 A-134 hi-performance units. If desired, the control unit can be hidden in the engine compartment to keep the factory points look. Ehrenberg sells the components direct, but some of them can be purchased through other vendors.

Important Adjustment Note: When adjusting the gap between the distributor cam and pickup, use a brass feeler gauge since a steel gauge will create drag and can also disrupt the magnetism.

hirev7500 distributor kit
Figure 4: HiRev 7500 Kit (photo and parts by Richard Ehrenberg)

Points Adjustment and Distributor Inspection/Cleaning

What do contact points even do? Essentially, they function as a type of trigger that helps direct coil current. The coil ground is connected to one side of the points. When the points are closed, the coil primary circuit is completed and lets the primary current flow for the coil to build up its charge; when the points open, the circuit is broken, primary current stops flowing through the coil, and the magnetic field in the coil collapses. Voltage builds high enough to move through the coil wire, transfer from the distributor cap to the rotor, jump from the rotor to the cap terminal, and flow down the spark plug wire where it eventually arcs at the spark plug. For this process to function properly, the points need to be clean and properly adjusted.

There are two adjustments for points: gap and its related dwell angle. Referring to Figure 5, points gap refers to the gap between the two contact points when the rubbing block is up on the distributor cam lobe at full lift. Gap in measured while the engine is off. The dwell angle is measured with the engine running and measures how long points stay closed in degrees out of a 360° rotation of the distributor shaft. Dwell is the important measurement of these two, and gap simply puts the points in the ballpark so the engine will run in order to adjust dwell precisely.

Here is the process I use for inspecting and cleaning the distributor and setting gap and dwell:

  1. With the ignition switch off, unplug the coil-to-distributor wire and remove the distributor cap. Mark and unplug the distributor wires as necessary, making sure to mark the #1 plug on the cap for reassembly.
  2. Remove the rotor. Inspect the rotor tip for corrosion. If lightly corroded, use fine-grit sandpaper to lightly clean the tip. If heavily corroded/pitted/melted, replace.
  3. Inspect the cap terminals for corrosion. Clean or replace similar to the rotor.
  4. Inspect the points rubbing block for excessive wear or damage. Plan on replacing if excessively worn or damaged.
  5. Inspect the cam lobe for debris. Wipe off the cam if necessary and wipe on a very, very small dab of distributor cam lube. Excessive or incorrect lube can fling off onto the points and cause spark issues.
  6. Inspect the ground and condenser wires for damage. Address if necessary.
  7. Push and pull the distributor shaft side to side and up/down looking for excessive play. If there is excessive play, the bushings and/or shaft are worn and will need replacing.
  8. Using a small, clean flathead screwdriver, gently push the points arm away from the stationary point about 1/4″ to visually inspect the points. Do not bend the arm, use excessive force, or pry the arm beyond its pivot range. Use a flashlight to inspect both sides of the points for burning, corrosion, and pitting.
  9. If the points are corroded to where they require heavy filing, remove the points from the distributor since debris will fall into the distributor. Place a clean points file in a vice and sandwich the points around the file. Holding the points so they do not flex or contort, move them back and forth across the file until the points are clean and trued up. The maximum allowed amount of pitted surface area is 30% after filing. Any more pitting, the points need replacing. Blow off the points with compressed air and reinstall being careful not to strip the screw threads or round off the screw heads.
  10. Rotate the engine clockwise when facing the front of the engine using a ratchet on the crankshaft bolt until the points arm is at full lift on top of the cam lobe. Once close to full lift, gripping the mechanical advance mechanism at the top of the shaft and rotating it by hand will give enough slop to precisely open/close the points.
  11. Factory poly A-block and LA points gap is 0.017”. Take a 0.017” feeler gauge (I use a wire sparkplug gauge since it is easier to get into tight spots) and gently try to insert it between the points. The goal is to be able to insert the gauge with slight drag/resistance but not enough to push open the points farther.
  12. If the gap needs adjusting, locate the locking screw attached to the stationary point. Loosen the screw keeping some light tension on it. Locate the prying points on the baseplate and the point bracket, which are often a small nub or notch. Using a clean screwdriver, gently pry the point bracket away from the other point to increase gap or toward the other point to decrease gap. Never pry on the point itself.
  13. Once the gap is set, tighten the lock screw snugly and confirm one last time the gap is properly set. Don’t over-tighten the screw.
  14. Reinstall the rotor and wipe off any grease and fingerprints with lacquer thinner or brake clean, for grease can inhibit proper electricity transfer. Reinstall the cap and wires.
  15. Hook up a dwell meter to the coil (I use the dwell meter on my timing gun). If using a two-lead meter, the black goes to a confirmed ground and the red goes to the coil negative post. If using a timing gun equipped with a dwell meter, the gun black lead goes to a confirmed ground and the dwell lead goes to coil negative. Ensure the wires are clear of any exhaust, fans, pulleys, and belts.
  16. Start the engine and bring it up to running temperature, holding your hand on top of the radiator near the intake port to feel when the thermostat opens and hot coolant rushes into the radiator. Note the dwell meter reading. An 8-cylinder engine should have 26° – 28° dwell ideally. If necessary, shut off the ignition switch, follow the above procedure for removing distributor parts, and adjust the points. If you increase the points gap, the dwell angle goes down since the points remain closed for a shorter amount of shaft rotation; if I decrease the gap, the dwell angle goes up. For example, if dwell is 30°, I would increase the gap very slightly (as in by .002”) and recheck dwell. If the dwell is 24°, I would decrease the gap very slightly and recheck dwell. Yes, this process can take multiple adjustments, which is why Chevy invented points that could be adjusted with the engine running using a long hex T-wrench through a door in the distributor cap. Unfortunately, Mopar did not use a similar system.
  17. Once dwell is set between 26° – 28°, confirm that the points screws are tight and that the rotor, cap, and wires are properly installed.

distributor points system
Figure 5: Distributor Points System